Rainwater Harvesting Calculator

A rainwater harvesting calculator returns the gallons a roof can collect from a rainfall amount. It multiplies the roof area by the rainfall depth and by about 0.6234 gallons per square foot per inch of rain. It then applies a runoff coefficient for losses such as evaporation and splash-off.

Last reviewed September 2026Method & sources Report an error

Gallons Collected

794.8 gal

Breakdown
Gallons Collected	794.8 gal

How to Calculate Rainwater Harvesting

1 in. of rain falling on 1 sq ft of roof is exactly 1/12 of a cubic foot of water (since depth in feet is inches ÷ 12). That converts to gallons using the exact 7.48052 gallons-per-cubic-foot factor, giving about 0.6234 gallons per sq ft per inch of rain. Not all of that water actually reaches a collection barrel. Some evaporates, splashes off, or is lost to the first-flush diverter many systems use to skip contaminated initial runoff. A runoff coefficient (commonly 0.75-0.90 for a typical pitched roof) scales the theoretical maximum down to a realistic collected volume.

Example: 1,500 sq ft roof catching 1 in. of rain

A 1,500 sq ft roof catching 1 in. of rain, with a 0.85 runoff coefficient: 1. Gallons per sq ft per inch of rain: (1 ÷ 12) × 7.48052 ≈ 0.6234 gal 2. Theoretical maximum: 1,500 × 1 × 0.6234 ≈ 935.1 gal 3. With the 0.85 runoff coefficient: 935.1 × 0.85 ≈ 794.8 gal. That roof could realistically collect about 795 gallons from 1 in. of rain.

Rainwater Harvesting: Compare Scenarios

Scenario	Gallons Collected
Small (800 sq ft roof)	423.9 gal
Medium (1,500 sq ft roof)	794.8 gal
Large (3,000 sq ft roof)	1,589.6 gal

What Is Rainwater Harvesting?

A rainwater harvesting calculator estimates how many gallons a rain barrel or cistern system can realistically collect from a roof's catchment area and a given rainfall amount. It is useful for sizing a collection system or estimating how much of your outdoor watering a rain barrel could offset.

Rainwater Harvesting: Pros and Cons

Pros

Uses the exact cubic-foot-to-gallon conversion for the theoretical maximum, not a rounded approximation
Includes an adjustable runoff coefficient rather than assuming 100% collection efficiency, which no real roof achieves
Works for any roof size and rainfall amount, useful for both sizing a new system and estimating a specific storm's yield

Cons

Roof area here means the catchment's horizontal footprint, not its sloped surface area. Use the same flat footprint your gutters actually drain, not the roof's true (pitched) surface area
The runoff coefficient is a general planning figure. Actual collection efficiency varies by roof material, gutter condition, and whether a first-flush diverter is used
Doesn't size the storage tank or barrel itself. Compare this result against the Tank Volume calculator to check whether your storage capacity can hold what a given storm collects

Rainwater Harvesting: What a Pro Checks

Use your roof's flat, horizontal footprint, the same area your gutters actually drain, not its larger sloped surface area. Rainfall is measured as a vertical depth over a horizontal area.
Check this estimate against the Tank Volume calculator before you buy a barrel or cistern. A system that collects more than it can store just overflows during a big storm.

Frequently Asked Questions

How much rainwater can I collect from my roof?

Roughly 0.6234 gallons per sq ft of roof for every inch of rain, before runoff losses. A 1,500 sq ft roof at 1 in. of rain theoretically sheds about 935 gallons, or about 795 gallons after a typical 0.85 runoff coefficient.

What is a runoff coefficient?

The fraction of rainfall that actually reaches your collection system after evaporation, splash-off, and any first-flush diversion. It is commonly 0.75-0.90 for a typical pitched roof, rather than the 1.0 (100%) a purely theoretical calculation would assume.

How big of a rain barrel do I need?

It depends on your roof's catchment area and how much rainfall you want to capture per storm. Use this calculator's gallon estimate for a typical rain event, then compare it against the Tank Volume calculator to size a barrel or cistern that can hold that amount without overflowing.

Does roof material affect how much rainwater I can collect?

Yes, indirectly. Smoother materials (metal roofing) tend to have a higher runoff coefficient than rougher materials (some shingle types) that hold onto slightly more water. Any roof material used for drinking-water collection needs separate consideration for contaminant runoff, which this calculator doesn't address.

Should I use my roof's footprint or its actual sloped area for catchment?

Use the flat, horizontal footprint (the same area your gutters drain). Rainfall is measured as a vertical depth over a horizontal area, so a pitched roof's larger sloped surface area isn't the relevant catchment figure here.

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