About this calculator
Total drop for a drain or waste pipe at a given gradient, so you can set invert levels at each end.
How it’s calculated
Fall = length ÷ gradient ratio. 1 in 50 = 2%, about ¼" per foot (exactly ¼" per foot is 1 in 48).
Worked example
With these inputs:
- Pipe run length
- 40 ft
- Gradient
- 1 in 50 (2%), about ¼" per foot
the result is:
- Total fall
- 9-5/8"
- Gradient
- 1:50 = 2%
- Angle
- 1.15°
- Fall per foot
- 1/4"
Tips from the trade
- The plumbing code asks for a minimum slope of 1 in 50 (2%) for drains 3" and smaller and 1 in 100 (1%) for 4" and larger. Provinces adopt the National Plumbing Code with their own changes, and drain work usually needs a permit.
- Too steep can be as bad as too flat: water outruns the solids.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and the building code adopted in your province before ordering.