Drainage Slope Calculator

Work out drainage slope instantly with clear inputs, formula shown and shareable results.

About Drainage Slope Calculator

Drainage slope is fall divided by length, expressed either as a percentage or the traditional one-in-N ratio. Velocity follows from Manning's equation with a hydraulic radius of one quarter the diameter for a half-full pipe, and the self-cleansing minimum is generally taken as 0.75 m/s.

Formulas

Slope

Slope = fall / length; gradient = 1 in (length / fall)

Velocity

V = (1/n) x R^(2/3) x S^(1/2), with R = D/4 half full

Worked example

With the default values below, Drainage Slope Calculator returns Slope of 0.500%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Pipe run lengthminimum 0.1 m: 50 m
Total fallminimum 0.1 mm: 250 mm
Pipe diameterminimum 10 mm: 100 mm

Results

Slope: 0.500% %
Gradient: 1 in 200
Velocity when half full: 0.465 m/s
Fall per metre: 5.00 mm/m

Step by step

1Values used
2Slope
3Velocity
4Slope= 0.500 %
5Gradient= 1 in 200
6Velocity when half full= 0.465 m/s
7Fall per metre= 5.00 mm/m

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Drainage Slope figure worked out?

Slope = fall / length; gradient = 1 in (length / fall). Drainage slope is fall divided by length, expressed either as a percentage or the traditional one-in-N ratio.

What is the standard minimum gradient?

One in 80 for a 100 mm foul drain and one in 40 for smaller branches, but the real criterion is achieving self-cleansing velocity at the expected flow, not the gradient itself.

What values do I need to enter?

This calculator takes 3 inputs: Pipe run length, Total fall, Pipe diameter. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.