Gutter Size Calculator: What Size Guttering and Downpipes Do I Need? (UK)
By the Professional Roofers team
Updated 2026 · Independent cost guide
Guttering is sized by how much water a roof sheds in heavy rain, not by the length of the run. A gutter that is too small overspills down the wall in every downpour, so the rule that matters is matching gutter capacity and downpipe outlets to your roof’s drained area. Enter your roof plan dimensions and pitch below and this works out the profile and number of downpipes you need.
What size guttering do you need?
A guide for buying and quote-checking, based on the principles in BS EN 12056-3 for roof drainage. Real gutter performance also depends on falls, bends and outlet position, so treat the result as a sound minimum, not an exact hydraulic design.
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