Calculate the total water volume needed to irrigate your field to a given depth, and how long it will take with your pump's discharge rate.
How to Use the Irrigation Calculator
Enter your "Land Area (acres)".
Enter the "Irrigation Depth (mm)" you want to apply - typically 50-75mm per watering, depending on crop and soil type.
Optionally enter your "Pump Discharge Rate (liters/hour)" to estimate how long irrigation will take.
Click "Calculate Water Required" to see the total water volume and irrigation time.
How the Water Volume and Pump Time Are Found
The land area is converted to square metres using 1 acre = 4,046.86 m².
Volume in litres = area in m² × depth in mm, because a 1 mm layer of water on one square metre is exactly one litre. Dividing by 1,000 gives cubic metres.
If you enter a pump discharge, Irrigation Time (hours) = Volume in litres ÷ discharge in litres per hour.
Worked Example
You want to give a 2-acre field a 50 mm watering using a pump that delivers 36,000 litres per hour (10 litres per second). Area = 2 × 4,046.86 = 8,093.72 m². Volume = 8,093.72 × 50 = 404686 litres, or 404.69 m³. Time = 404,686 ÷ 36,000 = 11.24 hours.
That is the water that must reach the crop root zone. If you estimate that only about 70% of pumped water does so with your method, divide by 0.7: you would need to pump about 5,78,000 litres, which takes roughly 16 hours with the same pump.
Water Needed per Acre at Different Depths
Depth applied
Litres per acre
m³ per acre
10 mm
40,469
40.47
25 mm
1,01,172
101.17
50 mm
2,02,343
202.34
75 mm
3,03,515
303.51
100 mm
4,04,686
404.69
For reference, one acre-inch (25.4 mm over an acre) is a little under 1,03,000 litres.
Finding Your Pump's Discharge
The rated discharge on a pump nameplate is measured under set conditions. Actual output in the field drops as the water level falls and the delivery pipe gets longer, so a simple bucket test is more reliable. Time how many seconds the outlet takes to fill a container of known size, such as a 20-litre bucket. If it fills in 2 seconds, the discharge is 10 litres per second, which is 10 × 3,600 = 36,000 litres per hour.
If your figure is in litres per second, multiply by 3,600. If it is in cubic metres per hour, multiply by 1,000. Bore well output often falls during summer, so repeat the test in the season you are planning for.
Using the Result Wisely
Sandy soils hold little water, so they need lighter, more frequent irrigation. Clay soils hold more and can take a deeper watering less often.
Flood irrigation through open channels loses water to seepage and evaporation. Drip and sprinkler systems deliver a larger share of pumped water to the roots, and support for them is available under the Per Drop More Crop component of PMKSY through state agriculture or horticulture departments.
For power planning, compare the hours needed with the hours of three-phase supply your feeder gets for agriculture. An 11-hour job may need two days on a limited supply schedule.
Rainfall counts too. A 25 mm shower shortly before a planned watering may let you skip or reduce it.
Frequently Asked Questions
Conveniently, applying 1mm of water depth over 1 square meter equals exactly 1 liter. So the total water volume (liters) = Area (m²) × Depth (mm), where the area is first converted from acres to square meters.
This varies by crop, growth stage, soil type, and climate. Many field crops are irrigated with 40-75mm per watering - check your crop's specific water requirement guidelines or your local agriculture extension office for precise figures.
No, this calculates the theoretical water volume delivered to the field. Actual pumped water needed will be higher due to conveyance and application losses, especially with flood irrigation versus drip or sprinkler systems.
Multiply it by 3,600 to get litres per hour. A pump giving 5 litres per second delivers 18,000 litres per hour. If the figure is in cubic metres per hour, multiply by 1,000 instead.
An acre-inch is a 25.4 mm layer of water over one acre, which is about 1,02,790 litres or 102.8 cubic metres. Enter 25.4 as the depth and 1 as the area to see it in the calculator.