Drain Slope & Fall Calculator
Calculate the slope (gradient), fall (drop), or run (horizontal length) of a drain pipe. Enter any two values to find the third.
Formulas Used
Core relationship:
Slope (fraction) = Fall (m) ÷ Run (m)
Conversions:
Slope ratio (1:X) = 1 ÷ slope_fractionSlope (%) = slope_fraction × 100Slope (degrees) = arctan(slope_fraction) × (180 ÷ π)Fall (mm) = Run (m) × slope_fraction × 1000Run (m) = Fall (m) ÷ slope_fraction
Example: A 6 m drain at 1:60 slope → Fall = 6 ÷ 60 = 0.1 m = 100 mm
Assumptions & References
- Slope is expressed as a uniform gradient along the entire pipe run.
- Run is the horizontal (plan) distance, not the pipe length along the slope.
- Fall is the vertical drop from inlet invert to outlet invert.
- Minimum slope: 1:80 (1.25%) — AS/NZS 3500.2, UK Building Regulations Part H, and general plumbing codes to ensure self-cleansing velocity.
- Maximum slope: 1:12 (8.33%) — steeper gradients risk liquid running ahead of solids (self-siphonage / blockage).
- Recommended range: 1:40 to 1:60 (1.67%–2.5%) for most domestic drains (100 mm diameter).
- For large-diameter or commercial drains, consult a hydraulic engineer and local authority requirements.
- References: AS/NZS 3500.2:2021, UK Building Regulations Approved Document H, BS EN 12056-2.