Gutter Size Calculator
Enter the roof area that drains to the gutter, the roof pitch, the gutter length and the heavy rainfall rate for your area. The calculator gives the rain flow, the gutter size and how many downpipes to fit.
- Roof area and pitch
- Rainfall intensity
- Downpipe count
Use the plan area of the roof slope draining into this run of gutter, measured flat as if seen from above.
Gutters must cope with the heaviest storms, not average rain. A short cloudburst can drop 75 mm of rain an hour or more. On a 60 m² roof that is over a litre every second. A gutter that is too small overflows down the walls, soaking brickwork and foundations.
Gutter capacity chart
Flow with one end outlet. Search for a size.
| Gutter | Capacity (L/s) | Roof area served (m²) | Typical downpipe |
|---|---|---|---|
| 75 mm half-round | 0.38 | 14 | 50 mm |
| 100 mm half-round | 0.8 | 30 | 68 mm |
| 112 mm half-round | 1.1 | 41 | 68 mm |
| 125 mm half-round | 1.5 | 55 | 68 to 80 mm |
| 150 mm half-round | 2.3 | 85 | 110 mm |
| 170 mm box | 3.4 | 125 | 110 mm |
Roof areas are for 75 mm/h rain on a 30° to 45° roof. A centre outlet roughly doubles the area a gutter can serve.
Roof area and gutter fill
75 mm/h storm, one end outlet. Slide the roof area.
Most UK houses use 112 mm half-round gutters with 68 mm downpipes. Large roofs or extreme rain call for deep-flow or box gutters.
How to size gutters
Rain flow first. Step through it.
Measure the roof
Plan length × depth of each slope.
Fit gutters with a slight fall towards the outlet, about 1 in 600. Too much fall looks odd and too little leaves puddles.
Half-round, deep-flow or box
Profiles carry different amounts. Compare them.
The classic UK profile. Easy to clean and widely available.
Deeper section that carries about 50% more water.
Square section for large roofs and commercial buildings.
US K-style gutters are common at 5 and 6 inches. A 5 inch K-style carries about the same as a 125 mm half-round.
Gutter maintenance
Keep water flowing. Tick each item.
Tick each item as you go.
Blocked gutters are the most common cause of overflow. A gutter that is the right size still fails if it is full of leaves.
What you get
Size and layout. Each tile explains one result.
Smallest profile that copes.
Litres per second in a storm.
How many to fit.
Flow each downpipe takes.
Roof area with pitch allowance.
Effective roof area adds an allowance for wind-driven rain on pitched roofs, as recommended in BS EN 12056-3.
Frequently Asked Questions
No question matches that search.
What size gutter do I need?
Most houses need 112 mm half-round. Larger or steeper roofs need 125 to 150 mm.
How do I calculate gutter size?
Work out rain flow from roof area × rainfall ÷ 3,600, then choose a gutter with more capacity per outlet.
How many downpipes do I need?
About one for every 12 m of gutter, and at least one per roof slope.
What rainfall rate should I use?
75 mm/h is the UK design value. Use local data for tropical or storm-prone areas.
Does roof pitch affect gutter size?
Yes. Steep roofs catch more wind-driven rain, so add 15% to 50% to the area.
Where should the downpipe go?
Centre outlets carry most. End outlets are simpler but halve capacity.
What size downpipe for a 112 mm gutter?
68 mm round is standard.
Gutters that handle the storm
Measure your roof, allow for the pitch and design for heavy rain, not average rain. Choose a gutter with spare capacity and enough downpipes, and keep it clear, and your walls stay dry.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.