How this calculator works
A gradient of 1 in 80 means the pipe falls 1 unit in every 80 units of length: 12.5 mm in each metre. The total fall is the length of the drain divided by the gradient, and the depth at the lower end is the depth at the top plus the fall. The pipe sits on a bed of pea shingle 100 mm deep and is surrounded by it and covered by another 100 mm. The volume of shingle is the cross-section of the trench filled to that height, less the pipe, times the length, at 1.6 tonnes per cubic metre.
Worked example: a 100 mm drain of 15 m at 1 in 80 falls 188 mm (12.5 mm per metre), so a pipe at 0.6 m deep at the top is 0.79 m deep at the lower end. It takes 5 pipes of 3 m and 3.28 tonnes of pea shingle in a 450 mm wide trench (4.45 tonnes in a 600 mm trench). At 1 in 40 the same drain falls 375 mm, and at 1 in 150 it falls 100 mm. A 150 mm drain at 1 in 150 uses 3.58 tonnes of shingle for the same run. A drain of 30 m at 1 in 80 falls 375 mm, which means it is 0.98 m deep at the lower end if it begins at 0.6 m.
Fall of a drain for common lengths and gradients
The total fall in millimetres.
| Length | 1 in 40 | 1 in 60 | 1 in 80 | 1 in 100 | 1 in 150 |
|---|---|---|---|---|---|
| 5 m | 125 mm | 83 mm | 63 mm | 50 mm | 33 mm |
| 10 m | 250 mm | 167 mm | 125 mm | 100 mm | 67 mm |
| 15 m | 375 mm | 250 mm | 188 mm | 150 mm | 100 mm |
| 20 m | 500 mm | 333 mm | 250 mm | 200 mm | 133 mm |
| 30 m | 750 mm | 500 mm | 375 mm | 300 mm | 200 mm |
| 50 m | 1,250 mm | 833 mm | 625 mm | 500 mm | 333 mm |
Tips before you buy
- Check the fall with a string line or a laser level along the trench, and a spirit level on a board is not enough over a long run.
- Lay the pipe from the lower end, with the sockets facing uphill.
- Bed the pipe on 100 mm of pea shingle, and cover it with another 100 mm over the crown, so that it cannot be pinched by stones.
- Fit an inspection chamber or rodding point at each change of direction, so that it can be cleared.
- Pass on an air or water test before backfilling, and ask building control and the water company before you connect to the sewer.
Tools & materials for this project
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Frequently asked questions
What fall does a drain need?
For a 100 mm foul drain serving a WC, 1 in 80, which is 12.5 mm per metre: 188 mm over 15 m. A 150 mm drain needs 1 in 150, and the usual steepest is 1 in 40.
What is a fall of 1 in 80?
The pipe drops 1 unit for every 80 units of length: 12.5 mm in each metre, or 188 mm over a 15 m drain.
How deep will the drain be at the lower end?
The depth at the top plus the fall: a pipe 0.6 m deep at the top of a 15 m drain at 1 in 80 is 0.79 m deep at the lower end.
How much pea shingle do I need to bed a drain?
About 0.66 tonnes for each 3 m pipe in a 450 mm trench: 3.28 tonnes for 15 m, laid 100 mm under, round and over the pipe.
Is a steeper fall better?
Not above about 1 in 40: the water can run ahead of the solids and leave them behind to block the pipe. A steady fall is best.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.