Free Plumbing & Drainage Calculators

These are free calculators for specifying UK rainwater and drainage systems. They size gutters and downpipes using the effective area method from BS EN 12056-3 and the rainfall intensities published in Approved Document H of the Building Regulations, and they show every step of the working so you can check the result. There is no signup and no paywall, and you do not have to give us an email address to see an answer.

Why we built these

Most gutter calculators on the UK web do one of two things, and both of them produce the wrong answer. Some hardcode a rainfall figure of 75 mm/hour, which comes from BS 6367, a standard that has been withdrawn and should no longer be used here. Others ask you to type in a design rainfall intensity, which is a number almost nobody has to hand, because it has to be read off a contour map in Approved Document H.

Ours default to the highest intensity on that map, so they will never undersize a gutter, and they let you select your local figure if you know it. They also state plainly where the method stops applying (eaves gutters, England and Wales, half-round laid level), rather than returning a confident number for a case the standard does not cover. Everything is set out in the full methodology, including the sources and the assumptions we make.