How much rainwater will my roof collect?
Multiply roof area by rainfall depth converted to feet, convert cubic feet to gallons, then apply a runoff factor. This gives a planning estimate for water reaching gutters or storage.
Runoff volume is not gutter sizing
This calculation estimates total gallons from a storm amount. Gutter size, downspout count, local rainfall intensity, roof valleys, debris, and overflow paths still need separate planning.
Use actual roof surface area
Use roof surface area when possible, not only the house footprint. Pitch, overhangs, dormers, and multiple roof planes can change the collection area.
Roof area to roofing squares
One roofing square equals 100 sq ft. Examples include 10% waste and do not include accessories.
| Roof area | Squares before waste | Squares with 10% waste |
|---|---|---|
| 1,200 sq ft | 12 squares | 13.2 squares |
| 2,000 sq ft | 20 squares | 22 squares |
| 2,500 sq ft | 25 squares | 27.5 squares |
| 3,000 sq ft | 30 squares | 33 squares |
Before you calculate
- Use roof area rather than house footprint when the calculator asks for area.
- Account for pitch, waste, hips, valleys, dormers, and roof complexity when applicable.
- Use the coverage value that matches the selected roofing product.
Common mistakes
- Confusing roof squares with square feet.
- Ignoring pitch or complex roof geometry.
- Using bundle coverage without checking the actual product.
Formula
gallons = roofArea * (rainfallDepth / 12) * 7.48052 * (runoffPercent / 100)
Assumptions
- This estimates rainfall capture volume only, not gutter, barrel, cistern, overflow, or stormwater-code sizing.
- Runoff factor depends on roof material, slope, losses, first-flush diversion, and gutter routing.
- Use local rainfall data and qualified guidance before relying on the result for drainage design.
Example
Estimated runoff volume: 505 gallons
How to use this calculator
- Enter roof area in sq ft.
- Enter rainfall depth in in.
- Enter runoff factor in %.
- Review the live estimate and compare it with the example result.
- Check the formula, assumptions, product labels, and site conditions before using the Roof Runoff Rain Barrel Calculator result to plan materials.
Before you buy materials
- Roofing orders often round to bundles, squares, rolls, or panels.
- Verify quantities with roofing plans or a qualified roofer before purchasing.
FAQ
How many gallons do I need for roof runoff rain barrel?
Use roof area, rainfall depth, runoff factor, then round up to the buying unit when the result is sold as whole items. In the default example, the result is 505 gallons.
How do I calculate rainwater from a roof?
Use gallons = roof area x rainfall depth in feet x 7.48052 x runoff factor. A runoff factor below 100% allows for splash, wetting, and losses.
How much rainwater from 900 sq ft and 1 inch of rain?
With a 90% runoff factor, this example estimates about 505 gallons.
Does this size gutters or downspouts?
No. It estimates runoff volume only. Gutter sizing depends on rainfall intensity, roof layout, downspout placement, slope, and local conditions.
Should I use footprint area or roof area?
Use actual roof surface area when available. A pitched roof can have more surface area than the building footprint.
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This calculator is for planning estimates only. Verify final quantities with product labels, project conditions, and a qualified professional when accuracy matters.