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.
Gutter slope example drops
Examples use 1 inch of drop per 16.67 ft as a planning input. Local conditions and installer guidance can differ.
| Gutter run | Formula | Total drop |
|---|---|---|
| 20 ft | 20 / 16.67 | 1.20 in |
| 40 ft | 40 / 16.67 | 2.40 in |
| 60 ft | 60 / 16.67 | 3.60 in |
| 100 ft | 100 / 16.67 | 6.00 in |
Before you calculate
- Measure each roof edge or gutter run separately.
- Account for corners, downspouts, outlets, end caps, and miters outside the simple length estimate.
- For slope, use the run length and intended drop consistently.
Common mistakes
- Adding all roof edges together without separating gutter runs.
- Forgetting downspout locations and corner fittings.
- Treating gutter slope as a material quantity.
Formula
gallons = roofArea * (rainfallDepth / 12) * 7.48052 * (runoffPercent / 100)
Assumptions
- This estimates runoff volume only, not gutter sizing or drainage design.
- Runoff factor varies by roof material, slope, and losses.
- Downspout capacity, local rainfall intensity, and overflow paths should be checked separately.
Example
Estimated gutter runoff volume: 673 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 Rainwater Calculator for Gutters result to plan materials.
Before you buy materials
- Round up to full gutter sections or standard lengths.
- Confirm profile, color, hanger spacing, and accessory counts before ordering.
FAQ
How many gallons do I need for rainwater calculator for gutters?
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 673 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 1,200 sq ft and 1 inch of rain?
With a 90% runoff factor, this example estimates about 673 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.
Related calculators
This calculator is for planning estimates only. Verify final quantities with product labels, project conditions, and a qualified professional when accuracy matters.