Rainwater Harvesting Calculator
One thousand square feet, 1 inch of rain, and an example efficiency of 0.8 collect 498.40 gallons.
Last updated . Efficiency opens at zero point eight. Demand starts blank.
A rainwater harvesting calculator for collected gallons and a tank. The gallon product uses 0.623 from a university extension bulletin. Efficiency is a decimal you can replace. The tank is the smaller of supply and the demand you type.
Pond gallons from a mean depth: pond calculator. The leaders that bring the roof water down: gutter calculator.
Collected gallons and tank
0.623 is the factor printed in Arizona Cooperative Extension AZ1565. The 0.8 in the efficiency box is an example decimal, not that bulletin’s “about 90 percent.” The period menu only names the stretch of time your rainfall and demand describe. It does not split a year into months.
Results
The extension formula, then a tank
University of Arizona Cooperative Extension publication AZ1565, October 2012, by Cado Daily and Cyndi Wilkins, prints this product: square feet of catchment × inches of rain × 0.623 × efficiency = gallons. Their tip line is that 1,000 square feet of roof equals 623 gallons for each inch of rain, which is 1,000 × 0.623 with efficiency left out. They describe efficiency as approximately 90 percent for most roofs. The box on this page opens at 0.8, marked as an example, so the starter answer is 1,000 × 1 × 0.623 × 0.8 = 498.40 gallons. Type 0.9 when you want their approximate roof figure: 560.70 gallons from that same inch.
The bulletin’s own annual illustration multiplies 1,000 × 15 × 0.623 × 0.90 and then writes 8,140. Those four factors multiply to 8,410.50, so this page reports 8,410.50 and does not repeat the printed 8,140. The catchment in that bulletin is the horizontal roof area. A sloped surface is larger and is not the input. Monthly and annual use the same product. You type the rainfall and the demand for the period you selected. The form does not assign a percent of the year to each month.
AZ1565 says to consider a supply and demand water budget. It does not print a tank volume. This page’s tank line is the smaller of the collected gallons and the demand you type. Demand opens blank, and the tank line then reads Enter a demand while the gallons stay. At 498.40 gallons collected, a demand of 400 leaves a surplus of 98.40. A demand of 600 leaves a shortfall of 101.60, and the tank stays at 498.40. The PDF is extension.arizona.edu.
Worked catchment
| Catchment | Rain | Efficiency | Collected | Demand | Tank |
|---|---|---|---|---|---|
| 1,000 ft² | 1 in | 0.8 | 498.40 gal | blank | Enter a demand |
| 1,000 ft² | 1 in | 0.8 | 498.40 gal | 400 gal | 400.00 gal, surplus 98.40 |
| 1,000 ft² | 1 in | 0.9 | 560.70 gal | blank | Enter a demand |
| 1,000 ft² | 15 in | 0.9 | 8,410.50 gal | blank | Enter a demand |
Rainwater questions
How many gallons is 1,000 square feet at 1 inch and 0.8?
Leave the catchment at 1,000 ft², rainfall at 1 in, and efficiency at 0.8. Leave demand blank. Collected reads 498.40 gal · 1,886.65 L. Tank reads Enter a demand. Balance reads —. The 0.8 is an example decimal, and you are meant to type over it.
How big is the tank if demand is 400 gallons?
Leave the opening catchment, the 1 inch, and efficiency 0.8. Type 400 in the demand box. Collected stays 498.40 gal · 1,886.65 L. Tank reads 400.00 gal · 1,514.16 L. Balance reads Surplus 98.40 gal. The tank is the smaller of what fell and what you plan to use.
What if demand is 600 gallons?
Type 600 instead of 400, with the same 1,000 ft², 1 inch, and 0.8. Tank reads 498.40 gal · 1,886.65 L. Balance reads Shortfall 101.60 gal. The tank does not grow past the water the roof produced.
What does the bulletin’s 90 percent efficiency collect?
Set efficiency to 0.9 and leave 1,000 ft² and 1 inch. Collected reads 560.70 gal · 2,122.48 L. Arizona Cooperative Extension AZ1565 says efficiency is approximately 90 percent for most roofs. This form opens at 0.8 on purpose, as a labeled example, and 0.9 is what you type when you want their figure.
Why is the bulletin’s 15 inch example not 8,140?
AZ1565, October 2012, “Basic Components of a Rainwater Storage System,” by Cado Daily and Cyndi Wilkins, prints 1,000 × 15 × 0.623 × 0.90 and then 8,140. The product of those four numbers is 8,410.50. Set rainfall to 15 in, efficiency to 0.9, and the period to Annual. Collected reads 8,410.50 gal · 31,837.21 L. This page shows that product. The PDF is extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1565.pdf.
Does monthly versus annual change the formula?
No. Gallons = catchment square feet × rainfall inches × 0.623 × efficiency, for whichever period you meant when you typed the rainfall and the demand. The menu labels that period. It does not invent a monthly share of a yearly total. The catchment is the roof footprint, the same horizontal area the bulletin uses, not the sloped surface.
Where does 0.623 come from?
AZ1565 prints square feet × inches of rain × 0.623 × efficiency = gallons, and its tip says 1,000 square feet of roof equals 623 gallons per 1 inch of rain. That tip is 1,000 × 0.623. The form uses 0.623, the factor they printed, and not a longer decimal. Their tank advice is to budget supply against demand. The lesser-of tank line on this form is this page’s rule, not a second formula from the bulletin.