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Roof drainage area and local peak rainfall intensity determine how much water your gutters need to move.
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.
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.
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
Apply the roof-runoff formula
1500 × 4 × 0.0104= 62.4 GPMResult
Required capacity: 62.4 GPM
A 1500 sq ft roof at 4 in/hr peak rainfall needs gutters rated for about 62.4 GPM.