Find how much water to irrigate with. This free irrigation water calculator turns a watering depth and area — or crop ET minus rainfall — into volume in m³, litres and gallons, plus watering time.
Irrigation
mm
mm
mm
%
m³/h
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).
Water needed
—
Enter area & depth
Net depth
—
Volume (litres)
—
US gallons
—
Watering time
—
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.
Use this free irrigation water calculator to find how much water your field needs. Enter the area and a watering depth — or crop water use minus rainfall — to get the volume and watering time.
What this irrigation water calculator does
An irrigation water calculator converts a watering depth over an area into a water volume in cubic metres, litres and gallons — and, with a flow rate, the time to apply it.
How irrigation water is calculated
volume (m³) = depth (mm) × area (ha) × 10
ET mode: net depth = (ET − rainfall) ÷ efficiency
watering time = volume ÷ flow rate
So 25 mm over 2 hectares is 500 m³ (500,000 litres) — about 25 hours at a 20 m³/h flow.
Handy water conversions
Quantity
Equals
1 mm over 1 ha
10 m³ (10,000 L)
1 mm over 1 m²
1 litre
1 m³
1,000 L (≈ 264 US gal)
Crop need and efficiency
In ET mode, the net depth is the crop’s water use (evapotranspiration) minus effective rainfall, divided by your system efficiency. A drip system (high efficiency) wastes less than overhead sprinklers, so it needs less applied water for the same crop benefit.
How to use the irrigation calculator
Enter the area and units.
Choose a depth, or ET minus rainfall with efficiency.
Add a flow rate for the watering time.
Irrigation Water FAQ
How do I calculate irrigation water?
Multiply depth in mm by area in hectares by 10 for cubic metres. 25 mm over 2 hectares is 500 m³ (500,000 litres).
How much is 1 mm of water?
One millimetre of water is 10 m³ per hectare, or 1 litre per square metre — a simple way to convert depth to volume.
What is ET in irrigation?
Evapotranspiration (ET) is the crop's daily water use. Net irrigation need is ET minus effective rainfall, adjusted for system efficiency.
How does system efficiency change the amount?
Lower efficiency means more water is lost, so you must apply more to deliver the same to the crop. Drip systems are more efficient than sprinklers.
How long will watering take?
Divide the volume by your flow rate. 500 m³ at 20 m³/h takes about 25 hours.
Is the irrigation water calculator free?
Yes, this irrigation water calculator is completely free, needs no sign-up, and gives instant results in your browser.
