Rain Barrel Calculator

From roof rainfall to water in storage

The calculator works internally in square metres, millimetres and litres. The key identity is simple: 1 mm of rain on 1 m² of horizontal area equals 1 litre of water. Use only the projected roof area that drains to the selected barrels. Roof pitch increases cladding surface area, but it does not increase the horizontal footprint on which rain falls.

Let A be connected catchment area in m², P the rainfall event in mm, E the collection efficiency as a decimal, and F a fixed diversion in litres:

gross rain = A × P

collectible rain = max(0, gross rain × E − F)

The efficiency is a visible assumption, not a rating of the roof. It can represent proportional losses from splash, wetting, leaks and imperfect conveyance. A separate first-flush value is subtracted once after those proportional losses. If your efficiency already includes first-flush losses, leave the fixed diversion at zero to avoid double counting.

US customary inputs use exact conversions. One square foot is 0.09290304 m², one inch is 25.4 mm, and one US gallon is 3.785411784 L. This is equivalent to about 0.6233766 US gal per ft² per inch before losses. “US gal” is stated explicitly because an Imperial gallon is different.

Storage is a separate limit

Potential collectible rain is not the same as water captured. Let B be one barrel’s capacity, N the barrel count and W the amount already stored:

total capacity = B × N

free capacity = total capacity − W

captured = min(collectible rain, free capacity)

overflow = collectible rain − captured

storage after event = W + captured

The event-capture percentage compares captured water with collectible water. It is not shown when collectible yield is zero, because there is no event yield to divide by. The fill percentage compares ending storage with installed capacity. These two percentages answer different questions and should not be interchanged.

Worked metric example

A 100 m² connected roof receives 10 mm of rain. At 85% efficiency with no fixed diversion, gross rain is 100 × 10 = 1,000 L and collectible rain is 850 L. Two 200 L barrels provide 400 L of capacity. If 50 L is already stored, only 350 L is free, so the event captures 350 L, overflows 500 L and ends with 400 L stored. At an entered use of 25 L/day, the ending storage represents 16 days with no more rain; the newly captured portion represents 14 days.

Worked US example

A 1,000 ft² catchment receives 1 inch of rain. Gross yield is about 623.38 US gal. At 80% efficiency and after a 10 US gal fixed diversion, collectible yield is about 488.70 US gal. Two 55 US gal barrels holding 20 US gal initially have 90 US gal free. They capture 90 US gal, overflow about 398.70 US gal, and finish full at 110 US gal.

Use the estimate safely

• Treat harvested roof water as non-potable by default. Roof runoff may contain microbes, metals, chemicals, animal waste and debris. Drinking or indoor connections require appropriate local assessment, treatment and backflow separation.
• Water is heavy: a full 55 US gal barrel can weigh more than 450 lb (about 204 kg) including the container. Use a rated, covered container on a stable base and obtain structural advice for large or elevated storage.
• Screen openings, control mosquitoes, maintain gutters and first-flush equipment, and route overflow away from foundations, neighbouring property and unsafe discharge points.
• This event balance does not model rainfall timing, gutter or downspout flow capacity, wind, withdrawals during the storm, evaporation, leaks, freezing, irrigation demand or soil infiltration.
• Check local rules for rainwater harvesting, plumbing, mosquito control, overflow and discharge. A calculated barrel count is a volume comparison, not approval of a system.

The volume relationship and projected-area method are described by the Australian Bureau of Meteorology. Collection losses and efficiency are discussed in the US EPA environmental-benefits method. For health and system-separation precautions, see the CDC rainwater guidance.