Calculate ETc (crop evapotranspiration) using FAO-56 methodology. Enter your crop, growth stage, reference ET, and field area to get daily and seasonal irrigation water requirements instantly.
Select your crop, growth stage, ETo, and field area to calculate ETc
Step 1 — Select Crop & Growth Stage
Crop Type
Growth Stage
Step 2 — Enter Reference ET & Field Area
ETo (Reference ET)
Field Area
Duration (days)
days
Step 3 — Rainfall & Irrigation Method optional
Rainfall in Period
Irrigation Method
Typical field application efficiency (FAO). Use your own figure if you have a tested system.
Adjust Kc for a dry or windy climate (FAO-56 Eq. 62)
Min. Relative Humidity
%
Wind Speed (2 m height)
Leave blank for standard conditions (RHmin 45%, wind 2 m/s). Use mid-season averages. FAO-56 limits the correction to RHmin 20–80% and wind 1–6 m/s, and applies it to Kc mid and to Kc end when Kc end is 0.45 or higher.
Auto-Selected Kc Value
—
from FAO-56 Table 12
⚠ Please enter valid values for ETo, field area, and duration (days > 0). Rainfall cannot be negative.
Irrigation Requirement
Daily Crop Water Need (ETc)
—
mm / day
Weekly ETc
— mm/wk
Total Period ETc
— mm
Daily Water (m³/ha)
— m³/ha/d
Total Field Volume
— m³
Total Volume (L)
— litres
Crop Coefficient (Kc)
— —
Irrigation Requirement for the Period after rain & system losses
Effective Rainfall
— mm
Net Irrigation
— mm
Gross Irrigation
— mm
Gross Volume to Apply
— m³
Gross Volume (US gal)
— gal
Avg Daily Supply
— m³/day
Kc Curve & Daily Need by Growth Stage
📖 How To Use
How to Use This Crop Water Need Calculator
This calculator implements the FAO-56 single crop coefficient (Kc) method — the global standard for estimating crop water requirements. Here's how to use it in under a minute:
Select your crop type
Choose from 30 crops across field crops, vegetables, fruit trees, and forages. The calculator loads the FAO-56 Table 12 Kc values for that crop (mango is the one exception — FAO-56 does not list it, so the page labels its values as an estimate).
Choose the growth stage
Select Initial, Development, Mid-Season, or Late Season. Initial and mid-season use the flat FAO-56 values. Development and late season use the stage average, because FAO-56 changes Kc in a straight line through those stages. Mid-season usually gives peak water demand.
Enter reference evapotranspiration (ETo)
ETo is the water demand of a reference grass crop. Get it from your local meteorological station, FAO CLIMWAT database, or your state or national ET network. FAO-56 Table 2 puts typical values at about 6–9 mm/day in warm arid climates and 2–4 mm/day in moderate humid ones. Use the average for the period you are planning, not the yearly average. You can input in mm/day, mm/week, or inches/day.
Enter field area and duration
Your field size in hectares, acres, m², or dunams. Duration is the number of days in the growth stage you're planning for.
Add rainfall and your irrigation method (optional)
Enter the rain you expect in the period. If it is total rainfall, the calculator estimates the effective part with the FAO formula. Then pick drip, sprinkler or surface irrigation so the result includes system losses. Open the climate panel if your summers are very dry or windy.
Read your results
The top block is the crop water need (ETc) in mm and as a volume for your field. The irrigation block below it is what you actually have to deliver: net irrigation after rain, then gross irrigation after system losses, in mm, m³, US gallons and an average daily supply for tank sizing or pump specification.
Tip: ETc is the crop water need, not the irrigation application. You also need to account for rainfall, irrigation efficiency (typically 70–90% for drip, 60–75% for sprinkler), and soil water reserves. Net irrigation requirement = ETc − effective rainfall. Step 3 of the calculator does this for you.
📐 The Formula
Crop Water Need Formula (FAO-56)
The crop water need is calculated using the single crop coefficient approach from FAO Irrigation and Drainage Paper No. 56 (Allen et al., 1998):
ETc = Kc × ETo
ETc = Crop evapotranspiration (mm/day)
Kc = Crop coefficient (dimensionless)
ETo = Reference evapotranspiration (mm/day)
Volume = ETc (mm/day) × Area (m²) × Days ÷ 1,000
Result in cubic metres (m³)
Net irrigation (mm) = ETc total − Effective rainfall Gross irrigation (mm) = Net irrigation ÷ Efficiency
The crop coefficient (Kc) accounts for differences between the reference crop (clipped grass) and the actual crop in terms of crop height, aerodynamic resistance, albedo, and canopy resistance. Kc varies through the crop's life cycle — peaking at mid-season and dropping at senescence.
FAO-56 builds the Kc curve from three numbers. Kc ini is flat through the initial stage. Kc then rises in a straight line to Kc mid during development, stays flat through mid-season, and falls in a straight line to Kc end by harvest. That is why this calculator averages Kc ini and Kc mid for the development stage, and Kc mid and Kc end for the late stage. Using Kc end for the whole late stage is a common mistake. It under-budgets that stage, badly so for crops harvested dry like wheat (Kc end 0.30) — the stage average is 0.73.
FAO-56 Kc Values Reference Table
Crop
Kc ini
Kc mid
Kc end
Season (days)
Wheat
0.30
1.15
0.30
150–180
Maize / Corn
0.30
1.20
0.35
125–180
Rice (Paddy)
1.05
1.20
0.75
150–180
Soybean
0.40
1.15
0.50
135–150
Cotton
0.35
1.20
0.50
180–195
Tomato
0.60
1.15
0.70
135–180
Potato
0.50
1.15
0.75
105–145
Onion
0.70
1.05
0.75
150–210
Alfalfa*
0.40
1.20
1.15
Perennial
Citrus
0.70
0.65
0.70
Perennial
Date Palm
0.90
0.95
0.95
Perennial
Banana
0.50
1.10
1.00
300–365
Source: FAO Irrigation and Drainage Paper No. 56 (Allen et al., 1998), Table 12. Values are for a sub-humid climate (RHmin ≈ 45%, wind ≈ 2 m/s) and standard agronomic conditions. Citrus is the no-ground-cover, ~70% canopy row — its Kc mid is lower than Kc ini because citrus closes its stomata in hot weather. *Alfalfa and berseem clover use the individual-cutting values (peak between cuts); FAO-56 gives 0.95 mid / 0.90 end for alfalfa averaged over the whole season.
Adjusting Kc for Dry or Windy Climates
Table 12 values assume a sub-humid climate. In dry, windy places a tall crop pulls more water than the table suggests, and in humid, calm places less. FAO-56 Equation 62 corrects Kc mid (and Kc end when it is 0.45 or more):
Kc adj = Kc table + [0.04 (u₂ − 2) − 0.004 (RHmin − 45)] × (h ÷ 3)0.3
u₂ = mean wind speed at 2 m (m/s), 1–6
RHmin = mean daily minimum relative humidity (%), 20–80
h = mean crop height in mid-season (m), from FAO-56 Table 12
Example: tomato (h = 0.6 m) in a dry, breezy summer with RHmin 25% and wind 4 m/s. The correction is +0.10, so Kc mid goes from 1.15 to 1.25. At an ETo of 6 mm/day that is 7.5 mm/day instead of 6.9 — about 9% more water at peak. The optional climate panel in the calculator applies this for you.
Estimating Effective Rainfall
Not all rain reaches the root zone — some runs off and some drains below the roots. When you enter total rainfall, the calculator converts it to a monthly rate and applies the FAO dependable-rain formula from Irrigation Water Management Training Manual No. 3:
If monthly rain P > 75 mm: Pe = 0.8 P − 25
If monthly rain P ≤ 75 mm: Pe = 0.6 P − 10 (not less than 0)
So 50 mm of rain in a month counts as 20 mm of effective rain, and 100 mm counts as 55 mm. If your extension service or soil-water balance gives you an effective rainfall figure, choose "Already effective rain" instead.
Unit Conversions
1 mm/day over 1 ha = 10 m³/day
1 mm/day over 1 acre = 4.047 m³/day
1 m³ = 1,000 litres
1 inch/day = 25.4 mm/day
🌾 When to Use This
Use Cases & Applications
This calculator is used by farmers, agronomists, and irrigation engineers across a wide range of planning scenarios:
Irrigation System Design
When sizing drip lines, sprinkler heads, or furrow irrigation systems, you need peak ETc at mid-season to determine the required flow rate. A wheat field at peak demand in a 6 mm/day ETo climate needs 1.15 × 6 = 6.9 mm/day — that's 69 m³/day per hectare that your system must deliver.
Tank & Reservoir Sizing
Combine this calculator with the Farm Water Storage Calculator to size your on-farm storage tank. If you have 3 days of expected supply interruption, your tank must hold 3 × the average daily supply from the irrigation block (gross, so it includes system losses).
Pump Selection
Peak ETc divided by daily irrigation hours gives you the required flow rate. A field needing 69 m³/day running on 6-hour irrigation windows requires a minimum pump rate of 11.5 m³/hour — feed that into the Irrigation Pump Size Calculator.
Seasonal Water Budget Planning
Multiply daily ETc by the full season length to get total water commitment. This helps in pre-season water purchase agreements, borehole yield assessments, and rainwater harvesting system sizing.
Practical note: ETc is theoretical demand under no stress. To get actual irrigation amounts, subtract effective rainfall (rainfall contributing to soil moisture) and adjust for your system's application efficiency. (ETc − effective rainfall) ÷ efficiency = gross irrigation requirement. For drip at 90%, divide by 0.90; for sprinkler at 75%, divide by 0.75.
🧮 Worked Example
Worked Example: A Full Tomato Season on 1 Hectare
A 1-hectare drip-irrigated tomato field in a hot, dry summer with an average ETo of 6 mm/day. The stage lengths below are an example (check your variety and planting date). Tomato Kc values from FAO-56 Table 12 are 0.60 ini, 1.15 mid and 0.70 end.
Stage
Days
Kc used
ETc (mm/day)
Stage ETc (mm)
Volume (m³/ha)
Initial
30
0.60
3.60
108
1,080
Development
40
0.875
5.25
210
2,100
Mid-season
45
1.15
6.90
310.5
3,105
Late season
30
0.925
5.55
166.5
1,665
Season
145
—
5.48 avg
795
7,950
With 40 mm of effective rain over the season, net irrigation is 795 − 40 = 755 mm. Drip at 90% efficiency makes the gross requirement 755 ÷ 0.90 = 839 mm, or about 8,390 m³ (2.2 million US gallons) per hectare. That sits at the upper end of the 400–800 mm FAO gives for tomato, as you would expect in a hot, dry climate before the climate correction.
Two things to take from this. First, the peak is what sizes your pump and pipes: 6.9 mm/day ÷ 0.90 = 7.7 mm/day, or 77 m³ per hectare per day. Second, if the late stage had been planned with Kc end (0.70) instead of the stage average (0.925), it would have been short by 40.5 mm — 405 m³ per hectare in the weeks when the fruit is filling.
❓ FAQ
Frequently Asked Questions
What is crop water need (ETc) and how is it calculated?
Crop water need (ETc) is the amount of water a crop consumes through transpiration and evaporation from the soil surface under optimal growing conditions. It's calculated as ETc = Kc × ETo, where Kc is the crop coefficient (a dimensionless ratio specific to crop type and growth stage) and ETo is the reference evapotranspiration from a standard grass surface.
Where do I get the ETo (reference evapotranspiration) value?
ETo is available from several sources: (1) your nearest meteorological station, which may publish monthly ET data; (2) the FAO CLIMWAT 2.0 database, which covers thousands of locations worldwide; (3) online calculators that apply the Penman-Monteith equation using temperature, humidity, wind, and solar radiation; (4) local agricultural extension departments. Typical values range from 2–3 mm/day in cool humid climates to 7–10 mm/day in hot arid regions like the Middle East or Pakistan.
What is the crop coefficient (Kc) and why does it change with growth stage?
The crop coefficient (Kc) reflects how much a specific crop transpires relative to reference grass, accounting for canopy size, leaf area, and stomatal behavior. In the initial stage, Kc is low because the soil is bare and the canopy is small. It rises through development to peak at mid-season when the canopy is fully covering the ground. At the late season, Kc drops as leaves senesce and water uptake slows. Using the correct Kc for each stage is critical for accurate irrigation scheduling.
Is ETc the same as the irrigation requirement?
No. ETc is the total crop water demand. The net irrigation requirement (NIR) is ETc minus effective rainfall. The gross irrigation requirement (GIR) is NIR divided by the irrigation system efficiency (e.g., divide by 0.90 for 90% efficient drip irrigation). So for ETc = 5 mm/day, rainfall = 1 mm/day, and drip efficiency = 0.90: NIR = 4 mm/day, GIR = 4.44 mm/day applied at the field level.
How much water does one hectare of wheat need per day?
At mid-season (Kc = 1.15) with a typical ETo of 5 mm/day: ETc = 1.15 × 5 = 5.75 mm/day. Over 1 hectare (10,000 m²), this equals 57.5 m³/day or 57,500 litres/day. In a hot arid climate with ETo of 8 mm/day, this rises to 92 m³/day per hectare.
What is the difference between ETc, ETo, and ETa?
ETo (reference ET) is the evapotranspiration from a hypothetical, well-watered reference grass surface — a climate-driven baseline. ETc (crop ET) applies crop-specific coefficients to ETo to get the demand for a specific crop at a specific stage — the potential demand under no stress. ETa (actual ET) is what really happens in the field when water, nutrients, pests, or other stresses reduce growth — ETa ≤ ETc. This calculator computes ETc (the target to meet with irrigation).
How do I use this calculator to size a farm storage tank?
Run this tool with your peak-stage inputs, rainfall set to zero and your irrigation method selected, and read the Avg Daily Supply (gross m³/day). Then multiply by the number of days of storage autonomy you need (e.g., if supply is unreliable 3 days at a time, multiply by 3). Add 10–15% as a buffer. That gives your minimum tank volume. Feed that number into our Farm Water Storage Calculator to select tank dimensions.
Which crops have the highest water need per hectare?
High water demand crops include sugarcane (Kc mid ≈ 1.25), rice paddy (Kc mid ≈ 1.20), banana (Kc mid ≈ 1.10), alfalfa (Kc mid ≈ 1.20), and maize at silking (Kc mid ≈ 1.20). In a 6 mm/day ETo environment, these crops can demand 66–75 m³/ha/day at peak. Over a full season FAO puts sugarcane at 1,500–2,500 mm (15,000–25,000 m³/ha) and banana at 1,200–2,200 mm, against 450–650 mm for wheat. Efficient irrigation is critical for these crops to be economically viable.
✅ Sources & Review
Sources and review
The method, Kc values and conversion factors 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 crop heights, Equation 62 climate correction, and Table 2 typical ETo ranges.
FAO CLIMWAT 2.0 — long-term monthly ETo for weather stations worldwide.
Mango does not appear in FAO-56 Table 12; its values here are a planning estimate and the calculator marks them that way. For commercial orchards, use Kc values from your local extension service.
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.
