Drainage Fall Calculator
Calculate required fall over a drainage run from a 1-in-N ratio.
What this tool calculates
Calculate required fall over a drainage run from a 1-in-N ratio. The main inputs on this page are Run length (m), Fall ratio 1 in .... The result is intended to be readable immediately, with the explanation below available when you need to verify the method.
Method and calculation basis
Fall = run × gradient. For a 1:N gradient, fall equals run ÷ N after converting both to compatible units.
Worked examples
Measure irregular work
Split an irregular area into simple shapes, calculate each separately and add the quantities.
Allow for site reality
Apply waste or extra allowance after calculating the geometric base quantity.
Understanding the result
The output should be read in the units shown beside the result. If you are comparing alternatives, keep all unchanged assumptions identical and alter one input at a time. That makes it easier to see which value is responsible for the difference.
Assumptions, edge cases and accuracy
This is a quantity or geometry estimate, not structural design approval. Product yield, compaction, joints, substrate condition, reinforcement, waste and site tolerances can materially change the amount required.
Sources and methodology
- BSI / applicable British Standards — Relevant construction measurement and design standards where applicable
- Manufacturer technical data — Product coverage, yield, density and installation requirements
Method reviewed for Instant Solvers V10.0 · October 2026. If you find a calculation or explanation that needs correcting, use the site Contact page and include the tool URL and the values you entered.