Enrichment · Passion Projects · Home and garden

Every inch of rain,
counted in gallons
and in a tank.

Type the roof footprint, the monthly rainfall and the tank. The estimator works out what the roof sheds, then runs the tank month by month so you see when it spills and when the garden goes short.

Estimate the yieldWhat you get

The rainwater estimator

The worked example is made up: a 1,500 sq ft shingle roof, a 1,000-gallon tank and 2,800 gallons a month for the garden and toilets. Nothing you type leaves this browser.

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What you get

Enrichment · Passion ProjectsFree

What you finish with
Gallons collected a year, the tank month by month (in, used, overflow, left), the share of your demand met and notes on tank size. It prints as a summary and downloads as a spreadsheet.
Who it is for
Families planning a rain barrel or tank, gardeners, school garden clubs and students doing environmental science projects.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The Passion Projects space in Caveat Enrichment.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Measure the roof footprint under the eaves and pick its surface.
  2. Paste twelve monthly rainfall normals for your area, or one yearly total.
  3. Type the water you plan to use each month and the tank size.
  4. Read the yearly yield and the month-by-month tank table.
  5. Try a bigger or smaller tank and print the one that fits.

Estimate the yield

How the estimate works

Area times depth, then a tank that can only hold so much.

The worked example: a 1,500 sq ft shingle roofOne inch on one sq ft0.623 gal144 cu in ÷ 231× 1,500 sq ft934.5 galper inch of rain× 85% collected794 galper inch, after losses× 43.7 in a year34,712 galinto the tankThe worked example: a 1,500 sq ft shingle roofOne inch on one sq ft0.623 gal144 cu in ÷ 231× 1,500 sq ft934.5 galper inch of rain× 85% collected794 galper inch, after losses× 43.7 in a year34,712 galinto the tank
A roof turns every inch of rain into a fixed number of gallons. Whether you can use them depends on the tank: a full tank in a wet month spills.

The arithmetic

One inch of rain over one square foot is 144 cubic inches of water, and a US gallon is 231 cubic inches, so each square foot sheds 0.623 gallons per inch. Use the roof’s footprint, because rain falls vertically: a steep roof catches no more than a flat one over the same ground. Not every drop arrives; splash, evaporation and the first flush that washes the roof are lost, so the page applies a collection efficiency of 75 to 90 percent.

Why month by month

A yearly total hides the problem gardeners actually have: wet months fill the tank and spill, dry months empty it. The table starts with an empty tank in January, adds each month’s rain, takes out your use, and lets anything above the tank size overflow. Collected rain is for non-drinking uses unless it is properly treated, and some places regulate collection, so check local rules first.

Sources

  1. Texas Water Development Board. (2005). The Texas Manual on Rainwater Harvesting (3rd ed.): yield of 0.623 gallons per square foot per inch and collection efficiency (twdb.texas.gov).
  2. U.S. Environmental Protection Agency. Soak up the rain: rain barrels (epa.gov).
  3. National Oceanic and Atmospheric Administration, National Centers for Environmental Information. U.S. climate normals (ncei.noaa.gov).
  4. National Institute of Standards and Technology. Handbook 44, appendix C: the US gallon of 231 cubic inches (nist.gov).

The worked example is made up. The estimate is for planning; local rainfall varies from year to year. Your figures stay in this browser. Support: team@caveatacademic.com.