Rainwater Harvest Calculator

Try this rainwater harvesting calculator that helps determine the potential volume of rainwater that can be collected from a catchment area, typically a roof surface.

A home with a 2,000 square foot roof in an area receiving 30 inches of annual rainfall.

The calculator would factor in:

Roof surface area: 2,000 sq ft
Annual rainfall: 30 inches
Runoff coefficient: 0.8 (for standard sloped roof)
System efficiency: 0.85 (accounting for losses)

Roof Area (sq ft)	Annual Rainfall (inches)	Efficiency Factor	Annual Collection (gallons)
1,000	20	0.85	10,591
1,500	25	0.85	19,856
2,000	30	0.85	31,768
2,500	35	0.85	46,336

Rainwater Harvesting Calculation Formula

The formula for calculating rainwater harvesting potential is:

Volume of Water (gallons) = Catchment Area (sq ft) × Rainfall (inches) × 0.623 × Runoff Coefficient × System Efficiency

Where:

0.623 is the conversion factor to convert 1 inch of rainfall per square foot into gallons

Runoff coefficient typically ranges from 0.75 to 0.95

System efficiency usually ranges from 0.80 to 0.90

Volume = 2,000 sq ft × 30 inches × 0.623 × 0.8 × 0.85
Volume = 25,414 gallons annually

How do I calculate how much rainwater I will collect?

Follow these steps:

Measure your catchment area (usually roof surface)
Obtain local rainfall data (average annual rainfall)
Determine your system’s efficiency factors
Apply the formula: Volume of Water (gallons) = Catchment Area (sq ft) × Rainfall (inches) × 0.623 × Runoff Coefficient × System Efficiency

For a 1,500 sq ft roof with 25 inches annual rainfall:

Annual collection = 1,500 × 25 × 0.623 × 0.8 × 0.85
                  = 15,884 gallons
Monthly average = 15,884 ÷ 12 = 1,324 gallons

How many gallons is 1 inch of rain per square foot?

One inch of rain falling on one square foot of surface area yields approximately 0.623 gallons of water.

For a 100 sq ft area receiving 1 inch of rain:

Volume = 100 sq ft × 1 inch × 0.623
       = 62.3 gallons

How fast will a 50-gallon rain barrel fill up?

The filling rate of a rain barrel depends on:

Roof area draining to the barrel
Rainfall intensity
Gutter efficiency

For a 500 sq ft roof section during 1 inch/hour rainfall:

Filling rate = 500 sq ft × 1 inch × 0.623 × 0.8
             = 249.2 gallons per hour
A 50-gallon barrel would fill in: 50 ÷ 249.2 = 0.2 hours (12 minutes)

How to calculate rainwater collection from roof

To calculate roof collection potential:

Measure total roof area (length × width)
Account for roof pitch (multiply flat roof area by 1.05 for moderate pitch)
Apply the collection formula

For a 40′ × 30′ roof with moderate pitch:

Actual roof area = 40 × 30 × 1.05 = 1,260 sq ft
Monthly collection (4" rainfall) = 1,260 × 4 × 0.623 × 0.8 × 0.85
                                = 2,147 gallons

How much water does 1 inch of rain equal?

The water volume from 1 inch of rain depends on the collection area:

1 sq ft = 0.623 gallons
1,000 sq ft = 623 gallons
1 acre = 27,154 gallons

References

U.S. Environmental Protection Agency (EPA) – Water Management Resources https://www.epa.gov/green-infrastructure/manage-flood-risk
American Rainwater Catchment Systems Association (ARCSA) https://www.arcsa.org
Texas A&M AgriLife Extension – Rainwater Harvesting https://rainwaterharvesting.tamu.edu

Related Misc Tools:

Rainwater Harvest Calculator – Measure Rainwater Catchment