Drainage Pipe Size Calculator (DFU)
Add the fixtures that drain into a pipe and get the total drainage fixture units and the minimum pipe size from the IPC tables, in inches and DN. Modelling the installation too? Browse our MEP Revit families.
IPC 2021 / 2024 tablesBranches, stacks, drainsInches and DNFree, no sign-upFormula with your numbers
What are you sizing?
A horizontal fixture branch collects fixtures on one floor and runs to a stack or to the building drain.
About one per floor that connects to the stack (at least 8 ft or 2.4 m of stack each).
The heaviest floor. Leave empty to use the total (safe side).
If your slope falls between two values, pick the lower one.
Fixtures served by this pipe
Drainage fixture units (dfu) and minimum trap sizes from IPC Table 709.1. Count every fixture upstream of the pipe you are sizing.
Pipes draining into it (optional)
Drain pipes may not get smaller in the direction of flow (IPC 704.2). A 4 × 3 in closet bend does not count as a reduction.
Minimum pipe size
Add your fixtures to see the result.
Preliminary sizing only. This follows the International Plumbing Code (IPC 2021 and 2024, Sections 704, 709 and 710). The UPC, EN 12056-2 (discharge units) and many national codes use different load values and tables, so the sizes can differ. Venting, offsets, cleanouts and local amendments are not checked. Confirm the design with the code your authority adopts and a licensed engineer.
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Request a projectDrain pipe size in three steps
Choose the pipe
Pick a horizontal fixture branch, a stack (with its number of branch intervals) or a building drain or sewer (with its slope).
Add the fixtures
Add each fixture type and how many there are. The tool takes the dfu and the minimum trap size from IPC Table 709.1.
Read the size
Get the smallest size that carries the load and meets the water closet, trap and no-reduction rules, with the table row highlighted.
How the size is found
1. Total load. Total dfu = sum of (number of fixtures × dfu of each fixture), with dfu values from IPC Table 709.1. A continuous flow counts as 2 dfu per gpm (IPC 709.3).
2. Table size. The smallest size whose maximum dfu is equal to or greater than the total:
- Horizontal fixture branch: IPC Table 710.1(2), branch column.
- Stack: IPC Table 710.1(2), the column for three branch intervals or less, or for more than three. The heaviest branch interval must also pass the one-interval column.
- Building drain or sewer: IPC Table 710.1(1), in the column for your slope.
3. Minimums. The final size is the largest of the table size, 3 in when a water closet is connected, the largest fixture trap, and any pipe that drains into this one (no reduction in the direction of flow, IPC 704.2).
Example: water closet 1.6 gpf (3) + lavatory (1) + bathtub (2) + kitchen sink (2) + washer (2) = 10 dfu. A 2-1/2 in branch carries 12 dfu, but the water closet needs 3 in, so the branch is 3 in (DN80).
IPC maximum drainage fixture units by pipe size
Horizontal branches and stacks (IPC Table 710.1(2)), maximum dfu
| Pipe | Horizontal branch | Stack, 1 interval | Stack, up to 3 intervals | Stack, over 3 intervals |
|---|---|---|---|---|
| 1½ in DN40 | 3 | 2 | 4 | 8 |
| 2 in DN50 | 6 | 6 | 10 | 24 |
| 2½ in DN65 | 12 | 9 | 20 | 42 |
| 3 in DN80 | 20 | 20 | 48 | 72 |
| 4 in DN100 | 160 | 90 | 240 | 500 |
| 5 in DN125 | 360 | 200 | 540 | 1,100 |
| 6 in DN150 | 620 | 350 | 960 | 1,900 |
| 8 in DN200 | 1,400 | 600 | 2,200 | 3,600 |
| 10 in DN250 | 2,500 | 1,000 | 3,800 | 5,600 |
| 12 in DN300 | 3,900 | 1,500 | 6,000 | 8,400 |
Building drains and sewers (IPC Table 710.1(1)), maximum dfu by slope
| Pipe | 1/16 in/ft 0.5 % | 1/8 in/ft 1 % | 1/4 in/ft 2 % | 1/2 in/ft 4 % |
|---|---|---|---|---|
| 1½ in DN40 | – | – | 3 | 3 |
| 2 in DN50 | – | – | 21 | 26 |
| 2½ in DN65 | – | – | 24 | 31 |
| 3 in DN80 | – | 36 | 42 | 50 |
| 4 in DN100 | – | 180 | 216 | 250 |
| 5 in DN125 | – | 390 | 480 | 575 |
| 6 in DN150 | – | 700 | 840 | 1,000 |
| 8 in DN200 | 1,400 | 1,600 | 1,920 | 2,300 |
| 10 in DN250 | 2,500 | 2,900 | 3,500 | 4,200 |
| 12 in DN300 | 3,900 | 4,600 | 5,600 | 6,700 |
A building drain serving a water closet must be at least 3 in. DN sizes are approximate; plastic pipe is sold by outside diameter (for example 110 mm for 4 in).
Drainage pipe sizing FAQ
A drainage fixture unit is a load factor that compares how much and how often a fixture discharges into the drain. A lavatory is 1 dfu, a bathtub or kitchen sink 2 dfu and a private 1.6 gpf water closet 3 dfu (IPC Table 709.1). You add up the dfu of every fixture a pipe serves and look up the size in the IPC tables.
Under the IPC, a water closet is 3 dfu in a private bathroom (1.6 gpf), 4 dfu in a public one or with a flushometer tank, and 4 (private) or 6 (public) dfu when it flushes more than 1.6 gpf. A full bathroom group with a 1.6 gpf water closet counts as 5 dfu.
Under IPC 2021 and 2024, a 3-inch horizontal fixture branch carries up to 20 dfu. A 3-inch stack carries 48 dfu with three branch intervals or less, or 72 dfu with more, but no more than 20 dfu into one interval. A 3-inch building drain carries 36 dfu at 1/8 in per ft and 42 dfu at 1/4 in per ft.
At least 3 inches. The IPC requires a 3-inch minimum for any building drain serving a water closet, and pipe size may not get smaller in the direction of flow. A 4 × 3 in closet bend is not a reduction.
IPC Table 704.1 sets the minimum: 1/4 in per ft (about 2 %) for pipes 2-1/2 in and smaller, 1/8 in per ft (about 1 %) for 3 to 6 in, and 1/16 in per ft (about 0.5 %) for 8 in and larger. A steeper slope lets a building drain carry more fixture units.
Close, but not exactly. Metric plastic pipe is named by its outside diameter. A 110 mm PVC-U pipe has a bore of about 104 mm, a little more than 4 in (102 mm), and is the usual metric equal of a 4-inch drain. A 75 mm pipe is smaller than 3 in inside, so check what your authority accepts. Other codes, such as the UPC and EN 12056-2 (discharge units), use different values and give different sizes.
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