⚠ Please enter valid values for Field Area, ET₀, and System Efficiency (1–100%).
WaterTankCalculator.com — Irrigation Water Requirement Results
Your Results
Daily Irrigation Requirement (Gross)
—
m³ / day
Daily (Litres)
— L/day
Daily (US Gallons)
— gal/day
Net Crop Water Need
— mm/day
Weekly Requirement
— m³/week
Total for Period (m³)
— m³
Period (Acre-Inches)
— ac-in
Depth per Irrigation
— mm
Kc Used
— –
Water Balance Visualisation
📖 How To Use
How to Use the Irrigation Water Requirement Calculator
Get accurate irrigation requirements for any field in under a minute:
Enter your field area
Select hectares, acres, m², or square feet — whichever matches your land records. One hectare = 10,000 m² = 2.47 acres.
Enter reference evapotranspiration (ET₀)
ET₀ is a climate index in mm/day. Obtain it from your local agro-meteorology station, FAO CLIMWAT database, or weather apps that report ETo. FAO-56 Table 2 gives typical values of about 6–9 mm/day in warm arid climates, 2–4 mm/day in moderate humid ones and 1–2 mm/day in cool humid ones. Use the average for the month or stage you are planning.
Select your crop and growth stage
The calculator loads the FAO-56 Table 12 crop coefficients for your crop. Initial and mid-season use the flat Kc ini and Kc mid. Development and late season use stage averages, because FAO-56 changes Kc in a straight line through those stages.
Enter effective rainfall and system efficiency
Enter effective rainfall in mm/day, or total monthly rainfall and let the calculator apply the FAO effective-rain formula. Then set system efficiency, or pick your method to fill FAO's typical figure: 90% drip, 75% sprinkler, 60% surface.
Enter season length and calculate
Set the number of days at this growth stage. The calculator outputs daily, weekly and period totals in several units. Add an irrigation interval to see the depth to apply each time.
Tip: For annual crops like wheat or maize, run the calculator separately for each growth stage (initial, development, mid, late) using the Kc for that stage and the number of days in that stage. Sum the seasonal totals for a full-season figure.
📐 The Formula
Irrigation Water Requirement Formula (FAO-56)
This calculator follows the FAO-56 single crop coefficient method. Penman-Monteith is how ET₀ itself is calculated from weather data; here you enter ET₀ and the calculator applies the crop coefficient:
Where: Kc = crop coefficient, ET₀ = reference ETo (mm/day), P_eff = effective rainfall (mm/day), Ea = irrigation efficiency (%).
Multiply gross depth (mm) by field area and the factor 10 to convert mm × ha → m³ (since 1 mm over 1 ha = 10 m³).
FAO-56 Crop Coefficients (Kc) Reference
Crop
Kc Initial
Kc Mid-Season
Kc End
Wheat
0.30
1.15
0.25–0.40
Rice (paddy)
1.05
1.20
0.60–0.90
Maize / Corn (grain)
0.30
1.20
0.35–0.60
Cotton
0.35
1.15–1.20
0.50–0.70
Sugarcane
0.40
1.25
0.75
Tomato
0.60
1.15
0.70–0.90
Potato
0.50
1.15
0.75
Soybean
0.40
1.15
0.50
Alfalfa (individual cuttings)
0.40
1.20
1.15
Citrus (no ground cover, ~70% canopy)
0.70
0.65
0.70
Grape (table or raisin)
0.30
0.85
0.45
Source: FAO Irrigation and Drainage Paper No. 56, Table 12, for a sub-humid climate (RHmin ≈ 45%, wind ≈ 2 m/s). Where FAO gives a range, the calculator uses wheat 0.30, rice 0.75, maize 0.35 (dried in the field), cotton 1.20/0.50 and tomato 0.70. Mango is not in Table 12 and is shown as an estimate. In development and late season the calculator uses the stage average, because Kc changes in a straight line through those stages.
Unit Conversions Used
1 ha = 10,000 m² | 1 acre = 4,046.86 m²
1 mm depth over 1 ha = 10 m³ = 10,000 litres
1 m³ = 264.17 US gallons = 1,000 litres
1 acre-inch = 102.79 m³
🌾 Use Cases
When to Use This Irrigation Water Requirement Calculator
🚿
Drip System Design
Size emitters, laterals, and headers by knowing the exact m³/hour your field needs during peak ETc periods.
🏗️
Tank & Reservoir Sizing
Calculate seasonal water volume to size on-farm storage tanks or ponds before drilling a borehole or installing pumps.
💧
Water Licence Applications
Provide regulators with documented seasonal water demand in m³ and acre-inches per crop and season.
📊
Irrigation Scheduling
Determine how many mm to apply per irrigation event and set refill intervals based on soil water holding capacity.
🌍
Climate Change Planning
Compare irrigation requirements under different ET₀ scenarios (current vs. projected +1°C, +2°C) to future-proof your farm water strategy.
💰
Water Cost Forecasting
Convert seasonal m³ requirements into cost estimates using your utility rate or diesel pump running cost per cubic metre.
🧮 Worked Example
Worked Example: A Full Maize Season, Stage by Stage
10 ha of grain maize under sprinklers (75% efficiency), with no effective rain. ET₀ rises through the season. The stage lengths are an example — use the ones for your variety and planting date.
Stage
Days
ET₀
Kc
ETc (mm/day)
Gross (mm/day)
Volume (m³)
Initial
25
4.0
0.30
1.20
1.60
4,000
Development
40
5.0
0.75
3.75
5.00
20,000
Mid-season
40
6.5
1.20
7.80
10.40
41,600
Late season
30
5.5
0.775
4.26
5.68
17,050
Season
135
—
—
620 mm total
827 mm total
82,650
The crop uses about 620 mm over the season, inside the 500–800 mm FAO gives for maize, and the farm has to pump about 82,650 m³ (about 804 acre-inches). If you had run the calculator once with the mid-season Kc and peak ET₀ for all 135 days, you would get 140,400 m³ — 70% too much. Size pumps and pipes from the mid-season row, and use the stage-by-stage total for seasonal water budgets and licences.
❓ FAQ
Frequently Asked Questions
How do I calculate irrigation water requirements for my crop?
The standard method is the FAO-56 approach: multiply your reference evapotranspiration (ET₀) by your crop's coefficient (Kc) to get crop water demand (ETc). Subtract effective rainfall to get net irrigation need, then divide by system efficiency to get the gross amount to apply. This calculator automates all those steps.
Where do I get the ET₀ value for my area?
The best sources are: (1) your national or regional meteorological service — many publish monthly ETo tables by district; (2) the FAO CLIMWAT 2.0 database, which covers thousands of weather stations worldwide; (3) weather apps or APIs that include ETo (e.g., Agromonitoring, OpenET, or NASA POWER). As a guide, FAO-56 Table 2 lists about 1–2 mm/day for cool humid regions, 2–4 mm/day for moderate humid ones and 6–9 mm/day for warm arid ones. Hot desert summers can go higher on individual days.
What is a crop coefficient (Kc) and why does it matter?
Kc scales reference ET₀ (measured for a standardised grass surface) to the actual water demand of your specific crop and growth stage. A young seedling might have Kc 0.3, while the same crop at full canopy cover at mid-season might reach Kc 1.2. Ignoring growth stage is the most common error: using the mid-season Kc for a whole maize season overstates its water use by about 70% in the worked example above.
How do I estimate effective rainfall for my irrigation calculation?
Effective rainfall is the portion of total rainfall that actually stays in the root zone and benefits the crop — runoff and deep percolation are excluded. With monthly totals, FAO's Irrigation Water Needs manual gives a simple formula: if monthly rain P is over 75 mm, effective rain = 0.8 P − 25; if it is 75 mm or less, effective rain = 0.6 P − 10 (not below zero). So 50 mm in a month counts as 20 mm and 100 mm counts as 55 mm. Choose "Total monthly rain" in the calculator and it applies this for you. FAO CROPWAT and the USDA method give more detailed figures.
What irrigation efficiency should I enter?
Use a measured figure if you have one. Otherwise FAO's Irrigation Scheduling manual gives typical field application efficiencies of 90% for drip, 75% for sprinklers and 60% for surface (furrow, basin, border) — the method selector fills these in. Real systems vary: a badly maintained sprinkler system or a long, uneven furrow can be well below these. If you are unsure between two values, use the lower one; it gives the larger, safer water requirement.
How many litres per hectare per day does an irrigated crop typically need?
For a mid-season crop with ET₀ of 6 mm/day and Kc of 1.15, ETc = 6.9 mm/day. With drip at 90% efficiency the gross requirement is 7.67 mm/day, or 76,700 litres per hectare per day; with sprinklers at 75% it is 92,000 L/ha/day. In a cooler, wetter place — ET₀ 3 mm/day, Kc 0.9 and 1 mm/day of effective rain — sprinklers need (2.7 − 1) ÷ 0.75 = 2.27 mm/day, about 22,700 L/ha/day.
What size water storage tank do I need for drip irrigation on 1 hectare?
Calculate your peak-season daily requirement with this tool, then size the tank for 1–3 days of storage buffer. For a 1 ha tomato crop in a hot semi-arid climate (ETc 7 mm/day, drip at 90% efficiency, zero rainfall), that's ~78 m³/day gross. A 2-day buffer requires a 156,000-litre (156 m³) tank. Use our Drip Irrigation Tank Size Calculator and Farm Water Storage Calculator for the full sizing workflow.
Can I use this calculator for greenhouse or protected cultivation irrigation?
Yes, but with adjustments. With care. The cover cuts sunlight and wind, so evaporative demand inside is lower — a study in unheated plastic greenhouses in Almería, Spain measured it at 30–40% below outdoors, so multiply your outdoor ET₀ by about 0.6–0.7. Set rainfall to zero. For substrate or hydroponic growing, drain water also has to be counted, so use our Greenhouse Water Usage Calculator instead.
✅ Sources & Review
Sources and review
The method, crop coefficients, rainfall formula and efficiencies on this page come from these references:
FAO Irrigation and Drainage Paper No. 56 — Allen, Pereira, Raes & Smith (1998). ETc = Kc × ETo, the Kc curve, Table 12 Kc values and Table 2 typical ET₀ ranges.
FAO CLIMWAT 2.0 — long-term monthly ET₀ and rainfall for weather stations worldwide.
Orgaz et al. (2005) — Evapotranspiration of horticultural crops in an unheated plastic greenhouse, Agricultural Water Management 72; evaporative demand 30–40% below outdoors.
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
