Irrigation Water Calculator
Water volume in gallons and acre-feet to apply a depth over a field — and the cost
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Quick summary
Enter acreage, the depth of water to apply (in inches), and your water cost to get total gallons, acre-feet, and dollars. One acre-inch — a 1-inch depth over one acre — is 27,154 gallons, so applying 1 inch over 40 acres takes about 1.09 million gallons, or 3.3 acre-feet, costing roughly $2,715 at $2.50 per thousand gallons. This is water delivered to the field; add for system inefficiency to size actual pumping.
Reviewed by Simple Calculators TeamLast reviewed
How to use this calculator
- Enter the inputs requested by the calculator.
- Choose the units or options that match your situation and review the assumptions.
- Run or update the calculation to see the result.
- Review the result, breakdown, and formulas shown on the page.
- Check the inputs, units, dates, and limitations before saving or sharing the result.
Frequently asked questions
How many gallons is an acre-inch of water?
27,154 gallons. That's the volume to cover one acre to a depth of one inch. Twelve acre-inches make one acre-foot (325,851 gallons). This calculator scales it to your acreage and application depth.
How do I calculate irrigation water needs?
Multiply acres × inches to apply × 27,154 gallons per acre-inch. For 40 acres at 1 inch, that's 40 × 27,154 = 1,086,160 gallons. Divide inches by 12 and multiply by acres for acre-feet. This tool returns gallons, acre-feet, and cost together.
What is an acre-foot?
An acre-foot is the water covering one acre to a depth of one foot — 325,851 gallons, or about 12 acre-inches. It's the standard unit for large water volumes and water rights. The calculator shows your total in acre-feet as well as gallons.
Should I account for irrigation efficiency?
Yes. The figures here are water delivered to the crop's root zone. Systems lose water to evaporation, runoff, and deep percolation, so you must pump more than this. Drip is ~90% efficient, sprinklers ~70–85%, and flood/furrow less — divide by your system efficiency to get the actual amount to supply.
Key terms
- Acre-Inch
- Water covering one acre one inch deep — 27,154 gallons.
- Acre-Foot
- Water covering one acre one foot deep — 325,851 gallons.
- Irrigation Efficiency
- Share of applied water that reaches the crop; the rest is lost.
Methodology, sources & limitations
This page shows the formula, assumptions, and limitations used for this estimate. Rates, thresholds, and other time-sensitive inputs can change, so verify them against the relevant official source before relying on a result.
For our review approach and the primary sources we consult, see the methodology and source policy. If you spot an outdated figure or formula, please contact us with a correction.