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.
In NSW, sanitary and suspended pipe gradients are regulated under AS/NZS 3500.2 (the Australian sanitary plumbing and drainage standard, which specifies minimum pipe falls—like 1:50 or 2% for 65 mm and 80 mm waste pipes—to ensure that drainage discharges smoothly by gravity without self-siphoning or leaving waste solids behind). Contact our commercial plumbing team for site-specific design, compliance, and installation advice. As Sydney’s trusted plumbing contractor since 2011, Fixed Today provides clear consultative guidance and upfront fixed pricing for commercial and residential fit-outs. Informing our dispatch team whether your project involves suspended ceiling drainage, concrete slab penetrations, or commercial trade waste helps our licensed plumbers arrive equipped with the right optical levels, laser pipe-graders, and compliant bracket systems.