What this calculator works out
This calculator converts a roof catchment area and a design rainfall intensity into a flow rate in litres per second and per minute, with a safety allowance applied.
That flow rate is what you compare against the published capacity of a gutter and downpipe combination. Gutters overflow because the flow exceeds their capacity, not because they are the wrong colour.
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Before you size a gutter
- Catchment area for a pitched roof is the plan area plus an allowance for wind-driven rain — commonly half the vertical height multiplied by the length.
- 75 mm per hour is a widely used UK design intensity for domestic work. Higher figures are used where overflow would cause damage.
- Gutter capacity depends on shape, size, fall and the position of outlets. A gutter with an outlet at one end carries roughly half what the same gutter carries with a central outlet.
- Sharp bends, stopped ends and shallow falls all reduce effective capacity below the published figure.
A half-round gutter can often carry more than the downpipe serving it will accept. Adding a second downpipe frequently solves an overflow problem that replacing the gutter would not, and it is considerably cheaper.
How this calculator works
Area times intensity, converted to a flow rate:
Flow (litres/second) = area × rainfall in mm/hour ÷ 3,600 × (1 + allowance ÷ 100)
Litres per minute = litres per second × 60The 3,600 converts millimetres per hour over square metres into litres per second: one millimetre of rain on one square metre is one litre, and there are 3,600 seconds in an hour.
British Standard EN 12056-3 covers the design of roof drainage properly, including effective catchment areas, gutter capacities and outlet positions. This calculator gives the flow figure; the standard tells you what will carry it.
Worked example: a 70 m² catchment
Using the default figures — 70 m² of catchment at 75 mm/hour with a 20% allowance:
- Flow: 70 × 75 ÷ 3,600 × 1.2 = 1.75 litres per second
- Or 105 litres per minute
A standard 112 mm half-round gutter with a central outlet carries broadly this order of flow; the same gutter with the outlet at one end carries considerably less. That is why an extension or a converted garage frequently overflows on the same gutter profile that worked before — the catchment grew and the outlet arrangement did not.
Common mistakes
- Using the sloping roof area. Catchment is based on plan area plus a wind-driven rain allowance.
- Ignoring outlet position. An end outlet roughly halves effective capacity.
- Setting no fall. Gutters need a slight fall towards the outlet.
- Adding roof area without adding drainage. Extensions and conservatories change the catchment.
- Assuming blockages are the only cause of overflow. Undersized drainage overflows even when clean.
Frequently asked questions
What rainfall figure should I use?
75 mm per hour is a common design figure for domestic work in the UK. Where overflow would cause damage — over a doorway, against a wall, or where water would enter the building — a higher intensity of 100 mm/hour or more is often used, or an overflow route is designed in deliberately.
How much fall should a gutter have?
A slight but definite fall towards the outlet, commonly around 1 in 350 to 1 in 600. Too little and water stands and silts up; too much and the gutter looks wrong against the fascia and can overshoot at the outlet. Some systems are designed to be laid level, which relies entirely on the outlet capacity.
Should I use a bigger gutter or another downpipe?
Usually another downpipe. It halves the effective catchment on each length of gutter, which is a larger improvement than moving up one gutter size, and it is much less disruptive than replacing a whole run. It also gives redundancy if one downpipe blocks.
Why does my gutter overflow only in heavy rain?
Because capacity is only exceeded above a certain intensity. In ordinary rain the gutter copes; in a downpour the flow exceeds what the outlet can take and it backs up. That points to sizing or outlet arrangement rather than blockage, though it is always worth clearing them first to rule that out.
Is what I enter stored?
No. Areas and rainfall figures are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — Approved Document H guidance on rainwater drainage
- Health and Safety Executive — ladder and working at height safety when clearing or fitting gutters
Sources are checked at publication and can change — how I choose and check references.