Rain Barrel Calculator: Water per Inch of Rain
How much water can a roof collect? Enter your roof catchment area, a rainfall depth and a collection efficiency to see how much water you can capture and how many barrels hold it. The 80 percent efficiency is a typical allowance for splash, first flush and overflow; adjust it for your gutters.
Updated
Report a problem
Formula
- Imperial:
gallons = roof area (sq ft) x rainfall (in) x 0.623377 x efficiency - Metric:
litres = roof area (m2) x rainfall (mm) x efficiency(1 mm on 1 m2 is 1 litre) - Barrels needed = water collected / barrel size, rounded up
0.623377 gallons per square foot per inch is 144 cubic inches / 231 cubic inches per US gallon.
Worked example
A 1,000 sq ft roof, 1 inch of rain, 80 percent efficiency, 55 gallon barrels:
- 1,000 x 1 x 0.623377 x 0.80 = 498.7 gallons
- 498.7 / 55 = 9.07, so 10 barrels
Metric: 50 m2, 20 mm, 85 percent, 200 L barrels: 50 x 20 x 0.85 = 850 L, and 850 / 200 = 4.25, so 5 barrels.
When to use it
Use it to decide how many barrels to install, to see what one storm adds, or to compare the water you could collect with what your garden uses. A useful rule is about 0.62 gallons per square foot of roof for each inch of rain, before losses.
Common mistakes
- Using the sloped surface of the roof. Use the horizontal footprint.
- Forgetting that barrels overflow when full, so a big storm does not store more than the barrels hold.
- Using 100 percent efficiency. Typical values are about 75 to 90 percent.
- Mixing inches with millimetres or square feet with square metres.
How it differs from nearby calculators
The lawn watering calculator and irrigation flow calculator work out how much water your plants need and how fast it flows. This page works out the supply side: how much a roof can collect. The pond volume calculator sizes a water feature, not a roof.
Assumptions
- Efficiency of 80 percent is a typical allowance, not a measured value.
- Only the rain of one event is counted.
Frequently asked questions
How many gallons does a roof collect per inch of rain?
About 0.623 gallons per square foot per inch of rain, before losses. A 1,000 sq ft roof collects 1,000 x 1 x 0.623 = about 623 gallons from 1 inch of rain, or about 498 gallons at 80 percent efficiency.
How do I calculate rainwater harvest in litres?
Multiply roof area in square metres by rainfall in millimetres, because 1 mm on 1 m2 is exactly 1 litre. For 50 m2 and 20 mm that is 1,000 L, or 850 L at 85 percent efficiency.
Why is collection efficiency less than 100 percent?
Some water is lost to evaporation, splash, wind, leaks, overflow when barrels are full, and a first flush that is diverted to clean the roof. Typical values are about 75 to 90 percent; use a lower figure for flat or dirty roofs.
How many rain barrels do I need?
Divide the water collected by the barrel size and round up. At 498.7 gallons per inch of rain with 55 gallon barrels, 498.7 / 55 = 9.07, so 10 barrels hold one full inch. Many people store less and let the overflow drain away, so size for the storm you want to capture.
How many gallons from half an inch of rain on a 1,500 square foot roof?
Use 1,500 x 0.5 x 0.623377 = 467.5 gallons before losses. At the default 80 percent efficiency you collect 374 gallons, which fills 6.8 barrels of 55 gallons, so plan for 7.
How many 55 gallon barrels for a 2,000 square foot roof and 1 inch of rain?
A 2,000 sq ft roof collects 2,000 x 0.623377 = 1,246.8 gallons per inch, or 997.4 gallons at 80 percent. Divided by 55 that is 18.13, so you need 19 barrels to store the whole inch.
How many litres from 10 mm of rain on an 80 square metre roof?
80 x 10 = 800 L before losses, because 1 mm on 1 m2 is 1 litre. At 85 percent efficiency you collect 680 L, which fits in 4 barrels of 200 L.