Drainage Pipe Slope Calculator
Standard slope by diameter, fall over the run and velocity check.
Inputs
Result
Formula
| Fall | length × slope |
| Slope | fall ÷ length as a decimal |
| Velocity | (1/n) × R^(2/3) × i^(1/2) |
| Hydraulic radius | d ÷ 4 for a full circular pipe |
| Self-cleansing | v ≥ 0.6 m/s |
| Standard slopes | 2% at DN110, 1% at DN160 |
DN110 over 20 m at 2%: a 0.400 m fall, 20 mm per metre and 0.99 m/s full-bore — self-cleansing. Drop to 1% and velocity falls to 0.70 m/s, where solids settle over time.
Standard and minimum drainage slopes
| DN | Standard slope | Minimum slope | Fall per 10 m at standard | Fall per 10 m at minimum |
|---|---|---|---|---|
| DN50 | 3.5% | 2.5% | 350 | 250 |
| DN75 | 2.5% | 1.5% | 250 | 150 |
| DN110 | 2.0% | 1.2% | 200 | 120 |
| DN160 | 1.0% | 0.7% | 100 | 70 |
| DN200 | 0.8% | 0.5% | 80 | 50 |
| DN315 | 0.5% | 0.3% | 50 | 30 |
FAQ
Why is a steeper drain worse?
Because fast water outruns the solids. On a steep run the flow is swift but shallow, so solids roll along the invert and stall wherever the gradient eases or a joint interrupts them, building a blockage. Codes cap branch slopes at about 15%, beyond which you use a backdrop or a larger pipe. It is the opposite of supply pipe logic, which catches people out.
What matters with floor drains and traps?
Use a trapped gully with a seal at least 50 mm deep, or sewer gas comes back — the commonest cause of bathroom smells. Traps need the same 50 mm seal, and rarely used drains must be topped up. Branch runs beyond about 4-6 m need a rodding eye, or a blockage cannot be cleared.
Is rainwater pipe sloped like drainage?
No. Rainwater carries high flow and grit but little organic matter, so slopes can be gentler — 1% at DN100 — while the upper velocity limit is stricter, 5 m/s, to prevent erosion. The base of a roof downpipe needs an energy dissipator, since water falling ten metres or more hits hard. Storm and foul must be kept separate; cross-connection fails environmental inspection.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.