Gutter Size Calculator
Estimate the amount of rainwater a gutter must carry from the roof drainage area and design rainfall intensity.
You can enter the drainage area directly or calculate an effective roof area from roof dimensions.
Calculate roof runoff
Enter the roof area served by one gutter run.
Roof and rainfall details
Estimated runoff
- Effective drainage area
- 50.00 m²
- Total runoff
- 1.04 L/s
- Litres per minute
- 62.50 L/min
- US gallons per minute
- 16.51 US gpm
- Area per downspout
- 50.00 m²
- Flow per downspout
- 1.04 L/s
Worked calculation
Q = (50.00 × 75.00) ÷ 3600
Q = 1.04 L/s
How to use the gutter size calculator
- Choose whether to enter the roof drainage area directly or calculate it from roof dimensions.
- Enter the roof area that drains into the gutter run being assessed.
- Enter the design rainfall intensity for your location.
- Enter the number of downspouts serving that gutter run.
- Select Calculate to estimate the rainwater flow that the gutter and downspouts must handle.
Roof runoff formula
The calculator estimates roof runoff using:
A rainfall depth of 1 millimetre falling on 1 square metre of roof produces approximately 1 litre of water.
Effective roof drainage area
Gutter design is based on the effective area draining into the gutter rather than simply the total floor area or total roof area of the building.
For a pitched roof, this calculator estimates the effective roof drainage area using:
This allows for the additional rainfall exposure associated with a pitched roof rather than using only its horizontal plan area. Only the portion of roof draining into the gutter run being assessed should be included.
UK Approved Document H also uses effective roof area when assessing rainwater drainage and provides adjustment methods for pitched roofs.
For further guidance, see Approved Document H: Drainage and waste disposal .
Choosing a design rainfall intensity
Rainfall intensity is one of the most important inputs in the calculation. A higher design rainfall intensity produces a proportionally higher roof runoff rate.
The appropriate value depends on location and the drainage standard or design requirements being followed. Do not assume that the calculator's initial value of 75 mm/hr is suitable for every location or building.
Where the calculator is being used for an actual installation, obtain the required design rainfall intensity from the applicable building guidance, drainage standard, rainfall data or project specification.
Gutter runoff example
A gutter serves 50 m² of effective roof area and the design rainfall intensity is 75 mm/hr.
The calculated runoff is:
Q = (50 × 75) ÷ 3600 = 1.04 L/sThe gutter and downspout system therefore needs sufficient capacity for approximately 1.04 litres of water per second.
How downspouts affect gutter capacity
Adding more downspouts can divide the total runoff between multiple outlets and reduce the amount of water that must reach a single outlet.
The calculator divides the effective roof area and total runoff evenly between the selected number of downspouts. Actual flow distribution can vary with gutter slope, outlet position, gutter length and roof geometry.
From runoff flow to gutter size
The calculated runoff flow is the amount of rainwater that the gutter system needs to manage under the entered design conditions. It is not, by itself, an exact gutter-size recommendation.
Once the required flow is known, the next step is to compare that flow with the rated capacity of the proposed gutter, outlets and downspouts. Capacity depends on factors including gutter profile, dimensions, slope, outlet size and outlet position.
For example, if the calculator produces a design runoff of 1.04 L/s, the selected gutter and outlet arrangement should have sufficient capacity for that design flow under the applicable manufacturer guidance or drainage standard.
Approved Document H provides example capacities for common half-round eaves gutters and outlet sizes, while other gutter profiles and systems should be checked against their own manufacturer's capacity data.
Limitations
This calculator estimates rainwater runoff. It does not certify a particular gutter profile, outlet or downspout as suitable for a building.
Actual gutter capacity also depends on gutter shape, dimensions, slope, outlet position, number of outlets, downspout dimensions and local design requirements.
Manufacturer capacity tables and applicable building or drainage standards should take precedence when selecting the final gutter and downspout system.
Frequently asked questions
What roof area should I enter?
Enter only the roof area that drains into the particular gutter run you are sizing, rather than automatically using the entire roof area of the building.
Why does rainfall intensity matter?
A higher rainfall intensity produces a higher runoff rate. The gutter must therefore carry more water during the design storm.
Does a larger roof need a larger gutter?
A larger drainage area normally produces more runoff, but required gutter capacity also depends on rainfall intensity, gutter geometry, gutter slope and outlet arrangement.
Does adding downspouts increase capacity?
Additional outlets can reduce the amount of runoff directed to each individual downspout, although actual performance depends on their position and the gutter layout.
Does this calculator recommend an exact gutter size?
No. It calculates the runoff capacity that the gutter system needs to manage. Final gutter and downspout selection should be checked against manufacturer capacity data and applicable local standards.