Calculators/Drainage slope

Drainage slope and fall calculator

Total fall over a run, given a slope in inches per foot or as a percentage — with the ratio and the angle alongside, because every trade states it differently.

The run

The fall

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Check the result against the job. This does the arithmetic from the values you enter. Building codes, span tables and material specifications vary by jurisdiction and by product, and the governing document is your approved drawings and your local code. Nothing you type here leaves your browser: the calculation runs on your device and we never receive it.

How this is checked. Every formula on this site runs from one shared calculation engine, and every one has a test fixture whose expected answer was worked out from the geometry or taken from a figure the trade already publishes — never from what the code returns. Some land exactly, which is the point: a 6/12 hip over a 144″ run is exactly 216″, a 45° crown in a square corner bevels at exactly 30°, and a 30° conduit bend has a multiplier of exactly 2. Where a number is a convention rather than arithmetic — a code limit, a waste allowance, bundles per square — it is an input with a stated default rather than a hidden constant. Found something wrong? Tell us at [email protected] and we will fix it and say so.

Four ways to say the same thing

Slope gets stated differently depending on who is talking. Plumbers work in inches per foot, sitework and civil drawings use percentage grade, drainage specs often use a ratio, and only surveying instruments care about degrees.

  • ¼″ per foot = 2.08% = 1:48 = 1.19°
  • ⅛″ per foot = 1.04% = 1:96 = 0.60°
  • 2% = 0.24″ per foot = 1:50

Note how small the angles are. That is why slope is never specified in degrees on site: a fifth of a degree is not something anyone can set with a level, but a quarter inch on a two-foot level is.

Too much slope is a real failure mode

The intuition that steeper drains better is wrong for waste lines. If the liquid outruns the solids, material is left stranded in the pipe — which is why plumbing codes specify a practical maximum as well as a minimum.

Surface grading is the opposite problem: not enough fall away from a building sends water at the foundation, and a flat-looking finish can pond and hold it there.

Your local code governs. Minimum slopes vary by pipe diameter and by jurisdiction, and some allow shallower falls on larger lines where a quarter inch per foot would be impractical over the run. Check the code you are building to.

Common questions

How much fall is a quarter inch per foot?

A quarter inch of drop for every foot of run, which is a 1 in 48 ratio and a 2.08 percent grade. Over 40 feet that is 10 inches of total fall. It is the standard minimum slope for small-diameter drainage lines.

What is the minimum slope for a drain line?

A quarter inch per foot is the common minimum for smaller lines, and an eighth per foot is often permitted for larger diameters where a quarter would be impractical over the run. Your local plumbing code governs, and it may differ.

Can a drain have too much slope?

Yes. If the liquid outruns the solids the line can be left with material stranded in it, so plumbing codes set a practical maximum as well as a minimum. Steeper is not automatically better.

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