Water Formulas 

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Click here for an interactive Excel spreadsheet (XLS) for water use calculations. (Use your browser's "BACK" button to return to this page if you need to. You need not save changes to the spreadsheet when leaving.)  If you don't have Microsoft Excel, or simply want a bit more of a mathematical challenge, you can use the formulas below for these same calculations.

Calculating annual acre feet (AF) pumped (2 calculations)

The easiest way to calculate total acre feet used in a year is to multiply the gallons per minute rate you pumped the well, times the number of hours the well was pumped for the year, times the numerical factor .0001841.

The Formula is: (RATE) X (HOURS) X (.0001841)
Example: (600 gpm) X (1500 Hours) X (.0001841) = 165.7 AF per year.

An alternative calculation for the same answer is: multiply gallons per minute (GPM) X 60 (yielding gallons per hour (GPH)), then multiply GPH X hours pumped for the year (yielding Gallons per Year (GPY)), and finally dividing GPY by 325,851 (number of gallons in an AF)

The Formula is: (GPM) X (60) X (HOURS) / 325,851 (remember: do all multiplication first, then the division)
Example: (600 GPM) X (60) X (1500) / 325,851 = 165.7 AF per year.

Calculating hours to pump a known ammount

Figuring HOURS to pump a known amount of water at a known rate of diversion:

The question would be posed: "How many hours will it take me to pump 200 AF at 600 GPM?"


Example: 65,170,200 / 36,000 = 1,810 hours

The trick to this calculation is converting AF into GALLONS, and GPM into GPH. Remember that 1 AF = 325,851 Gal, so 200 AF = 200 X 325,851, or 65,170,200 GAL. Also, 600 GPM = 600 X 60, or 36,000 GPH.

Helpful Conversion

Some conversions for water are:

1 Acre foot = 325,851 gal
1 cubic foot of water = 7.48 Gal = 62.4 Lbs
1 Gal of water weighs 8.33 Lbs
1 Acre foot = 12 Acre inches
1 Acre inch = 27,154 Gal
1 Acre inch per Hour = 449 GPM = 1 cubic foot per second (CFS)