Drainage Gradient Calculator — Fall, Invert Levels & Compliance
Check or calculate drainage gradients for foul and surface water pipes against Building Regulations Part H minimum and maximum falls.
Horizontal distance of pipe run
Invert level at start (upstream)
Invert level at end (downstream)
How We Calculate This
This calculator checks drainage gradients against Part H minimum requirements or calculates the required end invert level for a compliant gradient.
The formula
Gradient = (Start Invert - End Invert) / Pipe Length
Fall per metre (mm/m) = Gradient × 1000
AD H 2015 Table 6 minimum (flattest) foul gradients
- 100mm foul: 1:80 (12.5mm/m) with ≥1 WC; 1:40 only if peak flow <1 l/s
- 150mm foul: 1:150 (6.7mm/m) with ≥5 WCs
- 100mm surface: 1:100 (10mm/m)
- 150mm surface: 1:150 (6.7mm/m)
Frequently Asked Questions
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Last updated: March 2026
All calculations are estimates. Verify with your supplier.