Calculators
Gutter and drainage calculators
Gutter length, downspout count and size from roof area and rainfall, hangers, slope, guards and rain barrels.
Gutters are sized by how much water the roof delivers to them, and that depends on three things: the area of roof draining to each run, how steep it is, and how hard it rains where you live. A 5-inch K-style gutter that is plenty for a bungalow in Seattle can overflow on the same roof in Houston, where a 10-year storm drops water four times as fast. These calculators use the method in the sheet-metal industry's manual: roof area times a pitch allowance times rainfall intensity, checked against the capacity of each gutter and downspout size.
The other half of the job is counting what to buy. Gutter comes in 10-foot sections or is run seamless on site; either way you need corners, end caps, hangers at a spacing that depends on snow, downspouts every 30 to 40 feet, and extensions that carry the water far enough from the foundation. The calculators here do that arithmetic from the eave lengths of your house.
7 calculators
Gutter Length Calculator
Gutter length, sections, joints, corners, end caps, outlets and sealant from the eave runs of the house.
Downspout Count & Size Calculator
How many downspouts a gutter run needs and whether the gutter is big enough, from roof area, pitch and local rainfall intensity.
Gutter Hangers & Brackets Calculator
Hangers, fasteners and boxes for a gutter from its length, the number of runs and the spacing your climate needs.
Gutter Slope Calculator
The drop over a gutter run at 1/16 to 1/4 in per foot, the high and low end heights and the chalk-line marks between.
Roof Runoff & Rain Barrel Calculator
Gallons of runoff from a roof area per storm or per year, barrels to hold it and the rain that fills one.
Gutter Guard Calculator
Gutter guard pieces, packs and screws from the gutter length, runs, size and piece length.
Downspout Extensions & Splash Blocks Calculator
Downspout pipe, elbows, straps, extensions, splash blocks or buried drain to carry roof water away from the foundation.
Guides for this topic
Downspouts: how many, where, and how far from the foundation
A gutter is only as good as the place its water ends up. Here is how to place downspouts so the water leaves the house and stays away.
Flat roof membranes: EPDM, TPO and PVC quantities
A flat roof is a membrane, a slope to the drains and as few seams as possible. Here is how to count each one.
Gutter sizing by roof area and rainfall
Gutters are sized for the worst few minutes of a storm on one face of the roof. Three numbers decide it, and one of them is local.
How to measure a roof from the ground (no ladder needed)
Everything a roofing order needs can be measured standing on the lawn. Here is the method, the one calculation that matters, and the checks that catch the usual mistakes.
Questions people ask
- How many feet of gutter does a house need?
- Add up the eaves, the horizontal edges where water runs off. A 40 by 30 foot gable house has two 40-foot eaves, so 80 feet of gutter; the same house with a hip roof has eaves on all four sides, 140 feet. Add a corner for every turn and two end caps for every run that does not go round a corner.
- Should I use 5-inch or 6-inch gutters?
- At one inch of rain per hour a 5-inch K-style gutter handles about 5,500 square feet of roof and a 6-inch about 8,000. Divide those capacities by your local 5-minute, 10-year rainfall intensity from NOAA Atlas 14 (it varies from a few inches per hour on the Pacific coast to more than double that along the Gulf) and compare with the roof area draining to the run, multiplied by its pitch allowance.
- How many downspouts does a roof need?
- A 2×3 inch downspout drains about 600 square feet of roof and a 3×4 about 1,200, and no gutter run should go more than about 40 feet without one. A 1,000 square foot roof face on a 40-foot eave needs two 2×3 downspouts or one 3×4, and a long run gets one at each end with the high point in the middle.
- How much should a gutter slope?
- The usual figure is 1/16 to 1/8 inch of fall per foot of run toward the downspout. A 40-foot run at 1/16 inch per foot drops 2.5 inches over its length, or 1.25 inches each way if it is split from a high point in the middle with a downspout at each end.
- How much rain does a roof collect for a rain barrel?
- One inch of rain on one square foot of roof is 0.623 gallons, so a 1,000 square foot roof sheds about 620 gallons per inch of rain, and the quarter of it that drains to one downspout fills a 55-gallon barrel in a tenth of an inch. Allow 10 to 25% for first-flush diverters, splash and evaporation.
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