In short: add up the drainage fixture units (DFU) for everything that empties into the pipe, then look up the smallest diameter in IPC Table 710.1 whose capacity at your slope meets or exceeds that total. A typical house bathroom group (two toilets, two lavatories, a shower, a kitchen sink and a washer — 15 DFU) needs a 3-inch drain at a 1/4-inch-per-foot slope, mainly because codes require at least 3 inches for any pipe carrying a water closet.
This is a planning aid based on published fixture-unit tables. Your local plumbing code may set stricter minimums — confirm with your inspector or a licensed engineer before installing.
How to size a drain pipe with fixture units
Plumbing codes don't size drains by adding up gallons per minute — they use a simpler, standardized measure called the drainage fixture unit (DFU). Each fixture is assigned a DFU value based on how much and how fast it typically discharges. You sum the DFU for everything on the pipe, then read the required diameter off a code table for the slope you're using.
Worked example. Two residential water closets (3 DFU each = 6), two lavatories (1 each = 2), one shower (2), one kitchen sink (2) and one clothes washer (3) add up to 6 + 2 + 2 + 2 + 3 = 15 DFU. At a 1/4-inch-per-foot slope, a 2-inch pipe is rated for 21 DFU — technically enough — but code requires at least 3 inches because water closets are present. A 3-inch pipe at that slope handles 42 DFU, so the minimum size is 3 inches, with 27 DFU of headroom for future fixtures.
Fixture units aren't flow rates. A DFU is a relative loading factor, not a gallons-per-minute number — the tables already build in the fact that fixtures rarely discharge at the same instant. Don't convert DFU to GPM and size by flow; use the code table directly.
Why slope changes the answer
A drain pipe relies on gravity, so its capacity depends on how steeply it falls as well as how wide it is — the same relationship captured in Manning's equation for open-channel flow. A steeper slope pushes waste along faster, so a given diameter can carry more fixture units before it needs to be upsized. That's why IPC Table 710.1(2) lists a separate DFU capacity for each of four standard slopes: 1/16, 1/8, 1/4 and 1/2 inch per foot.
Codes also set a minimum slope by pipe size, independent of fixture-unit sizing: pipes up to 2½ inches must fall at least 1/4 inch per foot, and 3-to-6-inch pipes at least 1/8 inch per foot. If your planned slope is below that floor for your pipe size, use the Pipe Slope Calculator to check it before relying on this tool's result.
Drain pipe sizing table (IPC Table 710.1(2))
Maximum drainage fixture units a building drain or sewer of each diameter can carry at each slope. This is the exact table the calculator reads from.
| Diameter | 1/16 in/ft | 1/8 in/ft | 1/4 in/ft | 1/2 in/ft |
|---|---|---|---|---|
| 1¼ in | — | — | — | 1 |
| 1½ in | — | — | — | 3 |
| 2 in | — | — | 21 | 26 |
| 2½ in | — | — | 24 | 31 |
| 3 in (3 in min. with a WC) | — | 36 | 42 | 50 |
| 4 in | 180 | 216 | 250 | 284 |
| 5 in | 390 | 480 | 575 | 650 |
| 6 in | 700 | 840 | 1,000 | 1,200 |
| 8 in | 1,400 | 1,600 | 1,920 | 2,300 |
| 10 in | 2,500 | 2,900 | 3,500 | 4,200 |
| 12 in | 3,900 | 4,600 | 5,600 | 6,700 |
| 15 in | 7,000 | 8,300 | 10,000 | 12,000 |
Common fixture unit (DFU) values
The values this calculator uses for each fixture type. For anything not listed, check your local plumbing code and enter it under "Additional fixture units."
| Fixture | DFU |
|---|---|
| Water closet, residential (gravity tank) | 3 |
| Water closet, flushometer / commercial | 4 |
| Lavatory (bathroom sink) | 1 |
| Shower or tub-shower | 2 |
| Kitchen sink or dishwasher | 2 |
| Clothes washer standpipe | 3 |
| Floor drain (2 in) | 2 |
Worked examples
Half-bath addition
3 in
1 water closet (3 DFU) + 1 lavatory (1 DFU) = 4 DFU. Even though a 2-inch pipe covers 21 DFU at 1/4 in/ft, the water closet forces a 3-inch minimum, giving 38 DFU of headroom.
Full house, three bathrooms
3 in
3 water closets (9) + 3 lavatories (3) + 2 showers (4) + 1 kitchen sink (2) + 1 washer (3) = 21 DFU. At a 1/8-inch-per-foot slope, 3 inches (36 DFU) is the minimum that clears both the fixture load and the water-closet floor.
Small commercial restroom
3 in
2 flushometer water closets (8) + 2 lavatories (2) + 1 floor drain (2) = 12 DFU. At a 1/2-inch-per-foot slope a 3-inch pipe (50 DFU) is more than enough, with room to add fixtures later.
What else affects the result
Local amendments. Many jurisdictions require a larger minimum than the base code — for example, a 4-inch minimum for any building sewer connecting to the public main. Always check with your local building department before finalizing a size.
Pipe material. The fixture-unit tables assume a typical smooth-wall drain pipe (PVC, ABS or cast iron). Material affects long-term roughness and self-cleansing velocity, but the code-minimum diameter for a given DFU load and slope doesn't change by material.
Venting. This calculator sizes the drain only. Every drain also needs adequate venting to protect trap seals and let waste flow freely — that's a separate calculation from drainage fixture units.
Horizontal branches vs. the building drain. This table applies to the building drain and sewer (the main horizontal run). Individual horizontal branches and vertical stacks use a related but separate IPC table with different limits — see the Drainage Fixture Unit Calculator if you only need the DFU total.
Frequently asked questions
What is a drainage fixture unit (DFU)?
A drainage fixture unit is a code-assigned number that represents how much a plumbing fixture loads a drain, based on its typical discharge rate and duration. A lavatory is 1 DFU, a shower is about 2, and a residential water closet is 3. Codes size pipes by adding up the DFU of everything connected, not by adding flow rates directly.
What is the minimum drain pipe size for a toilet?
Most plumbing codes require at least a 3-inch pipe for any drain that carries water closet (toilet) discharge, even if the fixture-unit table would technically allow a smaller size. This calculator applies that 3-inch minimum automatically whenever a water closet is included.
Does pipe slope affect drain pipe size?
Yes. A steeper slope moves waste faster, so the same pipe diameter can carry more fixture units before it needs to be upsized. IPC Table 710.1(2) gives a different fixture-unit capacity for each standard slope, from 1/16 inch to 1/2 inch per foot.
Can I use a 2-inch pipe for my whole house drain?
Only for a very light fixture load with no water closet connected — a 2-inch pipe is limited to 21 to 26 drainage fixture units depending on slope. Most single-family homes, once a toilet is included, need at least a 3-inch drain by code regardless of the fixture-unit total.
Does pipe material change the required size?
The fixture-unit sizing tables assume typical smooth-wall drain pipe (PVC, ABS or cast iron) and do not change by material. Material affects long-term flow performance and roughness, but the code-minimum diameter for a given fixture-unit load and slope stays the same.
What is the difference between drain pipe size and sewer pipe size?
The building drain is the horizontal pipe inside or under the building that collects fixture waste, sized by this calculator. The building sewer carries that same flow from the building to the public sewer or septic tank, and some jurisdictions set a larger minimum diameter for it. Use the Sewer Pipe Size Calculator for that segment.
Sources & references
- International Code Council (ICC). International Plumbing Code (IPC), Table 710.1(2) — Building Drains and Sewers.
- IAPMO. Uniform Plumbing Code (UPC), Table 703.2 — Building Sewer Pipe.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)