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Gutter Sizing Calculator

Roof drainage area and local peak rainfall intensity determine how much water your gutters need to move.

Roof drainage area (sq ft)

Peak rainfall rate (in/hr)

Required capacity

62.4 GPM

A standard 5-inch K-style gutter handles roughly 5,520 sq ft of roof at a 4 in/hr rainfall rate — larger roofs or heavier rainfall need 6-inch gutters or more downspouts.

What you entered

Roof drainage area (sq ft): 1500Peak rainfall rate (in/hr): 4

1
Apply the roof-runoff formula
1500 × 4 × 0.0104= 62.4 GPM

Result

Required capacity: 62.4 GPM

A 1500 sq ft roof at 4 in/hr peak rainfall needs gutters rated for about 62.4 GPM.

Gutter flow by roof area and rainfall intensity

Flow is roof plan area times rainfall depth per hour, converted to gallons per minute. One inch of rain on 1,000 square feet is about 623 gallons.

Roof area	Rainfall intensity	Flow to handle
500 sq ft	2 in/hr	10.40 gpm
1,000 sq ft	2 in/hr	20.80 gpm
1,500 sq ft	2 in/hr	31.20 gpm
1,000 sq ft	4 in/hr	41.60 gpm
1,000 sq ft	6 in/hr	62.40 gpm
2,000 sq ft	4 in/hr	83.20 gpm

Use the roof's plan area seen from above, not the sloped surface. Design to your local 5-minute peak intensity rather than an annual average: a 5-inch-per-hour cloudburst is what overflows a gutter, and that figure varies from about 2 in/hr in temperate regions to 8 or more in the subtropics.

Standard gutter capacity

A standard 5-inch K-style gutter can typically handle a roof area of roughly 5,520 sq ft at a 4 inch/hour rainfall rate — larger roofs, steeper pitches, or heavier regional rainfall may call for 6-inch gutters or additional downspouts.

Downspout count matters too

Gutter capacity is only half the equation — downspouts need enough total capacity to drain what the gutter collects, and are usually spaced no more than 35-40 feet apart along a gutter run.

Frequently asked questions

My roof drains 1,500 sq ft and our peak rainfall is 4 inches/hour — do I need 5-inch or 6-inch gutters?

Standard 5-inch K-style gutters handle about 5,520 sq ft at 4 in/hr rainfall. Your 1,500 sq ft is well within that capacity, so 5-inch gutters are sufficient. Upgrade to 6-inch if your area sees heavier peak rainfall or you want less frequent overflow.

How many downspouts do I need?

One downspout per 35-40 linear feet of gutter run, placed at the low points. A 60-foot gutter run needs at least 2 downspouts. More downspouts reduces the load on each and prevents overflow during heavy rain.

What's the difference between K-style and half-round gutters?

K-style gutters (the flat-backed, decorative-front type) hold more water per inch of width and are easier to mount. Half-round gutters are less common, hold less volume, but are preferred for historic homes and shed debris better.