Guttering Calculator

How This Is Calculated

Gutter lengths needed is the total run divided by your chosen stock length, rounded up. Brackets are spaced along that same run at your chosen centres, with one extra added to cover both ends. Corner fittings match the number of corners you enter directly.

Unions are worked out as whatever joints are needed beyond one length per straight section between corners - in other words, the extra pieces left over once each corner has "used up" one length either side of it. This is an approximation that assumes your run divides reasonably evenly between corners; if one section is much longer than the others, check that section on its own.

Downpipes needed is the roof's plan area divided by the roof area figure you've set per downpipe, rounded up.

Common Questions

How is the number of downpipes worked out?

By dividing the roof's plan area by how much roof area a single downpipe can drain, then rounding up. That capacity figure varies by gutter profile, downpipe diameter and the rainfall intensity assumptions used, so treat the default as a reasonable starting point and check the manufacturer's own rainfall capacity table for anything beyond a straightforward small house roof.

Why does the gutter need a fall?

Guttering needs a slight downward slope toward each downpipe outlet, typically around 1:350 to 1:600, so water actually runs off rather than sitting and overflowing. It's subtle enough that a spirit level with a fall gauge, or simple string-line and tape measure method, is normally used rather than trying to judge it by eye.

Do I need a union at every corner?

No. A corner fitting joins two lengths of gutter at an angle and does that joining job itself, so it replaces what would otherwise be a straight union at that point. Straight unions are only needed where two lengths join in a straight run between corners or outlets.

What are stop ends for?

A stop end caps off an open end of guttering that isn't joined to anything else - typically the two ends of a run that doesn't form a complete loop around the building, such as a lean-to or a single elevation being replaced on its own.