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Drain Slope Calculator

Work out the fall a drainage pipe needs — enter the run length and slope to get the total drop in inches (mm), or enter the fall you have to check the slope you get. Recommended slopes per pipe size are built in. Everything runs on your device.

Guide: What Slope Does a Drain Pipe Need?

Your drain run

Solve for
Fall needed
6.56 in
32.81 ft at 1:60 — set 16.7 mm of fall per metre.
Per metre
16.7 mm/m
As a percent
1.67%

Typical gradients — 4″ foul drain

Recommended range1:40 – 1:80
Typical target1:60

Check your local drainage rules

Recommended gradients are typical practice — the minimum (and sometimes maximum) that applies comes from your building regulations, which may tie gradients to fixture counts. Measure fall invert-to-invert, and keep the gradient constant along the run rather than averaging out a belly.

Tip: mental maths on site — 1:60 is about 17 mm per metre, 1:40 is 25 mm per metre. A torpedo level with a gradient vial, or a laser set to mm/m, beats eyeballing every time.

Questions & answers

Everything you need to understand the drain slope calculator.

How do I calculate the fall on a drain pipe?

Multiply the run length by the slope: fall = length × slope. A 20 ft run at ¼″ per foot needs 20 × ¼ = 5″ of fall (about 1:48). Working backwards, if you only have 2.5″ of drop over 20 ft, your slope is ⅛″ per foot (1:96). The calculator solves it in either direction.

What slope should a drain be laid at?

Standard US practice: ¼″ per foot (1:48) for drainpipes 2½″ and smaller, and ⅛″ per foot (1:96) for 3″ and larger sewer and building drains. Steeper is allowed but rarely needed. Your local plumbing code (IPC or UPC) sets the limits that actually apply — some tie the minimum slope to the pipe size and connected fixture-unit load.

What happens if a drain is too flat — or too steep?

Too flat and the flow can’t carry solids, so they settle and block the pipe. Too steep and the water outruns the solids, leaving them stranded — the classic problem on very steep runs. The sweet spot keeps the flow around self-cleansing velocity (about 2 ft/s), which is what the recommended ¼″- and ⅛″-per-foot slopes deliver.

How do I express slope — inches per foot, percent or ratio?

They are the same thing in different clothes. ¼″ per foot means ¼″ of fall per 12″ of run = about 2% = 1:48. The calculator shows all of them, so you can set a laser level in inches per foot or check a plan marked in percent or ratio without converting by hand.

Does this apply to stormwater too?

The arithmetic is identical, and storm drains are typically laid at similar or slightly flatter slopes since there are no solids to strand. Sizing the pipe diameter for a storm event is a separate calculation based on catchment and rainfall intensity — this tool handles the fall, not the flow.

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A calculation is just the start. ExequtechOS takes it from estimate to quote, job card, invoice and paid — for your whole team.

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