Checks a rainwater eaves gutter and its downpipes against BS EN 12056-3 (HR Wallingford SR620): catchment area and runoff from the roof's plan and pitch, then gutter design capacity and downpipe transport
capacity checked independently against the design flow.
Tender-stage only. Verify final design to project spec and a qualified drainage engineer.
For design guidance only. Always verify with a qualified engineer.
About this gutter and downpipe sizing calculator
This free gutter and downpipe sizing calculator checks a rainwater eaves gutter and its downpipes against BS EN 12056-3, using the UK implementation from HR Wallingford SR620. From the roof’s plan dimensions, pitch, any additional wall area and a design rainfall intensity, it works out the catchment area and runoff, then tests the gutter design capacity and the downpipe transport capacity independently against the design flow. It is aimed at UK M&E, public health and drainage engineers and contractors sizing roof drainage at tender stage. Everything runs in your browser. Nothing is uploaded.
How gutter and downpipe sizing is calculated
The effective catchment area is A = plan area + 0.5 × vertical (pitch) roof area + 0.5 × additional wall area, the halving being the SR620 26.6°-from-vertical wind-driven-rain allowance; the design runoff is Q = rainfall intensity × A. The gutter design capacity is QC = 0.9 × length factor × nominal capacity, half-round from SR620 equation 6.3 and box from equation 6.14 with the Table 5 freeboard, and the design flow is split evenly across the 2 × number of outlets gutter lengths. The downpipe transport capacity uses the UK-simplified Wyly–Eaton relation QRWP = 5.0×10−5 × d2.667 (f = 0.33), checked against the flow per outlet, independently of the outlet or hopper feeding it.
Frequently asked questions
What design rainfall intensity should I use?
Use a UK Category 1, 2-minute intensity for the location, typically about 0.010 l/s·m² in Scotland rising to around 0.022 l/s·m² in south-east England and East Anglia. The default is 0.020; higher-risk buildings may warrant a higher category. The value scales the runoff directly, so it drives the result.
Why is the pitched and wall area only counted at half?
SR620 treats wind-driven rain as arriving at 26.6° from vertical, so only half of the exposed vertical roof area (from the pitch) and half of any additional wall draining onto the gutter is added to the plan area. A flat roof (0°) adds no vertical area at all.
Half-round or box gutter: which profile?
Choose half-round for a standard eaves gutter (the design depth is the bore radius, no freeboard) and box for a non-eaves valley or parapet gutter, where SR620 Table 5 adds a freeboard allowance and the square-profile capacity equation applies. The tool switches the capacity formula and freeboard to match your selection.