Plumbing
Pipe Slope Calculator
Work out fall across a drainage run, and check it against the slope the pipe size actually needs.
Result
Pipe sizing, venting and drainage are set by the plumbing code adopted where you are, and the IPC and UPC do not agree with each other. Treat this as a planning figure and size the finished design from the code table your inspector works to. A quarter inch per foot is the usual minimum for pipe under 3 inches. Too little slope leaves solids behind; too much lets the water run away and strand them, which is why steeper is not automatically better.
Get a Free Business AuditWhat moves the number
- Pipe diameter, since the required slope differs by size.
- Total run length, which sets how much fall is needed end to end.
- Available height between the fixture and the connection point.
Where this one goes wrong
Assuming steeper is safer. Excessive slope lets water outrun the solids it should be carrying, leaving them behind to build up. Drainage has a target range rather than a minimum to exceed.
Questions
About this calculator
Can a drain be too steep?
Yes. Water runs away from solids at high slope and the solids stay behind. That is why codes specify a range rather than a minimum, and why more fall is not automatically better.
What slope does a drain need?
It depends on the diameter, and the figures are set by the code in force. Larger pipe generally needs less slope than smaller pipe to maintain carrying velocity.
What if there is not enough fall?
Then the run needs rethinking: a different route, a larger pipe at a flatter grade where code allows, or a pump. Installing below the required slope produces recurring blockages.
The formula
total fall = slope per foot x run length
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How to use this calculator
- Run length (ft). Enter your own figure; the page starts at 40.
- Slope. Enter your own figure; the page starts at 1/4 inch per foot, standard.
- Depth at the start (in) (in). Enter your own figure; the page starts at 12 in.
The result updates as you type, so you can try a figure, see what it does to total fall, 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 |
|---|---|
| Run length (ft) | 40 |
| Slope | 1/4 inch per foot, standard |
| Depth at the start (in) | 12 in |
What comes out
| Result | Value |
|---|---|
| Total fall main result | 10.0 |
| Depth at the far end | 22.0 |
| Grade | 2.08% |
| Fall per 10 feet | 2.5 |
Over 40 feet the line drops 10.0 inches, a grade of 2.08%. The far end sits 22.0 inches deep.
How sensitive the answer is
Holding everything else steady and changing only run length (ft), this is what happens to total fall. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Run length (ft) | Total fall |
|---|---|
| 20 | 5.0 |
| 30 | 7.5 |
| 40 example above | 10.0 |
| 60 | 15.0 |
| 80 | 20.0 |
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
- Run length (ft)
- A number, between 0 and 2000, starting at 40.
- Slope
- Choose from 1/8 inch per foot, 4 inch and larger, 1/4 inch per foot, standard, 1/2 inch per foot, short runs.
- Depth at the start (in) (in)
- Measured in in, between 0 and 400, starting at 12 in.
What the results tell you
| Result | What it is |
|---|---|
| Total fall | The headline result. A decimal figure. |
| Depth at the far end | A decimal figure. |
| Grade | A percentage. |
| Fall per 10 feet | 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.
Estimating is the easy part
The jobs you never hear about cost more than the ones you mis-measure. BDEVY builds the marketing, systems and automation that bring them in.