Surveying & site
Drainage Slope Calculator
Foul pipes need enough fall to carry solids but not so much that water outruns them - this gets the number right.
Calculator
Fill in the fields above to see results.
How it works
Fall = run x 1000 / x for a 1:x gradient (in mm per metre-run). Enter the pipe length plus either the target ratio or the fall you have, and read the other.
Frequently asked questions
What fall does a 100 mm soil pipe need?
UK practice for 100 mm foul drains is between 1:40 (25 mm per metre) and 1:80. Steeper is not better - liquids can outrun solids on extreme falls.
How do I measure existing fall on site?
A level and a tape: measure the height difference between two points a known distance apart, then enter run and fall here to get the ratio.