Gutter and Downspout Calculator

Enter the roof area that drains to the run and the design rainfall for the area. You get the peak flow, and with a downspout rating, how many outlets it takes and how hard the gutter works.

Units

Roof area draining to this run

The roof area that drains to this gutter run, measured flat: the footprint it covers, not the sloped surface. If your code adds an allowance for a steep roof or a wall that sheds onto it, enter the adjusted area.

Design rainfall intensity

The design rainfall intensity for where the building is, from the local code or the national rainfall data. The storm length and return period differ by market, so use the figure your code names.

Rated capacity of one downspout

What one downspout and its outlet can carry, from the manufacturer’s chart. Leave it empty to see the runoff alone.

Result

Fill in a few more boxes and the answer will show here.

Opens your print dialog. Choose Save as PDF. Your figures stay on this device.

Loading the calculator.

This is an estimate from the roof area and the rainfall figure you entered. Drainage rules differ between Canada, the US, Australia and New Zealand, so check the local code and the manufacturer’s ratings before you install.

A gutter is sized by the storm, not by the roof

The water a roof sheds is its area times the rainfall intensity. One inch of rain an hour on a thousand square feet of roof is about ten gallons a minute; a hundred millimeters an hour on a hundred square meters is 2.78 liters a second. That peak flow is what the gutters and downspouts have to carry, and it is exact arithmetic once you have the two inputs.

The rainfall figure is the one that matters, and it is local. Codes size for a short, heavy storm rather than an average day, and they do not agree on which storm: SMACNA uses a five-minute storm, AS/NZS 3500.3 a five-minute storm at one-in-twenty for an eaves gutter, and the New Zealand acceptable solution a ten-minute one. Use the figure your code names for the town the building is in.

Measure the area flat, as the footprint the roof covers. Some methods add an allowance for a steep pitch or a wall that sheds onto the roof, so check whether yours does and enter the adjusted area if it does.

The common mistake is counting downspouts by length of gutter instead of by flow. A long, low roof may need fewer outlets than a short, steep one over a big wall. The capacity of an outlet and a gutter profile comes from the manufacturer, at the slope you will hang it. Measure each run on the first visit; in Zeus the numbers stay with that address and the quote is built from them.

Built for Gutter work: see the Gutter job app.

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