Rain Barrel Calculator Formula

Understand the math behind the rain barrel calculator. Each variable explained with a worked example.

Use the Rain Barrel Calculator

Formulas Used

Gallons Collected

gallons_collected = roof_sqft * rainfall_inches * 0.623 * collection_efficiency / 100

Standard Barrels Needed (55 gal)

barrels_needed = ceil(roof_sqft * rainfall_inches * 0.623 * collection_efficiency / 100 / 55)

Liters Collected

liters_collected = roof_sqft * rainfall_inches * 0.623 * collection_efficiency / 100 * 3.785

Variables

VariableDescriptionDefault
roof_sqftRoof Catchment Area(sq ft)1500
rainfall_inchesRainfall Per Event(in)1
collection_efficiencyCollection Efficiency(%)80

How It Works

How Rain Collection Is Calculated

One inch of rain on 1 square foot of roof produces about 0.623 gallons of water.

Formula

Gallons = Roof Area (sq ft) x Rainfall (in) x 0.623 x Efficiency

Efficiency accounts for losses from gutters, splashing, and first-flush diverters.

Worked Example

1,500 sq ft roof, 1 inch of rain, 80% collection efficiency.

roof_sqft = 1500rainfall_inches = 1collection_efficiency = 80
  1. 01Raw collection = 1,500 x 1 x 0.623 = 934.5 gallons
  2. 02With 80% efficiency = 934.5 x 0.80 = 747.6 gallons
  3. 03Standard barrels = ceil(748 / 55) = 14 barrels

Frequently Asked Questions

Is rainwater collection legal?

Laws vary by state/country. Most US states allow rainwater collection, some with permits. Check local regulations before installing a system.

What can I use collected rainwater for?

Primarily garden watering, lawn irrigation, and car washing. With proper filtration and treatment, it can be used for non-potable household use.

How much rain does an inch represent?

One inch of rain on a 1,000 sq ft roof yields about 623 gallons. A typical rainstorm of 0.5 inches on a 1,500 sq ft roof gives about 467 gallons.

Ready to run the numbers?

Open Rain Barrel Calculator