# Drainage Pipe Slope Calculator

Standard slope by diameter, fall over the run and velocity check.

- Page: https://jianzhujisuan.com/en/drain-slope
- Category: M&E (https://jianzhujisuan.com/en/c/me)
- Chinese version: https://jianzhujisuan.com/zh/drain-slope.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Nominal diameter DN110 (2.0%), Pipe length 20 m, Actual slope 2.0%): **Fall over the run 0.400 m**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Nominal diameter | select | DN110 (2.0%) | — |  |
| Pipe length | number | 20 | m |  |
| Actual slope | number | 2.0 | % | Not less than standard, and not excessive |

## 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

## Worked example

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.

## Questions

### 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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