One inch of rain over 1,000 square feet represents about 623 gallons of water. Use this tool to understand roof runoff volume before discussing gutters and downspouts. It estimates volume, not the gutter size, pipe size, or safe discharge location your property needs.
Estimate your roof’s rainwater volume
About 623 gallons before collection losses.
Formula: area × rainfall ÷ 12 × 7.48052. Assumes all rain on the horizontal area becomes runoff; actual collected volume may be lower. Entries stay in your browser and are not submitted.
Use horizontal drainage area, not sloped roof surface
Rainfall depth describes water over a horizontal area. Use the roof’s plan-view area that feeds the gutter section you are considering. Do not substitute your home’s advertised living area or add the surface area of every sloped roof face without accounting for its projection.
A roof valley can concentrate runoff from more than one roof plane into a short section. If you do not know which areas drain to an outlet, use the calculator as an illustration and have an installer map the drainage areas. Do not climb onto the roof to obtain measurements.
Volume and flow rate answer different questions
One inch arriving gradually and one inch arriving in a short burst produce the same total volume over the same area, but very different demands on the gutter at any moment. Local design rainfall intensity, roof geometry, gutter slope, outlet placement, and discharge conditions affect system sizing.
| Horizontal area | Rainfall | Approximate volume |
|---|---|---|
| 500 sq ft | 1 inch | 312 gallons |
| 1,000 sq ft | 1 inch | 623 gallons |
| 2,000 sq ft | 2 inches | 2,494 gallons |
Use the result to ask better drainage questions
Ask where the water goes after it leaves the downspout and whether the planned outlet can function during rain. A buried line still needs an appropriate outlet; hiding the pipe does not solve an unsuitable discharge route. Compare surface extensions and underground drains and review overflow symptoms before your assessment.
Calculation basis: USGS rain-volume examples and USGS volume conversion factors. This tool is educational and does not certify drainage capacity.
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