DIY Cost Tools
Gutter Capacity Calculator
Estimate roof runoff flow that gutters and outlets must convey from roof plan area, rainfall intensity and runoff coefficient.
How to use this calculator
- Replace every example value with your own measurements, label values or planning assumptions.
- Keep each value in the unit printed beside its field.
- Select Calculate, then compare the result with the formula and limitations below.
Formula and method
Gallons per hour = roof plan area × rainfall inches per hour × 0.623 × runoff coefficient. Gallons per minute = gallons per hour ÷ 60.
Results are hidden whenever an input changes or validation fails, so an older answer cannot be mistaken for the current calculation.
Worked example
Roof plan area = 1000 sq ft; Design rainfall intensity = 3 in/hr; Runoff coefficient = 0.95 decimal. The calculator returns Design runoff = 29.6 gal/min; Design runoff = 1776 gal/hr; Water volume per hour = 237.4 cu ft.
The worked example uses the same default values displayed in the calculator.
Limitations and safety
Frequently asked questions
What should I enter first?
Start with measurements or rates from the project, product label, service quote or equipment specification. Replace every example value before relying on the estimate.
Why can the real result differ?
This estimates inflow, not the hydraulic capacity of a particular gutter profile. Roof geometry, valleys, debris, slope, outlet placement, local rainfall data and code requirements need professional design.
Does the calculator round purchases up?
Only outputs labeled as whole packages, vents, heaters, pails or similar counts are rounded up. Keep extra precision for planning and follow supplier packaging.
Review notes and reference
Reviewed October 5, 2026. The formula was independently checked against the displayed default example, and the calculator was tested for valid defaults, empty input, invalid formats, range boundaries and recovery. Reference: NIST SI Units. Explanations and examples are original.