Gutter size calculator

Last reviewed September 2026

Check whether a gutter and downpipe are big enough for the roof draining into them, using the method in Approved Document H. It’s useful for tracing overflowing gutters in a survey, checking the gutter after an extension or loft conversion, and sizing a replacement.

The roof

Length along the gutter (m)Depth on plan (m)Measured horizontally from the gutter to the ridge, not up the slope.Roof pitch (degrees)0 for a flat roof, up to 70°. A steeper slope counts as a wall.Wall draining onto the roof (m², optional)A wall above the roof, such as a dormer cheek or the side of a taller building. Half of it counts.

Design rainfall intensity (litres/second/m²)Read it from Diagram 1 of Approved Document H: 0.022 around London, Oxford and East Anglia, lower in the north and west.

The gutter

GutterOutlet positionAn outlet part way along takes water from both sides. If it isn’t central, enter the larger side as a separate roof with an end outlet.Stop end to outlet (m, optional)To check for a long run, which reduces what the gutter can carry.Water depth in the gutter (mm, optional)Leave blank to use 50 mm, about half the width of a half-round gutter.

The downpipe discharges onto a lower roof

Result

Enter the roof’s length, depth on plan and pitch to check the gutter.

How it’s worked out

The roof’s effective area is its plan area plus half its elevational area, which allows for the rain it catches when it’s pitched. That’s the basis of Approved Document H Table 1: ×1.29 at 30°, ×1.50 at 45° and ×1.87 at 60°. Half of any wall draining onto the roof is added.
The flow is the effective area times the design rainfall intensity from Diagram 1. It’s compared with the capacity of a half-round gutter laid level with a sharp-edged outlet, from Table 2.
A gutter laid to a fall, a deeper profile or a round-edged outlet carries more, so use the manufacturer’s capacity for those. Gutters should also be laid so that an overflow spills clear of the building.

When to use a full design

Approved Document H expects valley gutters, parapet gutters, flat roof and siphonic systems, and any gutter where an overflow could let water into the building, to be designed to BS EN 12056-3. The method here is for ordinary eaves gutters. Approved Document H covers England; for Scotland and Northern Ireland, check their own guidance. For damp that may come from a leaking gutter, the condensation and mould risk calculator helps rule condensation in or out.

Sources

Approved Document H: Drainage and waste disposal, 2015 edition, section H3 (GOV.UK)

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