A 2% slope in piping means the pipe falls 2 units vertically for every 100 units of horizontal run. This equals 20 mm per horizontal metre, or a 1 in 50 gradient.
The slope is calculated using the level horizontal distance, not the length measured along the sloping pipe:
Slope (%) = vertical fall ÷ horizontal run × 100
“Fall” means the downstream end is lower than the upstream end. A plumbing plan may show the required grades alongside the pipe layout and connection details.
Calculating the vertical fall
Multiply the horizontal pipe run by 0.02, using compatible units:
Vertical fall = horizontal run × 0.02
| Horizontal run | Fall at 2% |
|---|---|
| 2 m | 40 mm |
| 5 m | 100 mm |
| 10 m | 200 mm |
For example, over 5 m:
5 m × 0.02 = 0.10 m = 100 mm
Dividing the horizontal run by 50 gives the same result because 2% is equivalent to 1 in 50. For sanitary drainage, check the required sewer-pipe fall rather than assuming this example applies.
Is a 2% slope the same as 2 degrees?
No. A 2% slope is a vertical-to-horizontal ratio, while degrees measure an angle from horizontal.
2% slope angle = arctan(0.02) ≈ 1.15°
A 2-degree angle is therefore steeper than a 2% slope.
A 2% slope describes geometry; it does not set the required pipe gradient for every plumbing system. Requirements depend on the application and the plumbing standards that apply in NSW. For regulated plumbing work, have a licensed plumber confirm the required gradient before installation or alteration.
Contact our commercial plumbing team for site-specific design, compliance and installation advice.