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Rain Barrel Water Collection Calculator

Estimate how many liters or gallons of rainwater your roof can collect from a given rainfall.

Result

1,275 L
336.8 gal

How it is calculated

One millimetre of rain falling on one square metre is exactly one litre, so the harvest is litres = roof area (m²) × rainfall (mm) × efficiency ÷ 100. The efficiency (runoff coefficient) is the percentage of rain that actually reaches the barrel after losses to splash, evaporation, first flush and overflow. US gallons come from gallons = litres × 0.264172.

Use the footprint area the roof covers on the ground, in square metres, and the rainfall depth in millimetres for the event or period you care about. Area and rainfall must be above zero. If you leave efficiency empty or set it to 0 or less, the tool assumes 90%. An efficiency above 100 gives no result, since it is a percentage. Litres are rounded to whole numbers and gallons to one decimal. The result is how much water the roof could deliver, not how much your barrel can hold.

Worked example

A shed roof covers 60 m² and a storm drops 25 mm of rain. With an efficiency of 85%, litres = 60 × 25 × 0.85 = 1,275 L. In US gallons: 1,275 × 0.264172 = 336.8 gal. Without an efficiency entered, the default of 90% would give 60 × 25 × 0.90 = 1,350 L, or 356.6 gal. So a typical 200-litre barrel would overflow after about the first 4 mm of that storm.

Try this example →

Common mistakes

  • Entering the area in square feet. The tool expects square metres, so a 650 sq ft roof is about 60 m².
  • Entering rainfall in inches or centimetres. The field is in millimetres: 1 inch is 25.4 mm and 1 cm is 10 mm.
  • Typing the efficiency as 0.85 instead of 85. A value between 0 and 1 is read as 0.85%, so the example drops from 1,275 L to 13 L.
  • Using the sloped surface area of the roof instead of the area it covers on the ground. Rain is collected over the footprint.
  • Treating the volume as what you will actually store. A barrel's capacity, first-flush diverters and overflow cap the real amount.

About this calculator

Rainwater harvesting potential from a roof follows a straightforward relationship: Collected volume = Roof area × Rainfall depth × Runoff coefficient, where the runoff coefficient (typically 0.8–0.9 for standard roofing) accounts for the small fraction of rain lost to evaporation, absorption, and splash-off rather than reaching the gutter.

This is the same basic calculation used in professional rainwater harvesting system design and stormwater management, where a roof's catchment area and local rainfall data determine tank sizing, and municipal stormwater engineers use nearly the same math in reverse to estimate runoff volumes that drainage systems need to handle.

Homeowners sizing a rain barrel or cistern, gardeners planning irrigation from harvested water, and sustainability-minded builders estimating a property's water-collection potential all use this formula to translate roof size and local weather into a concrete number of liters or gallons.

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Comments (1)

  • Editorial team

    Try a 50 m² roof with 20 mm of rain at 80 percent efficiency: 800 L, or 211.3 gal. If your barrel holds 200 L, that single shower already fills four of them, so the question is often where the overflow goes rather than how much you collect. Leaving the efficiency blank or at zero makes the tool assume 90 percent, which is optimistic for a dirty or rough roof. What size barrel are you planning around?

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