How to use the rain barrel capacity calculator
- Enter the roof area that drains to your gutters, or choose Length and width and enter its dimensions in feet. Use the footprint seen from above, not the sloped surface.
- Enter the rainfall in inches that you want to plan for, from one storm or a month.
- Set the runoff factor, which is the share of the rain that actually reaches the barrel. The default of 90 percent is an example. If only part of the roof drains to this downspout, enter that share.
- Enter the barrel size and read the gallons collected and the barrels needed to hold them.
Formula
1 inch of rain on 1 sq ft = 144 cu in / 231 cu in per gallon = 0.6234 gallons
Gallons = catchment area x rainfall x 0.6234 x runoff factor
Catchment area = roof area x share draining to the barrel
Barrels = gallons / barrel size, rounded up
A square foot is 12 by 12 = 144 square inches, so one inch of water over it is 144 cubic inches. A US gallon is 231 cubic inches.
A 1,000 square foot roof and 1 inch of rain
With no losses the water is 1,000 x 1 x 0.6234 = 623.4 gallons. With the example runoff factor of 90 percent it is 561.0 gallons. A 55 gallon barrel holds a tenth of that, so 11 barrels of that size would hold all of it, and one barrel would overflow by about 506 gallons.
A 1,500 square foot roof in a 2.5 inch storm at 85 percent runoff gives 1,500 x 2.5 x 0.6234 x 0.85 = 1,987 gallons. If only half the roof drains to one downspout, that downspout sends half of what the whole roof would.
Where the number 0.623 comes from
Rainfall depth is a height, so the volume on a flat area is the area times the depth. One inch of rain on one square foot is a slab 12 inches by 12 inches by 1 inch, which is 144 cubic inches. A US gallon is exactly 231 cubic inches, so the slab holds 144 / 231 = 0.6234 gallons. That is the well-known figure of about 0.62 gallons per square foot per inch.
Roof slope does not change it, because rain falls vertically and the footprint seen from above is what catches it.
Why a runoff factor
Not all rain reaches the barrel. Some splashes over the gutter, some evaporates or soaks into the roof surface, and a first-flush diverter or a full barrel sends some away. The factor lets you account for that. It depends on your roof, your gutters and the storm, so treat 90 percent as an example and replace it with your own judgment.
The barrel needs an overflow that carries water away from the house, a cover, and a screen for debris. Check local rules on collecting rainwater before you install one, and do not drink the water. For gutter sizing see the gutter and downspout calculator, and for containers the tank volume calculator. The roofing calculator and the sprinkler run time calculator cover the roof and the watering.
Assumptions and limits
- The catchment area is the horizontal footprint of the roof, and rain falls evenly on it.
- The runoff factor of 90 percent is an example, not a measured value for any roof.
- The barrel starts empty, and the result is for one rainfall event.
- Evaporation from the barrel, first-flush diversion and local rules on collection are not modeled.
Frequently asked questions
How many gallons does a roof collect per inch of rain?
About 0.623 gallons per square foot for each inch of rain, before any losses. A 1,000 square foot roof gathers about 623 gallons from one inch.
How do I calculate rain barrel capacity needs?
Multiply the roof area that drains to the barrel by the rainfall in inches, by 0.623 and by a runoff factor, then divide by the size of your barrel and round up.
What runoff factor should I use?
The page starts at 90 percent only as an example. Your roofing material, gutters, first-flush diverter and the intensity of the storm all change it, so choose a value you can justify.
Does roof slope change the amount of water?
No. Use the footprint of the roof as seen from above, because that is the area that rain falls on. A steeper roof sheds water faster but does not catch more.