Rain Barrel Calculator: Gallons from a Roof

This calculator estimates how much rainwater a roof can send to a rain barrel. Enter the roof area that drains to the downspout, the rainfall in inches, a collection efficiency and the barrel size; you get the gallons and litres collected and how many barrels they would fill.

Formula

Gallons = roof area (ft²) × rainfall (in) ÷ 12 × 7.4805 × efficiency ÷ 100. Dividing by 12 turns inches into feet; one cubic foot holds 1,728 ÷ 231 = 7.4805 US gallons. Barrels = gallons ÷ barrel size. Litres = gallons × 3.785.

• a: the roof’s plan (footprint) area that drains to this downspout, not the sloped surface
• r: rainfall depth in inches
• eff: share of the rain that reaches the barrel after splash, evaporation and gutter overflow
• bar: capacity of one barrel in gallons

Worked Example

1,000 ft² of roof, 1 in of rain, 80% efficiency, 55 gallon barrels.

1. Water = 1,000 × 1 ÷ 12 × 7.4805 × 0.80 = 498.70, about 499 gal
2. Barrels = 498.70 ÷ 55 = 9.07 barrels
3. Litres = 498.70 × 3.785 = 1,888 L

How to use the result

Even a small storm fills a single barrel quickly, so plan an overflow route away from the house foundation, or link several barrels. Use the water for gardens and lawns; local rules may control rainwater collection.

Limits and common mistakes

• Use the footprint area, measured on the ground under the roof, not the area of the sloped roof.
• Only the part of the roof that drains to this downspout counts.
• Untreated roof water is not drinking water.

Frequently Asked Questions

How much water comes off a roof in 1 inch of rain?

About 623 gallons per 1,000 ft² before losses (0.623 gal per ft² per inch).

How fast will a 55 gallon barrel fill?

From 1,000 ft² at 80%, in about 0.11 in of rain.

What efficiency should I use?

About 75-90% is common for a gutter and downspout system; use less for leaky gutters.

Is collecting rainwater allowed?

Rules differ by place; check yours.

in a | Roof area draining to the downspout (footprint) | ft² | 1000 | 0
in r | Rainfall (one storm or period) | in | 1 | 0 | 400
in eff | Collection efficiency | % | 80 | 1 | 100
in bar | Barrel size | gal | 55 | 1 | 10000
out gal | Rainwater collected | gal | a * r / 12 * 1728 / 231 * eff / 100 | 0
out n | Barrels filled | barrels | gal / bar | 2
out lit | Rainwater collected | L | gal * 3.785411784 | 0