HesapHanem

Gutter Downspout Calculator

Estimate your roof's peak water flow and the number of downspouts needed to drain it.

How it is calculated

The tool works in metric units and treats the whole roof area as draining at the rainfall rate (runoff coefficient 1):

flow (L/min) = area × rain / 60 and downspouts = ceil(flow / 50), at least 1

area is the roof's plan area in m² (the footprint seen from above, not the sloped surface) and rain is the design rainfall intensity in mm per hour. One millimetre falling on one square metre is one litre, so dividing by 60 turns litres per hour into litres per minute. Each downspout is assumed to carry about 50 L/min, and the count is rounded up. If rain is left empty or zero, 75 mm/h is used. Area must be above zero.

Worked example

A roof with a 120 m² plan area and a design rainfall of 100 mm/h.

  1. Flow = 120 × 100 / 60 = 200 L/min
  2. Downspouts = 200 / 50 = 4, which is already whole

Result: peak flow 200.0 L/min and 4 downspouts. Leaving the rain field empty would use 75 mm/h instead, giving 150.0 L/min and 3 downspouts.

Try this example →

Common mistakes

  • Entering the sloped roof surface instead of the plan area: rain falls on the horizontal footprint, so a steep roof does not need a larger figure.
  • Entering the rainfall of a whole day or month: the rain field is an hourly intensity in mm/h for a short heavy storm.
  • Entering feet or inches: area must be in m² and rain in mm/h.
  • Putting in an average rainfall for the town: gutters are sized for peak storms, and the default of 75 mm/h applies only if you leave the field blank.

About this calculator

This calculator estimates peak roof runoff with the simple rational-method form flow (L/min) = roof plan area (m²) × rainfall intensity (mm/h) ÷ 60, treating the roof as shedding all the rain that falls on it. One millimetre of rain on one square metre is one litre, so the division by 60 turns litres per hour into litres per minute. If you leave the intensity blank it assumes a heavy storm of 75 mm/h. It then divides the flow by about 50 L/min, a typical capacity for one standard downspout, and rounds up to give the number of downspouts.

Roofing contractors and gutter installers use this sizing to determine how many downspouts a given roof needs, since too few downspouts cause gutters to overflow during heavy storms, leading to fascia rot and foundation water damage.

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Comments (1)

  • Editorial team

    The downspout count is a total for the roof, so split the roof into roughly equal drainage areas before deciding where each one goes. With 4 downspouts on a 120 m² plan area, each should serve about 30 m². The 50 L/min per downspout is a fixed planning assumption, and the 75 mm/h default only applies when the rain field is blank. Which design rainfall figure do you use for your area? Share your region and the intensity.

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