Calculators / Rainwater and drainage
How much rainwater can you collect?
Look up your rainfall by ZIP, then put in the roof area that drains to your barrel. You get the gallons a year, the gallons in one inch of rain, and how many barrels that fills.
Updated · How we check our numbers
You can collect
9,490 gallons a year
A 500 sq ft asphalt shingle roof sheds about 237 gallons in a 1 inch rain. Your 110 gallons fill in 0.46 in of rain, so plan the overflow.
- A year
- 9,490 gal40 in of rain
- Per inch of rain
- 237 gal
- First flush
- 5 galdivert before storage
- Your storage fills at
- 0.46 inof rain, 110 gal
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Estimate for planning only, not professional advice. Check the manufacturer’s instructions, your local code and a licensed professional before you buy, build or install. Use at your own risk. Terms.
Is it legal in your state?Rainwater rules for all 50 states and DC
Size a rain garden for the overflow500 sq ft of roof
per inch of rain = 500 sq ft × 0.62 gal × 77% = 237 gal
a year = 237 gal × 40 in = 9,490 gal
first flush to divert = 500 ÷ 1,000 × 10 gal = 5 gal
55 gal barrels to hold a 1 in storm = 237 ÷ 55 = 5, rounded up
rain that fills 110 gal = 110 ÷ 237 = 0.46 in
Barrels fill fast. Run the overflow away from the house foundation, or into a second barrel, a rain garden or a dry well. Keep it off septic drain fields.
Keep the lid tight and screen every opening with mesh finer than a mosquito, as the CDC advises.
See rain barrel screens on Amazon (opens Amazon in a new tab)The first water off the roof carries dust, pollen and droppings. A first flush diverter sends about 5 gallons aside before the barrel fills.
See first flush diverters on Amazon (opens Amazon in a new tab)Asphalt shingles can leach into the water. Texas says shingle roofs suit irrigation only; Oregon State says it's fine around plants. Water the soil, not the leaves, and never drink it.
Colorado allows two barrels holding 110 gallons in all. Utah allows two containers of up to 100 gallons each without registering, and up to 2,500 gallons once registered. Check your state before buying a big tank.
How we work it out
One inch of rain on one square foot of roof is 1/12 of a cubic foot, about 0.62 gallons. Not all of it reaches the barrel: some splashes out, evaporates or goes to the first flush. The Texas Water Development Board's rainwater manual uses 85% collection efficiency, and shingle and tile roofs lose about 10% more than metal.
gallons = roof sq ft × inches of rain × 0.62 × 85% (× 90% for shingle or tile)
Measure the roof's footprint, the area it covers seen from above, not the sloped surface. Rain falls straight down, so a steep roof catches no more than a flat one of the same footprint. Only count the part that drains to your downspout.
First flush. The first water off the roof carries dust, pollen and droppings. The Texas manual's rule of thumb is to divert at least 10 gallons for every 1,000 sq ft of roof before the water goes into storage.
Rain barrels. Most are 50 to 75 gallons, often 55. Link two or more with the overflow port, or a single 1 in rain on a small roof will overfill one. Cover them tight and screen every opening against mosquitoes.
Rainfall comes from NOAA's 1991 to 2020 climate normals at the station nearest your ZIP. Snow counts as the water it melts to. Your actual year will be wetter or drier.
Where the numbers come from
Rainwater and drainage
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