Drainage Slope Calculator
Work out a drain's fall and slope — from the length and the fall, or the length and the slope — as a percentage and as a gradient.
A drain that is too flat blocks; one that is too steep leaves the solids behind. This calculator turns length and fall into slope, or length and slope into fall.
Enter the length and one of the two; the other follows, as a percentage and as a gradient.
Your results
Enter your measurements and press Calculate.
Quantities are rounded up to whole tiles, planks, boxes or bags, after the wastage allowance. Cost appears only when you enter a price.
Formula
All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.
How to use this calculator
- Enter the pipe length.
- Enter either the fall or the slope percentage.
- Read the slope, the fall and the gradient.
- Use the Copy results button to copy the answer.
Worked example
| Pipe | 10 m long, 0.25 m fall |
|---|---|
| Slope | 0.25 ÷ 10 × 100 = 2.5% |
| Gradient | 1 in (10 ÷ 0.25) = 1:40 |
| Other direction | 20 m at 1% → fall = 20 × 0.01 = 0.2 m |
Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.
Frequently asked questions
What slope should a drain have?
Common guides are 1:40 (2.5%) for 110 mm drains and 1:100 (1%) as a minimum — too flat and solids settle, too steep and water outruns the solids. Local code decides the final design.
What does 1:40 mean?
The pipe falls 1 unit for every 40 units of length — 0.25 m over 10 m, or 2.5%. This calculator shows the slope as a percentage and as a gradient.
How do I measure the fall?
With a level and a tape: the height difference between the high end and the low end of the pipe run. Enter the length and the fall here to get the slope.
Is this a substitute for plumbing design?
No. This is a slope calculation for planning. Drainage design and code compliance depend on local regulations and real installation conditions — see the safety notice.
Related calculators
Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.