Irrigation water calculator using FAO-56 crop coefficients
How much water a crop needs depends on the weather and on the crop’s growth stage. This irrigation water calculator uses the FAO-56 method that irrigation engineers use worldwide: crop water use ETc = Kc × ET₀, where ET₀ is the reference evapotranspiration of the day and Kc is the crop coefficient for the crop and stage.
Choose your crop and stage (or days after sowing), enter ET₀ and the number of days, and the calculator gives the net irrigation depth, the gross depth after losses for surface, sprinkler or drip irrigation, the volume in litres and cubic metres for your field, and how long your pump must run. It covers 18 field and vegetable crops with Kc values from FAO-56 Table 12.
How to use the irrigation water calculator
- Pick the crop. The FAO-56 stage lengths are shown when the tables list them.
- Choose the growth stage — or switch to By days after sowing to let the calculator find the stage.
- Enter ET₀ in mm per day from a weather station or agromet advisory, the period (for example the days since the last irrigation) and any effective rain.
- Choose the irrigation method to set the application efficiency, and enter the field area.
- Add the pump discharge to see the running time. Tick the climate option if your area is very windy or very dry.
Irrigation Water Requirement Calculator features
FAO-56 crop coefficients
Initial, mid-season and end-of-season Kc for 18 crops, with linear change through the development and late stages.
Days after sowing
Finds the stage from FAO-56 Table 11 stage lengths.
Climate adjustment
Corrects Kc mid and Kc end for wind and minimum humidity with FAO-56 equation 62.
Net and gross depth
Subtracts effective rain and divides by surface (60%), sprinkler (75%) or drip (90%) efficiency.
Volume and pump hours
Litres, m³, acre-inches and US gallons, and running time for a pump in L/s, L/min, m³/h or gpm.
Formula and worked example
- Crop water use: ETc = Kc × ET₀. Wheat at mid-season has Kc = 1.15; with ET₀ = 5 mm/day, ETc = 5.75 mm/day.
- Net irrigation for the period: 5.75 × 7 days = 40.25 mm, minus any effective rain.
- Gross irrigation: with surface irrigation at 60% efficiency, 40.25 ÷ 0.6 = 67.1 mm.
- Volume: 1 mm on 1 m² is 1 litre, so 67.1 mm on one acre (4,047 m²) is 271,500 litres — 671 m³ per hectare.
- Pump time: at 5 litres per second (18,000 L/h), 271,500 ÷ 18,000 = 15.1 hours.
Crop coefficients (FAO-56 Table 12)
Kc initial values left blank in Table 12 take the crop group value. Where FAO gives a range, the calculator uses the middle. The values describe crops in a sub-humid climate with moderate wind; the climate option adjusts them. For drip-irrigated orchards and vegetables, also see the drip irrigation calculator, which works per plant.
| Crop | Kc initial | Kc mid | Kc end |
|---|---|---|---|
| Wheat | 0.3 | 1.15 | 0.25–0.4 |
| Rice | 1.05 | 1.20 | 0.90–0.60 |
| Maize (grain) | 0.3 | 1.20 | 0.60–0.35 |
| Cotton | 0.35 | 1.15–1.20 | 0.70–0.50 |
| Sugarcane | 0.40 | 1.25 | 0.75 |
| Soybean | 0.4 | 1.15 | 0.50 |
| Groundnut | 0.4 | 1.15 | 0.60 |
| Chickpea | 0.4 | 1.00 | 0.35 |
| Potato | 0.5 | 1.15 | 0.75 |
| Tomato | 0.6 | 1.15 | 0.70–0.90 |
| Onion (dry) | 0.7 | 1.05 | 0.75 |
Where to get ET₀ and what the numbers mean
- ET₀ is the water a well-watered grass surface would lose. Agromet advisories, university weather stations and many weather services publish it daily. FAO-56 Table 2 gives typical averages from 2–3 mm/day in cool humid tropics to 6–8 mm/day in warm arid tropics and subtropics.
- Effective rain is the part of rainfall that stays in the root zone — light showers and runoff from heavy storms count for little.
- Application efficiency covers losses to deep percolation, runoff and evaporation; FAO’s indicative values are 60% for surface, 75% for sprinkler and 90% for drip irrigation.
- A handy check: 8.64 mm/day equals a continuous flow of 1 litre per second per hectare.
Irrigation Water Requirement Calculator FAQ
How do I calculate crop water requirement?
Multiply the reference evapotranspiration ET₀ by the crop coefficient Kc for the crop’s stage: ETc = Kc × ET₀. Multiply by the number of days, subtract effective rain, and divide by the irrigation efficiency to get the water to apply.
How much water does an acre need?
1 mm of water on one acre is about 4,047 litres. A crop using 5.75 mm a day for 7 days at 60% efficiency needs about 271,500 litres per acre.
What is Kc in irrigation?
The crop coefficient: the ratio of a crop’s water use to that of the reference grass. It is low after sowing (0.3–0.5 for most field crops), peaks at 1.0–1.25 at full cover and drops as the crop matures.
How do I convert irrigation depth to volume?
Volume (litres) = depth (mm) × area (m²). 50 mm on one hectare is 500,000 litres or 500 m³.
How long should I run my pump?
Divide the volume by the pump discharge. 271,500 litres with a 5 L/s pump (18,000 litres an hour) takes about 15 hours.
Does the irrigation water calculator work for drip?
Yes — choose Drip (90%). For dripper counts, run time per plant and system flow, use the drip irrigation calculator.