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. 1:40 = 2.5%, 25 mm per metre. The load check uses SANS 10400-P table 14 for 100 mm (110 mm outside diameter) and 150 mm (160 mm) drains.
Worked example
With these inputs:
- Pipe run length
- 12 m
- Gradient
- 1:40
- Drain size
- 100 mm (110 OD)
- Load carried (fixture units)
- 24
the result is:
- Total fall
- 300 mm
- Gradient
- 1:40 = 2.5%
- Angle
- 1.43°
- Fall per metre
- 25 mm
- Maximum load at this fall
- 4,500 fixture units
Tips from the trade
- SANS 10400-P: no drain smaller than 100 mm. Table 14 sets the largest load (in fixture units) a drain may carry at each fall; a 100 mm drain flatter than 1:60 needs special permission from the local authority, and flatter than 1:120 is not permitted (1:200 for 150 mm).
- 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 SANS 10400 (the National Building Regulations) and your local municipality before ordering.