A plumbing fixture unit calculation can give two honest answers for one room. Take a restroom group of five water closets, four lavatories, two floor drains and one mop basin. It adds up to 30 drainage fixture units under the 2021 International Plumbing Code (IPC). Under the 2021 Uniform Plumbing Code (UPC) it adds up to 31. The water supply side sits further apart: 36.0 supply fixture units on the IPC scale and 19.5 on the UPC scale. Neither answer is wrong. They come from different tables, and a sizing sheet that does not say which table it used cannot be checked.
This guide is for US plumbing contractors and design firms. It helps whoever sizes pipe from fixture counts, or reviews sizing done by someone else. It explains what a fixture unit is, where the two codes differ, and how to size drains, vents and supply pipe from the totals. It then runs the same restroom group through both codes and lists what the drawings must show. For the wider question of handing a package like this to an outside team, read our MEP design outsourcing guide for US contractors. The MEP design services hub describes the disciplines behind it.
Quick answer. A fixture unit is a code-assigned load factor, not a flow rate. Add drainage fixture units (DFU) from IPC Table 709.1 or UPC Table 702.1 to size drains and vents. Add water supply fixture units (WSFU) from IPC Appendix E or UPC Table 610.3 to size supply pipe. The two codes use different scales, so never mix tables across codes. Values here are from the 2021 editions, with 2024 checked.
Sources were checked on 31 October 2026. The International Code Council site (codes.iccsafe.org) returned an access error from our research environment. IPC text was therefore read in UpCodes viewers of state adoptions. Chapter 7 came from the Maryland and Colorado adoptions. Chapters 9 and 11 and Appendices B and E came from Colorado: Chapter 9, Chapter 11, Appendix B and Appendix E. Both states adopt with amendments. The three drainage tables used below read the same in the two states. They read the same again in the 2024 IPC pages for Maryland, Colorado, Utah and Georgia, such as the Maryland 2024 Chapter 7.
UPC text was read in the read-only viewers of the 2021 UPC and 2024 UPC. The International Association of Plumbing and Mechanical Officials (IAPMO) publishes both. We cross-checked it against the Hawaii Plumbing Code 2021 on UpCodes, which UpCodes labels as adopted without amendments. Values below are 2021 editions unless marked. The edition your jurisdiction has adopted, and its local amendments, control the design. Confirm both with the authority having jurisdiction (AHJ).
What is a fixture unit, and why do the two codes disagree?
A fixture unit weights a fixture by how much load it puts on a pipe. IPC Section 709.1 says the values “designate the relative load weight of different kinds of fixtures”. They are used with Tables 710.1(1) and 710.1(2) to size soil, waste and vent pipe. A water closet, a lavatory and a floor drain each get a number. You add the numbers for everything connected to a pipe and read the total against a capacity table.
There are two separate scales. DFU size the drain and vent side. WSFU size the water supply side. The codes do not link the two: a fixture can carry one value for drainage and a very different one for supply. A private bathtub is 2 DFU in both codes. On the supply side it is 1.4 WSFU in IPC Table E103.3(2) and 4.0 WSFU in UPC Table 610.3. The scales belong to their own code, so a WSFU total only means something next to the table that produced it.
A fixture unit is also not a flow rate. Continuous or pumped flows convert to DFU at the same rate in both codes. IPC Section 709.3 and UPC Section 702.3 both set 2 DFU for each gallon per minute (gpm). The UPC names air conditioning equipment among the sources. The gpm comes from the equipment submittal, so the plumbing designer needs the mechanical schedule that follows the load calculation in our HVAC load calculation guide. That last point is practitioner judgement, not code text.
Where do the IPC and UPC differ for fixture-unit sizing?
The table below is the map. The sections after it give the values behind each row. Each row names the table or section it came from, and the editions are 2021 unless a row says otherwise.
| Topic | IPC 2021 | UPC 2021 |
|---|---|---|
| Drainage fixture unit table | Table 709.1 | Table 702.1 |
| Use classes | One value per fixture row, with separate rows for private and public water closets | Three columns: private, public and assembly |
| Water closet, 1.6 gpf, public | 4 DFU | 4.0 DFU (assembly 6.0) |
| Service sink or mop basin | 2 DFU, 1 1/2 in. trap | 3.0 DFU public, 2 in. trap |
| Shower | 2, 3, 5 or 6 DFU by total flow rate | 2.0 DFU for a single-head trap, plus 1.0 for each additional head |
| Drain and vent capacity | Table 710.1(1) for building drains and sewers, Table 710.1(2) for horizontal fixture branches and stacks, Table 906.1 for stack vents and vent stacks | Table 703.2 covers drains, branches and vents |
| 3 in. horizontal pipe | 42 DFU as a building drain at 1/4 in. per ft, 36 at 1/8; 20 as a horizontal fixture branch | 35 DFU at 1/4 in. per ft, and no more than five water closets (six in the 2024 edition) |
| Minimum slope, 3 in. pipe | 1/8 in. per ft (Table 704.1) | 1/4 in. per ft; 1/8 in. only for 4 in. and larger, with AHJ approval (Section 708.1) |
| Vent sizing | Individual and branch vents at least one-half the drain served; Table 906.1 for stack vents and vent stacks | Table 703.2 vent rows by units vented and length |
| Supply fixture unit table | Table E103.3(2), in Appendix E, with cold, hot and total columns | Table 610.3 in Chapter 6 (Table A 103.1 in Appendix A is a parallel table), with private, public and assembly columns |
| Where the supply sizing method sits | Chapter 6 leaves it to the designer; Appendix E offers methods and is not mandatory unless the adopting ordinance references it | Table 610.4 and Section 610.8 in Chapter 6 |
| Length allowance in the table method | Actual developed length multiplied by 1.2 (Section E104.1) | Developed length of supply pipe from meter to the most remote outlet (Section 610.7) |
| Flushometer systems | Flushometer column of Table E103.3(3); public flushometer water closet 10.0 WSFU | Table 610.10 assigns 40, 30, 20, 15, then 10 each to successive water closets |
The practical lesson is simple. If a calculation names a pipe size and no table, it cannot be reviewed. If it names a table but not the code edition, it can be reviewed only until the AHJ asks which edition applies.
How do you calculate drainage fixture units and size drains?
Start with the fixture schedule. Count each fixture, pick the use class, read the DFU from the code table and add. The values below are the restroom fixtures most layouts need.
| Fixture | IPC 2021 Table 709.1 (DFU) | UPC 2021 Table 702.1 (private / public / assembly DFU) |
|---|---|---|
| Water closet, 1.6 gpf | 3 private, 4 public | 3.0 / 4.0 / 6.0 |
| Urinal, 1 gpf or less | 2 | 2.0 / 2.0 / 5.0 |
| Lavatory | 1 | 1.0 / 1.0 / 1.0 |
| Floor drain, 2 in. | 2 | 2.0 / 2.0 / 2.0 |
| Service sink or mop basin | 2 (1 1/2 in. trap) | not listed / 3.0 / 3.0 (2 in. trap) |
| Drinking fountain | 1/2 | 0.5 / 0.5 / 1.0 |
| Bathtub | 2 | 2.0 / 2.0 / not listed |
Sources: IPC 2021 Table 709.1 as adopted in Maryland and Colorado, read on UpCodes (see the Chapter 7 links above). UPC 2021 Table 702.1, read in the IAPMO viewer and in the Hawaii Plumbing Code 2021 text of Section 702.
For a fixture the table does not list, the IPC assigns DFU by trap or outlet size in Table 709.2. A 1 1/4 in. trap is 1 DFU, 1 1/2 in. is 2, 2 in. is 3, 2 1/2 in. is 4, 3 in. is 5 and 4 in. is 6. The UPC bases the unit equivalent of an unlisted fixture on its trap or trap arm size. Table 702.1(1) caps the load a trap and trap arm may carry.
What do the capacity tables allow?
Once you have a total, you read it against the pipe capacity. The IPC splits this over two tables, and which one applies depends on what the pipe is. The UPC uses one.
| Pipe size | IPC building drain, 1/4 in. per ft | IPC building drain, 1/8 in. per ft | IPC horizontal fixture branch | IPC stack, three branch intervals or less | UPC horizontal, 1/4 in. per ft | UPC vertical |
|---|---|---|---|---|---|---|
| 2 in. | 21 | not listed | 6 | 10 | 8 | 16 |
| 3 in. | 42 | 36 | 20 | 48 | 35 | 48 |
| 4 in. | 216 | 180 | 160 | 240 | 216 | 256 |
Sources: IPC 2021 Tables 710.1(1) and 710.1(2), Maryland and Colorado adoptions, unchanged in the 2024 pages for Maryland, Colorado, Utah and Georgia. UPC 2021 Table 703.2 in the Hawaii Plumbing Code 2021 text of Section 703 and its 2024 successor in the IAPMO viewer. The IPC stack and UPC vertical columns are the closest comparable pair, not an exact match.
Four footnotes can change the answer even when the table values agree:
- IPC minimum size. Table 710.1(1) states that the minimum size of any building drain serving a water closet is 3 inches.
- UPC water closet count. In the 2021 Table 703.2, note 4 limits a 3 in. drainage pipe to “five water closets or five six-unit traps”. The 2024 text reads six water closets and five six-unit traps. A sixth water closet therefore forces a 4 in. drain under the 2021 UPC but not under the 2024 UPC.
- UPC slope factor. Note 5 gives horizontal capacities for 1/4 in. per ft and says to multiply by 0.8 at 1/8 in. per ft. It is attached to the 4 in. and larger columns.
- IPC stacks. Table 710.1(2) sizes stacks on the total accumulated connected load at each story or branch interval. A stack may reduce in size as that load falls, but not below one-half of the largest stack diameter.
Slope also differs. IPC Table 704.1 sets a minimum of 1/4 in. per ft for pipe 2 1/2 in. and smaller, 1/8 in. for 3 to 6 in., and 1/16 in. for 8 in. and larger. UPC Section 708.1 requires 1/4 in. per ft and permits 1/8 in. only for 4 in. and larger pipe, where the AHJ approves it first. A long slab-on-grade run that is tight on invert depth can pass under one code and need a size change under the other.
How do you size vents under each code?
The two codes size vents on different logic, and a restroom group shows it quickly.
| Rule | IPC 2021 | UPC 2021 |
|---|---|---|
| Individual and branch vents | At least one-half the required diameter of the drain served, with the drain size read from Table 710.1(2), and never under 1 1/4 in. (Section 906.2) | Not less than 1 1/4 in. and not less than one-half the diameter of the drain (Section 904.1, Table 703.2 note 6) |
| What sets the size | Drain size for branch vents; Table 906.1 for stack vents and vent stacks, using stack size, DFU vented and developed length | Length and the total fixture units connected, read from the vent rows of Table 703.2 (Section 904.1) |
| Capacity rows | Table 906.1, row by stack size and DFU | 1 1/2 in.: 8 units, 60 ft. 2 in.: 24 units, 120 ft. 3 in.: 84 units, 212 ft. 4 in.: 256 units, 300 ft |
| Long vents | More than 40 ft developed length goes up one nominal size for the whole length (Section 906.2) | At most one-third of the permitted length may be horizontal. Increase the vent one pipe size for its entire length and the table length limits no longer apply |
| Common branch vents | Sized for the common horizontal branch that is, or would be, required for the total DFU vented (Section 906.4) | Section 904.1 sizes a vent on the total fixture units connected to it, so a common vent carries the combined units |
Sources: IPC 2021 Sections 906.1, 906.2 and 906.4, Colorado adoption, with Section 906.2 and Table 906.1 unchanged in the 2024 pages. UPC 2021 Section 904.1 and Table 703.2, IAPMO viewer (Section 904.1 reads the same in the 2024 viewer) and the Hawaii text linked above.
Table 906.1 reads by row. A 3 in. soil or waste stack has rows at 10, 21, 53 and 102 DFU. A stack venting 30 DFU falls in the 53 DFU row, where the table allows a 2 in. vent to a developed length of 94 ft or a 1 1/2 in. vent to 27 ft. The first row at or above your load is the one to use.
How do you calculate water supply fixture units and size the supply pipe?
Add WSFU the same way you add DFU, then convert the total into a pipe size. The WSFU values for the same restroom fixtures show how far the two scales sit apart.
| Fixture | IPC 2021 Table E103.3(2): cold / hot / total | UPC 2021 Table 610.3: private / public / assembly |
|---|---|---|
| Water closet, flush tank | Public 5.0 total; private 2.2 | 1.6 gpf gravity tank: 2.5 / 2.5 / 3.5 |
| Water closet, flushometer valve | Public 10.0 total; private 6.0 | See Table 610.10 |
| Lavatory | Public faucet 1.5 / 1.5 / 2.0; private 0.5 / 0.5 / 0.7 | 1.0 / 1.0 / 1.0 |
| Service sink or mop basin | Offices and similar, faucet: 2.25 / 2.25 / 3.0 | 1.5 / 3.0 / not listed |
| Bathtub | Private faucet 1.0 / 1.0 / 1.4; public 3.0 / 3.0 / 4.0 | 4.0 / 4.0 / not listed |
| Drinking fountain | 0.25 (3/8 in. valve) | 0.5 / 0.5 / 0.75 |
Sources: IPC 2021 Table E103.3(2), Colorado adoption of Appendix E, with the values unchanged in the 2024 pages. UPC 2021 Table 610.3, IAPMO viewer and the Hawaii Plumbing Code 2021 text of Section 610.3. Both codes state that separate hot and cold loads may each be taken as three-quarters of the total.
How do flushometer valves change the arithmetic?
They change it a lot, and the two codes change it differently. For five flushometer water closets in public use, IPC Table E103.3(2) gives 10.0 WSFU each, so 50.0 WSFU. UPC Table 610.10 assigns 40, 30, 20, 15 and then 10 to successive water closets, so five valves count as 115. The IPC then converts WSFU to gpm in the flushometer column of Table E103.3(3). The UPC feeds its figure into Table 610.4 or the methods in Section 610.5. Do not carry a flushometer total from one code into the other code’s demand table.
How does each code turn WSFU into a pipe size?
The IPC does not set a sizing method in Chapter 6. A user note in Appendix E says sizing is “left up to the designer of the system with the code official approving the design method”. Appendix E then offers two methods, and it is not mandatory unless the adopting ordinance references it. The segmented loss method in Section E103.3 converts WSFU to gpm with Table E103.3(3). It then adds up the pressure losses, including friction read from the Appendix E charts, to check the supply. Section E104.1 is the table method. It deducts 0.5 psi for each foot the highest outlet sits above the meter and multiplies the actual pipe length by 1.2 for fittings. You then pick a pressure band and a length column. Read the meter and distribution pipe from the first row whose limit is at or above the WSFU.
The UPC puts its table method in Chapter 6. Section 610.8 follows the same steps. Deduct or add 1/2 psi for each foot of elevation, then pick the pressure range in Table 610.4. Pick the length column equal to or longer than the run, and move down the column to a value at or above the fixture units. The building supply may not be less than 3/4 in. Section 610.4 permits the table or the methods in Section 610.5, which point to Appendices A and C.
Neither table replaces the pressure and velocity check. IPC Section E103.1 asks for at least 15 psi flowing at a group of flushometer valves and at least 8 psi flowing at flush tank supplies. It also warns that pressure and noise are the two most common complaints about water supply systems. A chart note in the 2024 Appendix E says fluid velocities above 5 to 8 ft per second are not usually recommended. UPC Sections 610.1 and 610.12 require piping to stay within the maximum velocities the code and the applicable standard allow.
How do you run a plumbing fixture unit calculation for a restroom group?
The example below is illustrative. It sizes by table only, so it does not replace the engineer of record or the AHJ review. Every number is traceable to a table named in the row.
Assumptions.
- One-story commercial tenant space, slab on grade, public-use restrooms.
- Women’s room: 3 water closets (1.6 gpf, gravity flush tank), 2 lavatories, 1 floor drain. Men’s room: 2 water closets, 2 lavatories, 1 floor drain. Janitor closet: 1 service sink or mop basin. No urinals, flushometer valves, kitchen or hose bibbs.
- Building drain at 1/4 in. per ft.
- Minimum daily static pressure of 58 psi at the meter, with the highest outlet 10 ft above it. The supply pipe runs 100 ft from the meter to the most remote outlet. There is no pressure-reducing valve, backflow preventer or filter in the path, and no hot water load.
Step 1: add the drainage fixture units
| Fixture | Qty | IPC 2021 Table 709.1, each | IPC total | UPC 2021 Table 702.1 (public), each | UPC total |
|---|---|---|---|---|---|
| Water closet, 1.6 gpf | 5 | 4 | 20 | 4.0 | 20.0 |
| Lavatory | 4 | 1 | 4 | 1.0 | 4.0 |
| Floor drain, 2 in. | 2 | 2 | 4 | 2.0 | 4.0 |
| Service sink or mop basin | 1 | 2 | 2 | 3.0 | 3.0 |
| Total | 12 | 30 | 31.0 |
By room, the women’s room carries 16 DFU under both codes: 12 from water closets, 2 from lavatories and 2 from the floor drain. The men’s room carries 12 under both codes. The janitor closet is the only place the codes split: 2 DFU under the IPC and 3.0 under the UPC.
Step 2: size the drains
| Pipe | IPC 2021 | UPC 2021 |
|---|---|---|
| Women’s room branch, 16 DFU | 3 in. (Table 710.1(2): 2 in. holds 6, 3 in. holds 20) | 3 in. (Table 703.2: 2 in. holds 8, 3 in. holds 35) |
| Men’s room branch, 12 DFU | 3 in. (3 in. holds 20) | 3 in. (3 in. holds 35) |
| Group drain, 30 DFU IPC and 31.0 UPC, at 1/4 in. per ft | 3 in. (Table 710.1(1): holds 42, so 12 spare) | 3 in. (Table 703.2: holds 35, so 4 spare; five water closets meets note 4) |
If the IPC run is a horizontal fixture branch, Table 710.1(2) governs. If it is a branch of the building drain, Table 710.1(1) governs. The result is 3 in. either way here. Add a sixth water closet and the totals become 34 DFU under the IPC and 35.0 under the UPC. The IPC drain stays at 3 in. The 2021 UPC forces 4 in. because of the water closet count, even though 35 DFU is exactly the 3 in. limit. Under the 2024 UPC, six water closets are allowed and 3 in. works with nothing spare. If site conditions force 1/8 in. per ft, the IPC 3 in. drain still carries 36 DFU against a 30 DFU load. The UPC does not permit 1/8 in. on a 3 in. pipe, so the drain moves to 4 in., where Table 703.2 gives 216 DFU at 1/4 in. and 172.8 after the 0.8 factor, subject to AHJ approval.
Step 3: size the vents
| Vent | IPC 2021 | UPC 2021 |
|---|---|---|
| Women’s room branch vent | 1 1/2 in. (one-half of the 3 in. drain, Section 906.2) | 2 in. (16 units: 1 1/2 in. holds 8, 2 in. holds 24) |
| Men’s room branch vent | 1 1/2 in. | 2 in. (12 units) |
| Common branch vent for both rooms | 2 in. (30 DFU needs a 4 in. branch because 3 in. holds 20, so one-half is 2 in., Section 906.4) | 3 in. (31.0 units: 2 in. holds 24, 3 in. holds 84, up to 212 ft) |
This is where the codes diverge most in this example. The IPC reads the drain size, so the common vent is 2 in. The UPC reads the units vented under Section 904.1, so the same group needs a 3 in. common vent. Check developed length too. Under the IPC, a vent longer than 40 ft goes up one nominal size. Under the UPC, a 2 in. vent is limited to 120 ft and a 3 in. vent to 212 ft unless the vent is increased one size for its full length.
Step 4: add the water supply fixture units
| Fixture | Qty | IPC 2021 Table E103.3(2), total each | IPC total | UPC 2021 Table 610.3 (public), each | UPC total |
|---|---|---|---|---|---|
| Water closet, flush tank | 5 | 5.0 | 25.0 | 2.5 | 12.5 |
| Lavatory, public faucet | 4 | 2.0 | 8.0 | 1.0 | 4.0 |
| Service sink or mop basin | 1 | 3.0 | 3.0 | 3.0 | 3.0 |
| Total | 36.0 | 19.5 |
Floor drains carry no supply load in either table. Neither total is wrong. They sit on different scales and each is meant to be read against its own code’s tables.
Step 5: read the supply pipe size from each table
| Step | IPC 2021 Appendix E, Table E104.1 | UPC 2021 Chapter 6, Table 610.4 |
|---|---|---|
| Pressure after elevation | 58 psi minus 0.5 psi x 10 ft = 53 psi | 58 psi minus 1/2 psi x 10 ft = 53 psi |
| Pressure band | 50 to 60 psi | 46 to 60 psi |
| Length | 100 ft x 1.2 = 120 ft, so the 150 ft column | 100 ft, so the 100 ft column |
| Load | 36.0 WSFU | 19.5 WSFU |
| Limits read down the column | 3/4 in. meter with 1 in. pipe: 25. 1 in. meter with 1 in. pipe: 30. 3/4 in. meter with 1 1/4 in. pipe: 32. All fall short. 1 in. meter with 1 1/4 in. pipe: 80, which passes | 3/4 in. meter with 3/4 in. pipe: 17, which falls short. 3/4 in. meter with 1 in. pipe: 33, which passes |
| Result | 1 in. meter, 1 1/4 in. distribution pipe | 3/4 in. meter, 1 in. building supply |
Dropping the IPC 1.2 factor, or applying it to the UPC, changes neither answer. At 100 ft the IPC still lands on 80 for the 1 in. meter and 1 1/4 in. pipe, and at 150 ft the UPC gives 28 for 3/4 in. and 1 in., above 19.5. The difference comes from the fixture-unit scales and the table design, not from an error in either lookup.
For the IPC segmented loss method, the demand comes from Table E103.3(3) in the flush tank column. The table has rows at 35 WSFU (24.9 gpm) and 40 WSFU (26.3 gpm). The 36.0 WSFU load sits between them. Taking the next row up gives 26.3 gpm, and straight-line interpolation gives about 25.2 gpm. Run that flow through the friction charts for your pipe material and keep velocity inside the manufacturer’s limit.
What the example shows
- The drainage totals differ by one unit, because the mop basin counts 2 under the IPC and 3.0 under the UPC. The capacity tables, notes and slope rules matter more than that unit.
- The vent diverges: 2 in. under the IPC and 3 in. under the UPC for the same group.
- The UPC supply total (19.5) is a little over half the IPC total (36.0). A reviewer who compares the two numbers without the table names will think one of them is an error.
- Each result is a table minimum. Pressure losses, velocity and the AHJ review still follow.
How is storm drainage sized, and where do the rainfall tables fit?
Storm drainage uses a design rainfall rate and a roof area instead of fixture units. Both codes’ city rainfall tables use the 100-year, one-hour storm. IPC Section 1106.1 sizes conductors, leaders and storm drains on that rate. The rate comes from Figures 1106.1(1) through 1106.1(5) or from approved local weather data. IPC Appendix B, Table B101, lists city rates but is marked informative and not part of the code. UPC Table D 101.1 lists a 60-minute, 100-year rate for each city, in inches per hour and in gallons per minute per square foot.
The tables disagree for the same city. Birmingham, Alabama is 3.8 in. per hour in IPC Table B101 and 3.7 in UPC Table D 101.1. Montgomery, Alabama is 4.2 in IPC Table B101 and 3.8 in UPC Table D 101.1. Use the rate the AHJ names, and write it on the drawing with its source.
The pipe tables work differently too. The IPC converts rainfall to a flow in gpm and checks it against Table 1106.2. It also checks the flow against the roof drain manufacturer’s published rate for the specific drain model. The UPC tabulates the maximum roof area by rainfall rate in Tables 1103.1 and 1103.2. For a rate that is not listed, divide the area in the 1 inch per hour column by the rate. The UPC’s own example in Section D 103.1 is a 6 in. drain at 1/8 in. per ft and 3.2 in. per hour: 21,400 divided by 3.2 is 6,688 square feet.
| Horizontal storm pipe, full flow | IPC 2021 Table 1106.2 (gpm) | UPC 2021 Table 1103.2 (gpm) |
|---|---|---|
| 4 in. at 1/8 in. per ft | 115 | 78 |
| 4 in. at 1/4 in. per ft | 163 | 110 |
| 6 in. at 1/8 in. per ft | 344 | 222 |
| 6 in. at 1/4 in. per ft | 487 | 314 |
Sources: IPC 2021 Table 1106.2, Colorado adoption, unchanged in the 2024 page. UPC 2021 Table 1103.2, IAPMO viewer and the Hawaii Plumbing Code 2021 text of Section 1103, unchanged in the 2024 viewer.
Two more rules are worth a line. IPC Section 1106.4 adds one-half of the area of any vertical wall that diverts rainwater onto the roof to the projected roof area. Section 1106.5 sends scupper ponding depth back to the roof design in IBC Section 1611.1. The International Building Code therefore controls that depth, not the plumbing code.
What do the plumbing drawings need to show?
A sized pipe has to land on a sheet a reviewer can check. A plumbing set for a commercial building normally carries these items. The code excerpts above support the DFU and WSFU columns. The remaining list is practitioner judgement, so check it against your AHJ’s checklist.
- Plans. Fixture locations with tags, supply and DWV routing, pipe sizes, slopes, cleanouts, and the vent route to the roof.
- Fixture schedule. One row per fixture type: tag, description, flow or gpf, trap size, DFU, WSFU, branch size and connection sizes. The example tables above are the schedule in outline.
- Drainage riser diagram. The stacks, vent stacks and branch intervals, with the accumulated DFU written at each interval. IPC Table 710.1(2) sizes stacks on the accumulated load at each story or branch interval. A riser that shows the running total lets a reviewer check every reduction in size.
- Water riser diagram. The risers and branches with WSFU and gpm at each segment, the pressure assumptions and the table method used.
- Calculation sheet. The code and edition, table numbers, fixture counts, totals and lookups, so the sizing can be rechecked without rebuilding it.
- Storm sheet. The rainfall rate and its source, roof areas per drain, and the leader and storm drain sizes.
For tenant work, the plumbing sheets a city asks for often sit inside a wider permit set. The MEP drawings for commercial tenant improvements guide shows how. After permit, trades issue shop drawings, covered in the MEP shop drawings guide. Model detail for plumbing at each stage is described in the Revit MEP LOD 300 vs LOD 400 deliverables guide. This article sizes domestic water only. Sprinkler systems use their own hydraulic method, set out in our fire sprinkler hydraulic calculation guide.
If the sizing and the sheets are the constraint on your team, the Heaven Designs plumbing design services page is the point of need. It describes domestic water layouts and risers, drainage and vent design with falls and stacks, and pump rooms and tanks. It also lists isometrics and details, fixture layouts, fixture and equipment schedules, and a plumbing BIM model. Storm or special drainage is included where project scope includes it. The page describes packages for design firms and for property-owner projects. Tell the team which code edition and AHJ govern the project when you scope it.
When does it make sense to outsource plumbing sizing and drawings?
It helps when plumbing packages arrive in clusters and in-house engineers are committed elsewhere. It works when scope, authority and inputs are written down: fixture list, utility pressure and invert information, adopted code edition, and who reviews and signs. The outsourcing control guide lays out those controls. Stamping requirements are confirmed at quote stage, and turnaround is agreed at scoping.
FAQ
What is the difference between DFU and WSFU?
DFU weight a fixture for the drain and vent side, and WSFU weight it for the water supply side. A private bathtub is 2 DFU in both codes but 1.4 WSFU in IPC Table E103.3(2) and 4.0 WSFU in UPC Table 610.3. They are separate scales and are never added together.
Can I use IPC fixture unit values with a UPC pipe table?
No. Each code’s pipe table is written for that code’s fixture-unit values. In the example, the same restroom group is 36.0 WSFU under the IPC and 19.5 under the UPC. Each total is read against its own code’s table.
Which code applies to my project?
The state or city sets it, and many adopt a model code with local amendments. Read the plumbing code and edition named in the AHJ’s permit checklist, and ask the AHJ about any amendment to the sizing tables or appendices.
Do the 2024 editions change the numbers?
For the tables used here, mostly not. The IPC 2024 pages for Maryland, Colorado, Utah and Georgia show the same Table 709.1, 710.1(1) and 710.1(2) values. The UPC 2024 viewer shows the same Table 702.1, 610.3 and 610.4 rows used above. The change we found is UPC Table 703.2 note 4, which moves the water closet limit on a 3 in. pipe from five to six. The 2024 IPC Appendix E also adds a chart note on velocity. We did not compare every section between editions.
Where do HVAC condensate and pumped discharge enter the drain count?
Both codes convert a continuous flow at 2 DFU for each gpm (IPC Section 709.3, UPC Section 702.3). Get the gpm from the equipment data and add the resulting DFU to the pipe that receives it.
Is the supply table method enough to size the pipe?
It gives a minimum from a table. The code text still asks for pressure and velocity checks, and the AHJ decides what is acceptable for the project. Treat the table result as the first line of the calculation sheet, not the last.
Name the code and edition before you size a pipe
Start every plumbing calculation sheet with the code, edition, adopting jurisdiction and table numbers. Then add the fixtures, sum the DFU and WSFU, and read each pipe from its own table. Keep a separate line for continuous flows, flushometer systems and storm drainage, because those follow different rules in the two codes.
If your team needs capacity for the sizing and the drawing set, the plumbing design services page lists the scope Heaven Designs describes. Use our contact page to share the fixture list, the AHJ and the code edition. Scope and deliverables are agreed at scoping, and stamping requirements are confirmed at quote stage.