Plumbing
Drainage Pipe Size Calculator
Size drainage pipe from fixture load and slope, as a planning check against the code that applies locally.
Result
Pipe and drain sizing is governed by plumbing code (IPC/UPC or local equivalent) and must be designed and installed by a licensed plumber. This is an educational estimate, not a design. Fixture unit values and sizing tables differ between the IPC and UPC and between jurisdictions. Venting, stack sizing and offsets all affect the answer and are not modelled here.
Get a Free Business AuditWhat moves the number
- Drainage fixture units on the run, which is the code method for combining fixtures.
- Slope, since drainage relies on gravity and velocity rather than pressure.
- Venting, without which the system will not drain properly regardless of pipe size.
Where this one goes wrong
Sizing drainage without planning the venting. A correctly sized drain that cannot breathe siphons traps and gurgles, and the customer experiences it as a smell rather than as a drainage problem.
Before you quote
- The drainage fixture unit values the local code assigns.
- Whether the available fall supports the required slope over the run.
- Whether the vent arrangement meets code for the fixtures served.
Questions
About this calculator
Does this replace the code calculation?
No. Drainage sizing, fixture unit values and venting requirements are set by the plumbing code where you work and vary between them. This is a planning check.
Why does venting matter so much?
Because water falling in a pipe pushes air ahead and pulls it behind. Without a vent, that pressure change empties trap seals, which is what lets sewer gas into the building.
Can a drain be too big?
Yes. An oversized drain at the same slope runs shallower and slower, which leaves solids behind. Drainage depends on carrying velocity, not just on capacity.
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How to use this calculator
- Toilets. Enter your own figure; the page starts at 2.
- Sinks and basins. Enter your own figure; the page starts at 3.
- Showers and baths. Enter your own figure; the page starts at 2.
- Other fixture units. Dishwasher 2, washing machine 3, floor drain 2.
- Horizontal run length (ft). Enter your own figure; the page starts at 30.
The result updates as you type, so you can try a figure, see what it does to recommended drain size, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Toilets | 2 |
| Sinks and basins | 3 |
| Showers and baths | 2 |
| Other fixture units | 4 |
| Horizontal run length (ft) | 30 |
What comes out
| Result | Value |
|---|---|
| Recommended drain size main result | 3" |
| Total drainage fixture units | 19 |
| Minimum slope | 1/8" per foot |
| Total fall over the run | 3.8 |
19 drainage fixture units suggests a 3" drain at 1/8" per foot, which needs 3.8" of fall across the 30 ft run.
How sensitive the answer is
Holding everything else steady and changing only toilets, this is what happens to recommended drain size. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Toilets | Recommended drain size |
|---|---|
| 1 | 3" |
| 2 | 3" |
| 2 example above | 3" |
| 3 | 4" |
| 4 | 4" |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Toilets
- A number, between 0 and 200, starting at 2.
- Sinks and basins
- A number, between 0 and 200, starting at 3.
- Showers and baths
- A number, between 0 and 200, starting at 2.
- Other fixture units
- Dishwasher 2, washing machine 3, floor drain 2.
- Horizontal run length (ft)
- A number, between 0 and 2000, starting at 30.
What the results tell you
| Result | What it is |
|---|---|
| Recommended drain size | The headline result. A category or planning selection, rather than a measured quantity. |
| Total drainage fixture units | A number. |
| Minimum slope | A category or planning selection, rather than a measured quantity. |
| Total fall over the run | A decimal figure. |
Before you rely on this number
Pipe sizing, flow and pressure figures depend on the actual fittings, the run, the material and local code. Code requirements vary by jurisdiction and override any general calculation, so check what applies where the work is happening.
Keep going
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