Fall and gradient calculator

Work out the drop over a run from a fall such as 1 in 40 or 1 in 80, or the fall from a drop you have, for drains, patios, paths and flat roofs.

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Worked example

An 18 m run of drain at 1 in 80:

225 mm drop over 18.00 m

A fall of 1 in 80: 1.25% or 0.72°
12.5 mm a metre, so a 3 m length drops 37.5 mm

How the sum works

A fall of 1 in 80 means the run drops 1 unit for every 80 along it, so the drop is the length divided by 80: 18,000 mm / 80 = 225 mm. Working the other way, the fall is the length divided by the drop. As a percentage it is 100 divided by the 80, and the angle is the inverse tangent of 1/80. If you give the depth at the start, the calculator adds the drop to it for the far end: for a drain, put in the invert depth at the top of the run to get the depth at the bottom. That assumes the ground is level, so if it slopes, work from levels instead.

What falls to use

The fall a job needs comes from its own rules, not from the calculator. For below-ground drains, Approved Document H gives minimum falls by pipe size and flow; for paving, the paving maker gives the fall it needs; for a flat roof, the membrane maker and BS 6229 set the design fall. Check the rule for your job before you dig: GOV.UK: Approved Document H.

Guides that go with it

How deep should a patio sub-base be?A patio usually needs about 100 mm of compacted Type 1 and a 30 to 40 mm mortar bed; a drive about 150 mm. Dig depth, falls, DPC level and tonnes.How do I do a quantity take-off from drawings?How to take off quantities from drawings for a small job: scale check, set order, deductions, waste and pricing, with a worked garden room example.Do you need building regulations approval? Common jobs in EnglandWhich common jobs in England need building regulations approval: extensions, loft and garage conversions, outbuildings, windows, boilers and electrics.

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