Drain fall calculator: drop, gradient and invert levels
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| What do you know? | The gradient |
| Run length | 12 m |
| Gradient as | 1 in n |
| Gradient | 40 |
| Pipe size | 100 mm |
| What the drain carries | Foul water, with at least one WC |
Drop: 300 mm over the run (1 in 40, 2.50%)
- Limit not yet confirmed Gradient (1 in n) for 100 mm, foul water with at least one WC: 40 (1 in n) The limit for this check hasn't been confirmed yet: check the source. England: Approved Document H (2015), H1 section 2 (minimum gradients). Check with the source
| Result | Value |
|---|---|
| Drop over the run | 300 mm |
| Gradient | 1:40 |
| As a percentage | 2.5% |
| In degrees | 1.43 ° |
| Fall per metre | 25 mm/m |
Show the working
- Gradient: 1 in 40 = 2.5% = 1.43° = 25 mm per m
- Drop: 12 m × 0.025 = 300 mm
How it works
- Slope = 1 ÷ n for 1 in n; % ÷ 100; mm per m ÷ 1000
- Drop = run length × slope
- From invert levels: drop = top invert − bottom invert; slope = drop ÷ run length
- Degrees = arctan(slope)
Drains need enough fall to be self-cleansing: too flat and solids settle. Approved Document H gives minimum gradients by flow and pipe size.
Invert levels are measured to the bottom inside of the pipe.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
This calculator has no built-in defaults: every figure comes from you or your product's data.
Regulations and standards
- Approved Document H (2015), H1: minimum gradients for drains
Explained: Approved Document H (drainage).
Approved Document H covers England. Wales, Scotland and Northern Ireland have their own drainage guidance.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Minimum drain gradients (as 1 in n) by use and pipe size | Approved Document H (2015), H1 section 2 (foul drainage: minimum gradients by peak flow and pipe size) | England | 0/6 values entered Not yet checked against the source |