To apply a target depth of water (inches) over a field, convert to volume: an acre-inch is the water covering one acre one inch deep. Acres times inches gives acre-inches; ×27,154 gives gallons.
The example
50 acres, 1.5 inches.
| A | B | |
|---|---|---|
| 1 | Item | Value |
| 2 | 50 × 1.5 | 75 ac-in |
| 3 | Gallons | → 2,036,550 |
The formula
The formula:
How it works
How it works:
- One acre-inch = water one inch deep over one acre = 27,154 gallons.
- Multiply acres × inches for total acre-inches.
- Convert: ×27,154 for gallons, ÷12 for acre-feet.
- Divide gallons by flow rate (GPM) for the hours of pumping needed.
Inches of depth, not gallons, is how irrigation is planned. Crops need a depth of water per week (an inch or so), and rain is also measured in inches — so working in acre-inches lets you net rainfall against your irrigation target directly. Convert to gallons only at the end, to size pumps and runtime: gallons ÷ GPM ÷ 60 is hours of set time.
Try it: interactive demo
Acres and inches applied.
Variations
Gallons
Acre-inches × 27,154:
Acre-feet
Divide by 12:
Pumping hours
Gallons ÷ GPM ÷ 60:
Pitfalls & errors
Acre-inch = 27,154 gal. Use the constant for the gallon conversion.
Net out rainfall. Subtract rain inches from the target depth first.
Efficiency. Divide by system efficiency for gross water if you want applied-at-source.
Practice workbook
Frequently asked questions
How do I calculate irrigation water requirement in Excel?
What is an acre-inch?
How many hours of pumping is that?
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