Gutter Slope Calculator UK

The Gutter Slope Calculator UK exists for British installers who work in metric and quote slope as a 1:X ratio or millimetres per metre — the conventions used on UK rainwater jobs and in BS EN 12056-3. Rather than converting between the imperial in/ft figures common in American tools and the metric ratios on British specifications, this page works natively in metres and millimetres from the first input. It checks the slope of an existing eaves gutter from a measured run and drop, designs the drop for a target ratio, finds the maximum run a given fall supports, and plans outlets for long walls — all with in/ft and degrees alongside for reference.

UK gutter slope is expressed as a 1:X ratio, where X is the horizontal run per unit of vertical drop:

Slope ratio = Run ÷ Drop (e.g. 6 000 mm ÷ 17 mm ≈ 1:353, rounded to 1:350)

Converting a ratio to mm per metre (the preferred site figure):

mm per metre = 1000 ÷ X

So 1:350 = 1000 ÷ 350 ≈ 2.9 mm/m, and 1:600 = 1000 ÷ 600 ≈ 1.7 mm/m. To find the total drop for a chosen ratio and run: Drop (mm) = Run (m) × (1000 ÷ X). For a 7 m run at 1:350: 7 × 2.857 ≈ 20 mm.

The UK gutter slope calculator turns a measurement into a slope: enter the run in metres and the height drop in millimetres between the high end and the outlet, and it reports the slope as a 1:X ratio, mm per metre, a grade percentage and an angle, then flags whether it looks too shallow. It is the companion to the UK fall calculator, which goes the other way — from a chosen fall rate to the drop to build in.

Treat these as guidance figures and confirm against BS EN 12056-3, Building Regulations Part H and the manufacturer's instructions for your specific gutter and rainfall.