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Drainage Fall, Gradient and Invert Calculator

Check a drainage run from two invert levels, or calculate the downstream invert required for a chosen 1:n gradient.

Enter your data

Use horizontal plan length, confirm the intended flow direction and keep all levels on one datum.

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Calculate gradient from levels

Use known upstream and downstream inverts.

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m
m
m
m

Design downstream invert

Use a known upstream invert, length and falling ratio.

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m
1 : n
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m
Inputs are processed in this browser. This calculator does not send project data to a server.
Drainage limitation: This checks geometric fall and invert levels only. It does not verify hydraulic capacity, cover, pipe class, bedding, chamber details or approval against the drainage design.

Drainage gradient conventions

A falling run has an upstream invert higher than its downstream invert. For a ratio of 1:n, the vertical fall is the horizontal length divided by n. A 1:80 gradient therefore falls 12.5 mm per horizontal metre.

fall = upstream invert − downstream invert
ratio denominator = horizontal length ÷ fall

Manhole depth

When a ground level is supplied, the page calculates depth as ground level minus invert level. This is a simple level difference and does not add pipe wall thickness, bedding, cover requirements or chamber construction allowances.

Plan length and slope length

The formulas use horizontal plan length, which is the normal convention for gradient. For ordinary drainage gradients, the difference between plan and slope length is small, but project conventions should still be followed.

Frequently asked questions

What does 1:80 mean?

It means one unit of vertical fall for every 80 units of horizontal run. That is 12.5 mm per metre or 1.25 percent.

Why does the result say rising?

The downstream invert entered is higher than the upstream invert. Check that the flow direction and level order are correct.

Does the depth equal pipe cover?

Not necessarily. Ground-to-invert depth is not the same as cover to the outside crown of a pipe. Pipe diameter and wall thickness must be considered.