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Sanitary drain pipe size calculator
A sanitary drain is sized from the drainage fixture units, or DFU, of the fixtures that discharge into it. This page takes that load, reads the smallest listed diameter from Table 710.1(1) of the International Plumbing Code for a building drain by its slope, or from Table 710.1(2) for a horizontal fixture branch, and applies the 3 inch minimum for a drain that serves a water closet. 20 DFU at 1/4 inch per foot with a water closet on the drain takes 3 inch, which carries 42 DFU, and the same load with no water closet fits 2 inch. The page sizes horizontal drains only. Vent pipe is sized on its own table, and water supply pipe on water supply fixture units, which are a different count. For other pipe, see the hydronic pipe size calculator and the gas pipe sizing calculator.
- Take the DFU value of each fixture from Table 709.1 and add them for the pipe. The page opens on a two-bath house with a kitchen and a laundry, 20 DFU.
- Choose a building drain, which is read by slope, or a horizontal fixture branch, which has no slope. Say whether a water closet drains to the pipe, since a drain that serves one is 3 inch or larger.
- The page takes the smallest diameter whose limit is at or above the load. 20 DFU at 1/4 inch per foot with a water closet takes 3 inch, which carries 42 DFU.
Example result for the default inputs
Building drain, 20 DFU at 1/4 inch per foot: 3 inch.
Pipe size
3 inch
Capacity at that size
42 DFU
Load on that size
47.6 %
| Pipe | Building drain, building sewer or horizontal branch of the building drain, Table 710.1(1) |
|---|---|
| Load | 20 DFU at 1/4 inch per foot, a water closet on the drain |
| Pipe size | 3 inch, the smallest listed size whose limit of 42 DFU is at or above 20 DFU |
| Same load at each slope | 1/8 inch per foot: 3 inch (36 DFU); 1/4 inch per foot: 3 inch (42 DFU); 1/2 inch per foot: 3 inch (50 DFU) |
- The load is the sum of the DFU values of Table 709.1 for the fixtures on the pipe, not a count of fixtures. This page takes that sum as typed and does not check it.
- Table 704.1 sets the minimum slope of a drain, 1/4 inch per foot for 2-1/2 inch and smaller pipe, 1/8 inch per foot for 3 to 6 inch, 1/16 inch per foot for 8 inch and larger. Enter the slope the pipe will really be laid at.
- A water closet on the drain makes 3 inch the minimum under the footnote of Table 710.1(1). The load alone would fit a smaller pipe.
- Stacks, vents, sumps and ejectors, storm drainage and the water supply pipe are not sized here. The supply side counts water supply fixture units, which are not drainage fixture units.
For estimation purposes only. Consult a licensed engineer for final designs.
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How the drain pipe size is read
This page reads a pipe size from a table and does not derive one. The load is the sum of the drainage fixture units of the fixtures on the pipe, from Table 709.1 of the International Plumbing Code. A building drain, a building sewer or a horizontal branch of the building drain is read from Table 710.1(1) by slope, and a horizontal fixture branch from Table 710.1(2). The size is the smallest listed diameter whose limit is at or above the load. Building drains of 2 to 10 inch and fixture branches of 1-1/2 to 8 inch are covered, and stacks, vents and the water supply are not.
- Load in DFU
load = sum of (fixtures x DFU per fixture), Table 709.1Each fixture carries a drainage fixture unit value, and the pipe carries their sum. A private water closet of 1.6 gpf is 3 DFU, a lavatory 1 and a bathtub 2. The opening example, two baths with a kitchen sink, a dishwasher, a clothes washer and a laundry tray, adds up to 20 DFU.- Building drain, sewer or branch of the drain
size = smallest diameter in Table 710.1(1) whose limit at the slope is at or above the loadThe table lists a limit for each diameter at 1/8, 1/4 and 1/2 inch per foot. A 3 inch drain carries 36, 42 and 50 DFU at those slopes, and a 4 inch drain carries 180, 216 and 250. 20 DFU at 1/4 inch per foot is 47.6 percent of the 42 DFU of a 3 inch drain.- Water closet minimum
size = at least 3 inch when a water closet drains to the pipeA footnote of Table 710.1(1) sets 3 inch as the smallest building drain that serves a water closet. The same 20 DFU with no water closet fits a 2 inch drain, whose limit is 21 DFU at 1/4 inch per foot.- Horizontal fixture branch
size = smallest diameter in Table 710.1(2), horizontal fixture branch column, at or above the loadTable 710.1(2) has no slope. A 3 inch branch carries 20 DFU and a 4 inch branch 160. 24 DFU on a branch takes 4 inch, while the same 24 DFU on a building drain at 1/4 inch per foot with no water closet takes 2-1/2 inch.- Slope
the slope is an input, and the minimum slope depends on the diameterThe sources read print the minimum slope of Table 704.1 as 1/4 inch per foot for 2-1/2 inch and smaller pipe, 1/8 inch per foot for 3 to 6 inch, 1/16 inch per foot for 8 inch and larger. Nothing here checks the slope against the diameter, and it has no entry for 1/16 inch per foot. A flatter slope lowers the limit, so 200 DFU takes 4 inch at 1/4 inch per foot and 5 inch at 1/8.
Worked examples from the tables
The default case is a house with two baths, a kitchen sink, a dishwasher, a clothes washer and a laundry tray. Using the Table 709.1 values below it adds up to 20 DFU. At 1/4 inch per foot with a water closet on the drain it takes 3 inch, which is 47.6 percent of the 42 DFU limit. With no water closet the same load takes 2 inch, and at 1/8 inch per foot it takes 3 inch, because Table 710.1(1) lists no capacity for 2 or 2-1/2 inch pipe at that slope.
Fixtures behind the default load, with the DFU values of Table 709.1 for private use.
| Fixture | Count | DFU each | DFU |
|---|---|---|---|
| Water closet, 1.6 gpf | 2 | 3 | 6 |
| Lavatory or bidet | 2 | 1 | 2 |
| Bathtub, with or without shower head | 2 | 2 | 4 |
| Kitchen sink, domestic | 1 | 2 | 2 |
| Dishwasher, domestic | 1 | 2 | 2 |
| Clothes washer, residential | 1 | 2 | 2 |
| Laundry tray or utility sink | 1 | 2 | 2 |
Table 710.1(1), the largest DFU on a building drain, as this page reads it.
| Diameter, inch | 1/8 per foot | 1/4 per foot | 1/2 per foot |
|---|---|---|---|
| 2 | none listed | 21 | 26 |
| 2-1/2 | none listed | 24 | 31 |
| 3 | 36 | 42 | 50 |
| 4 | 180 | 216 | 250 |
| 5 | 390 | 480 | 575 |
| 6 | 700 | 840 | 1,000 |
| 8 | 1,600 | 1,920 | 2,300 |
| 10 | 2,900 | 3,500 | 4,200 |
Table 710.1(2), the largest DFU on a horizontal fixture branch.
| Diameter, inch | DFU |
|---|---|
| 1-1/2 | 3 |
| 2 | 6 |
| 2-1/2 | 12 |
| 3 | 20 |
| 4 | 160 |
| 5 | 360 |
| 6 | 620 |
| 8 | 1,400 |
Eight cases below run through the same functions the form uses. 200 DFU takes 4 inch at 1/4 inch per foot and 5 inch at 1/8. 24 DFU takes 2-1/2 inch on a building drain with no water closet and 4 inch on a horizontal fixture branch, since the two tables differ.
| Case | Setup | Result |
|---|---|---|
| Two-bath house, water closet on the drain | 20 DFU, building drain at 1/4 inch per foot, water closet on it | 3 inch, limit 42 DFU, 47.6% loaded |
| Same load, laundry drain with no water closet | 20 DFU, building drain at 1/4 inch per foot, no water closet | 2 inch, limit 21 DFU, 95.2% loaded |
| Same load with no water closet, at 1/8 inch per foot | 20 DFU, building drain at 1/8 inch per foot, no water closet | 3 inch, limit 36 DFU, 55.6% loaded |
| Small commercial building, 100 DFU | 100 DFU, building drain at 1/4 inch per foot, water closet on it | 4 inch, limit 216 DFU, 46.3% loaded |
| 200 DFU at 1/4 inch per foot | 200 DFU, building drain at 1/4 inch per foot, water closet on it | 4 inch, limit 216 DFU, 92.6% loaded |
| 200 DFU at 1/8 inch per foot | 200 DFU, building drain at 1/8 inch per foot, water closet on it | 5 inch, limit 390 DFU, 51.3% loaded |
| 1,000 DFU at 1/2 inch per foot, the printed limit of 6 inch | 1,000 DFU, building drain at 1/2 inch per foot, water closet on it | 6 inch, limit 1,000 DFU, 100.0% loaded |
| 24 DFU on a horizontal fixture branch | 24 DFU, horizontal fixture branch | 4 inch, limit 160 DFU, 15.0% loaded |
Standards and data sources
- Maryland Board of Plumbing, Plumbing Code 2024 (based on the 2024 International Plumbing Code), section 710.1 - Table 710.1(1), the maximum drainage fixture units on a building drain or sewer by diameter and by slope of 1/16, 1/8, 1/4 and 1/2 inch per foot, with the footnote that a building drain serving a water closet is 3 inch or larger. The horizontal fixture branch column of Table 710.1(2) runs from 1-1/2 to 8 inch. This page uses the rows of 2 to 10 inch at three slopes and the branch rows to 8 inch.
- Open Exam Prep, Drainage Fixture Units and Sizing Branches, Stacks and Building Drains (2021 IPC tables) - A second print of Tables 710.1(1) and 710.1(2) and of Table 709.1, the fixture DFU values. Every cell used on this page agrees with the Maryland adoption of the code.
- Open Exam Prep, Drainage Fixture Units (DFUs) and Sizing, commercial plumbing course - A third print of Table 710.1(1) for 1-1/2 to 8 inch, with the dashes where the table lists no capacity, and the DFU values of the common fixtures.
- Trussly, Drain Pipe Sizing with DFU Tables (IPC 710.1) - Table 710.1(1) for 2, 2-1/2, 3, 4 and 6 inch with 2 and 2-1/2 inch marked as not allowed at 1/8 inch per foot, the horizontal branch limits of Table 710.1(2) to 4 inch, and the minimum slopes of Table 704.1, 1/4 inch per foot for 2-1/2 inch and smaller pipe, 1/8 inch per foot for 3 to 6 inch, 1/16 inch per foot for 8 inch and larger.
- WaterCalculator.us, Drainage Fixture Unit Calculator - The 1/4 inch per foot column of Table 710.1(1) from 2 to 15 inch, the 4 inch drain at 1/8 and 1/2 inch per foot, and the private-use fixture values of Table 709.1.
Limitations of this calculator
- Horizontal drains only. Stacks are not sized, although the stack columns of Table 710.1(2) exist, because a stack is sized by its branch intervals and its offsets and the page takes neither.
- Building drains from 2 to 10 inch at 1/8, 1/4 and 1/2 inch per foot, and fixture branches from 1-1/2 to 8 inch. The 1/16 inch per foot column, the 1-1/4 and 1-1/2 inch building drain rows and the 12 and 15 inch rows are left out because the sources read did not confirm them across each other. A load above 4,200 DFU on a building drain, or 1,400 DFU on a branch, is refused.
- The tables were read in the Maryland adoption of the 2024 IPC and in two study guides that reproduce the 2021 tables. ICC's own site returned an error to the reader, so the model code text itself was not read, and the 2018 edition was not compared cell by cell. Check the table in the edition your jurisdiction has adopted.
- You type the load in. The page does not look up fixtures, apply the public-use values of Table 709.1, or add a continuous or intermittent flow from a pump or an indirect waste. A fixture with a larger trap carries a larger DFU value.
- The 3 inch water closet minimum comes from the footnote of the building drain table. No source read prints it for the fixture branch table, so the page applies it there as the cautious reading, and it does not check the outlet of the fixture or its trap.
- The slope is not checked against the diameter. Table 704.1 sets minimum slopes, and the page does not apply them, but it never names a 2 or 2-1/2 inch pipe at 1/8 inch per foot.
- Venting, the length of the drain and the number of fittings, backwater valves, the size of a building sewer outside the building, sump and ejector discharge, storm drainage and grease interceptors are not covered. Local amendments change these tables in some jurisdictions, and the Universal Plumbing Code uses its own table.
- This is a planning estimate, not a design drawing. A licensed plumber or engineer and the authority having jurisdiction decide the pipe sizes and the installation.
Common drain sizing mistakes
- Counting fixtures and not DFU. A water closet is 3 DFU, a lavatory 1 and a bathtub 2, so those three fixtures add to 6 DFU counted one by one, and not 3 fixtures. Table 709.1 also lists a bathroom group as a single line at a lower value, which this page does not use. The 20 DFU of the opening house comes from 10 fixtures.
- Taking 2 inch pipe at 1/8 inch per foot. Table 710.1(1) has a dash for 2 and 2-1/2 inch at 1/8 inch per foot. 20 DFU with no water closet takes 2 inch at 1/4 inch per foot, and 3 inch at 1/8, where the limit is 36 DFU.
- Forgetting the water closet minimum. 20 DFU fits a 2 inch drain at 1/4 inch per foot, whose limit is 21 DFU, but a drain that serves a water closet is 3 inch or larger. The same load takes 3 inch with a water closet on it.
- Reading a building drain from the branch table. Table 710.1(2) is for horizontal fixture branches and has no slope. 24 DFU on a branch takes 4 inch, with a limit of 160 DFU, and on a building drain at 1/4 inch per foot with no water closet it takes 2-1/2 inch, with a limit of 24 DFU. The wrong table moves the answer from 2-1/2 to 4 inch.
- Ignoring the slope. A flatter drain carries less. 200 DFU takes 4 inch at 1/4 inch per foot, where the limit is 216 DFU, and 5 inch at 1/8, where the limit of a 4 inch drain is 180 DFU.
- Using drainage units for the vent or the supply. A vent is sized on its own table and the water supply pipe is sized on water supply fixture units, which are a different count from drainage fixture units. Any DFU total from this page answers the question of the drain only.
Common questions about drain pipe size
How do I size a sanitary drain pipe?
Add the drainage fixture units of the fixtures on the pipe, then take the smallest diameter in Table 710.1(1) whose limit at your slope is at or above that total. 20 DFU at 1/4 inch per foot with a water closet on the drain takes 3 inch, which carries 42 DFU. A horizontal fixture branch is read from Table 710.1(2) instead.
What is a drainage fixture unit?
Drainage fixture units, or DFU, are the values Table 709.1 gives a fixture for the load it puts on a drain. A water closet of 1.6 gpf is 3 DFU, a lavatory 1, a bathtub 2, and a drinking fountain is 0.5. The pipe carries the sum of its fixtures.
What size drain does a toilet need?
A building drain that serves a water closet is 3 inch or larger, by the footnote of Table 710.1(1). At 1/8, 1/4 or 1/2 inch per foot a 3 inch drain carries 36, 42 or 50 DFU.
How many DFU can a 4 inch drain carry?
180 DFU at 1/8 inch per foot, 216 at 1/4 and 250 at 1/2, from Table 710.1(1). As a horizontal fixture branch, a 4 inch pipe carries 160 DFU in Table 710.1(2).
What slope should a drain have?
Table 704.1 gives the minimum, which the sources read print as 1/4 inch per foot for 2-1/2 inch and smaller pipe, 1/8 inch per foot for 3 to 6 inch, 1/16 inch per foot for 8 inch and larger. The capacity at 1/8, 1/4 and 1/2 inch per foot is listed in Table 710.1(1), and a steeper slope carries more. 200 DFU takes 4 inch at 1/4 and 5 inch at 1/8.
What is the difference between Table 710.1(1) and Table 710.1(2)?
Table 710.1(1) sizes building drains, building sewers and the horizontal branches of the building drain by slope. Horizontal fixture branches and stacks are sized by Table 710.1(2), which has no slope. This page uses the first for a drain and the horizontal branch column of the second for a fixture branch.
Is this the same as a vent pipe or water pipe calculator?
No. This page sizes the pipe that carries waste out of the building. A vent is sized from its own table and the supply pipe from water supply fixture units, so a DFU total does not carry over to either.
Does the page size stacks?
No. The page covers horizontal drains only. A stack is sized by its branch intervals and its offsets, and the page takes neither.
Is there a limit on how many times I can run this?
No. This calculator runs in your browser with no daily limit and no signup.
Used on the same job
Count the fixture units, then read the table
The load comes first, as the sum of the Table 709.1 values of the fixtures on the pipe, and the table then gives the smallest diameter that carries it. Vent pipe, stacks and the water supply are sized separately. For the hot water side of the same fixtures, see the water heater size calculator, and for the condensate line of an air conditioner, the condensate drain calculator. You can also ask the assistant to go over a drain layout. Slope matters. A flatter run carries fewer fixture units in the same pipe.
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