Work out how much irrigation water a field, bed or lawn needs. Enter a watering depth and area, or crop ET minus rainfall, and get the volume in m³, litres and gallons, plus watering time.
Enter the area and either a set application depth, or the crop’s daily water use (ET) minus rainfall. One millimetre of water over one hectare is 10 m³ (10,000 litres).
Volume (m³) = depth(mm) × area(ha) × 10. With ET mode, net depth = (ET − rainfall) ÷ efficiency, so a less efficient system needs more water to deliver the same to the crop. Watering time = volume ÷ flow rate.
How to Calculate Irrigation Water Requirement
Multiply the depth of water needed by the area. Since 1 mm over 1 m² is 1 litre, a 25 mm watering over 100 m² needs 2,500 litres (2.5 m³, about 660 US gallons). At a flow of 20 litres per minute, that takes 125 minutes to apply.
| Depth over 100 m² | Litres | US gallons |
|---|---|---|
| 5 mm | 500 | 132 |
| 10 mm | 1,000 | 264 |
| 25 mm | 2,500 | 660 |
| 50 mm | 5,000 | 1,321 |
- Depth: 30 mm ET minus 5 mm rain = 25 mm.
- Volume: 25 mm × 100 m² = 2,500 litres.
- Run time: 2,500 ÷ 20 L/min = 125 minutes.
How Many Gallons to Water a Lawn per Square Foot
About 0.62 US gallons per inch of water. So irrigating a 1,000 sq ft lawn takes roughly 623 gallons per inch.
Turning the Result into a Watering Plan
The volume on screen is the water the root zone needs, not necessarily the water you have to pump. A few adjustments bring it closer to what actually happens in the field.
Allow for application efficiency. Some water evaporates, drifts, runs off or drains below the roots. Drip generally loses less than sprinklers, and surface flooding usually loses the most. Divide the net requirement by your system’s efficiency as a decimal to get the gross amount; your installer can give a realistic figure.
Count only effective rainfall. Heavy rain on dry ground partly runs off, and a light shower may evaporate almost at once. A rain gauge on site beats a regional forecast.
Before relying on a fixed schedule, consider the soil and crop:
- Sandy soils hold little water, so they need smaller, more frequent applications. Clay soils hold more but take water in slowly, so long runs can puddle and run off.
- Crop water use changes through the season, rising as the canopy fills out and usually falling as the crop matures.
- Watering in the early morning or evening cuts evaporation losses compared with midday.
- A soil moisture probe, or simply digging down with a trowel, tells you whether water is reaching the roots.
How to Calculate Irrigation Water Usage From Your Flow Rate
Multiply your real flow rate by the run time, and measure that flow rather than trusting the pump rating. Time how long it takes to fill a container of known size, or read a water meter before and after a run. Long pipes, uphill runs and worn nozzles all reduce delivery, which is why a sprinkler water usage calculator is only as good as the flow figure you give it. Rerun the numbers whenever your crop stage, weather or equipment changes. The evapotranspiration article on Wikipedia explains the crop water use figure behind the estimate.