Irrigation Water Calculator
Find the water needed for an area and a depth, and how many hours the pump has to run.
Part of the 🌾 Farmers toolkit, with more tools made for the job.
The share of the water that reaches the crop. Leave empty for 100%.
Gives the pumping time.
Type the area and the depth of water, and the volume you need appears here.
One millimetre of water on one hectare is 10 cubic metres (10,000 litres). How much your crop needs depends on the crop, the soil and the weather.
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Type the area, the depth of water you want to apply and, if you know it, the efficiency and the pump flow. This gives the volume in cubic metres and litres, or acre-inches and gallons, and the hours of pumping.
One millimetre of water on one hectare is 10 cubic metres, which is 10,000 litres. That is why 30 mm over 20 hectares is 6,000 cubic metres before you allow for any losses.
How it’s worked out
- Metric: volume in cubic metres is the area in hectares times the depth in millimetres times 10.
- Imperial: acre-inches is the area in acres times the depth in inches. One acre-inch is about 27,154 US gallons.
- Efficiency raises the amount you must apply: volume divided by the efficiency as a fraction. Pumping hours are the volume divided by the pump flow.
Applying 30 mm to 20 hectares is 6,000 cubic metres. At 75% efficiency you need 8,000 cubic metres. A pump giving 100 cubic metres an hour takes 80 hours.
Common mistakes
- Ignoring losses. Evaporation, run-off and uneven spread mean you have to apply more than the crop receives.
- Using the pump's rated flow when it is old or the lift is high. Measure the real flow if you can.
- Mixing millimetres with inches, or hectares with acres.
- Confusing the depth you apply with the depth the roots need. Soil type and rainfall change how much to apply.
Questions
How much water does my crop need?
This tool does not say. That depends on the crop, the stage, the soil and the weather. Use local irrigation advice, then use this to turn the depth into volume and pumping time.
What is application efficiency?
The share of the water you pump that actually reaches the crop. It is lower for flood irrigation and higher for drip. Leave it empty to use 100%.
What is an acre-inch?
The amount of water that covers one acre to a depth of one inch. It is about 27,154 US gallons, or 102.8 cubic metres.
Where the method comes from
Estimates only
These are estimates and general information, not agronomic, financial or professional advice. Rates, timings and costs depend on your crop, land, weather and local rules.
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