Gutter Downspout & Drainage Calculator
Calculate gutter downspout count and drain capacity in GPM from roof drainage area and rainfall intensity.
Returns count, slope, and downspout size.
Gutter drainage capacity is calculated using the Manning Equation adapted for open-channel flow in a horizontal trough. The core formula is:
Q = (1.49 / n) × A × R^(2/3) × S^(1/2)
Where Q is the flow rate in cubic feet per second, n is Manning’s roughness coefficient (typically 0.011 for aluminum gutters and 0.013 for vinyl), A is the cross-sectional area of the gutter in square feet, R is the hydraulic radius (area divided by wetted perimeter) in feet, and S is the slope of the gutter run as a decimal.
For a simpler rule-of-thumb used by most contractors, the formula becomes:
Drainage Area (sq ft) = Gutter Capacity (gallons/min) × 96
This assumes a moderate rainfall intensity of 1 inch per hour. For heavier rain regions (2–4 in/hr), divide the calculated drainage area by the rainfall intensity multiplier.
Worked Example: A 5-inch K-style aluminum gutter with a 1/16-inch-per-foot slope has a rated capacity of roughly 23 gallons per minute. Multiplied by 96, it can handle approximately 2,208 sq ft of roof in a 1-in/hr rainfall zone. In Miami (4 in/hr zone), that same gutter handles only about 552 sq ft of roof.
Reference Values: 4-inch round gutters handle ~2,000 sq ft; 5-inch K-style ~2,500 sq ft; 6-inch K-style ~3,840 sq ft. Downspouts should be sized at one 2×3 inch outlet per 600 sq ft of roof drained. Always add a 10–15% safety margin for debris and sag.