Irrigation Calculator Widget
Give growers, greenkeepers and gardeners a water budget based on the FAO-56 method. They enter today's reference evapotranspiration, choose a crop, subtract the rain that fell and get litres or gallons to apply, the depth in millimetres or inches and the system run time.
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How it works
FAO Irrigation and Drainage Paper 56 separates weather from crop. The weather part is reference evapotranspiration, ET0 - the water a well-watered grass surface would lose - which weather services and agricultural ET networks publish daily. The crop part is the coefficient Kc, and crop water use is ETc = Kc x ET0 (FAO-56 equation 58). Presets use the mid-season values in FAO-56 Table 12: 1.20 for grain maize, 1.15 for spring wheat, tomato, potato and soybean, 0.95 for cool-season turf, 0.65 for citrus without ground cover; any other Kc can be typed. Seven days of ETc minus the week's effective rainfall (never below zero) is the net need; dividing by system efficiency gives the gross water to apply. One millimetre over one square metre is one litre and one inch over one square foot is 0.623 US gallons. A flow rate turns the volume into run time: 4,000 L at 20 L/min is 200 minutes a week.
Calculation method
- ETc = Kc x ET0 (FAO-56, eq. 58)
- Net weekly need = 7 x ETc - effective rainfall, not less than 0
- Gross depth = net need / system efficiency
- Volume = gross depth (mm) x area (m2) litres; imperial: gross depth (in) x area (ft2) x 0.623377 gal
- Run time = volume / flow rate
Worked examples
Vegetable bed, metric
Inputs: ET0 5 mm/day; Kc 1.2; 10 mm rain; 100 m2; 80% efficiency; 20 L/min
Result: 4,000 L a week (40 mm gross); 571 L a day; run time 3.3 hours a week
ETc 6 mm/day x 7 = 42 mm - 10 mm rain = 32 mm net; / 0.8 = 40 mm; x 100 m2 = 4,000 L; / 20 = 200 min.
Cool-season lawn, imperial
Inputs: ET0 0.2 in/day; turf Kc 0.95; 0.5 in rain; 5,000 ft2; 75% efficiency; 10 gal/min
Result: 3,449 gal a week (1.11 in gross); 5.7 hours a week
0.19 x 7 = 1.33 in - 0.5 = 0.83 in net; / 0.75 = 1.11 in; x 5,000 x 0.623 = 3,449 gal.
Limitations
- Uses a single Kc; it does not model the growth stages or the dual (basal plus soil evaporation) coefficient method of FAO-56.
- ET0 must be supplied by the reader; the widget does not fetch weather data.
- Does not account for soil water holding capacity, salinity leaching or deficit-irrigation strategies.
Where publishers use it
- A drip-irrigation supplier's page sizing weekly water for customers' beds
- A golf-course or sports-turf blog on ET-based watering
- An agricultural extension service publishing local ET0 values each week
- A smart-controller company explaining how its schedule is calculated
- A school or university practical on crop water requirements
Questions
Where do I get ET0?
From your national weather service, an agricultural ET network or an irrigation-district bulletin; many publish daily reference evapotranspiration in mm or inches. Summer values are commonly 4-7 mm (0.15-0.3 in) a day in warm climates. Use the value for your location and week.
What is a crop coefficient?
Kc is the ratio of a crop's water use to that of the reference grass. It changes with growth stage: FAO-56 gives maize 0.3 at the start, 1.20 at mid-season and 0.35-0.60 at the end. The presets are mid-season values, the peak demand period.
What does system efficiency mean?
The share of the water applied that actually reaches the root zone. Drip systems lose little, sprinklers lose more to wind drift, evaporation and uneven coverage. At 80% efficiency, a 32 mm net need means applying 40 mm.
What is effective rainfall?
The part of the rain that stays in the root zone. A light shower that evaporates from leaves, or a downpour that runs off, contributes less than the gauge reading; enter the share you believe soaked in.
How many litres is 1 mm of water?
Exactly one litre per square metre, so 1 mm on 100 m2 is 100 L and on one hectare is 10,000 L (10 m3). In imperial units, 1 inch on 1,000 ft2 is about 623 US gallons.
Should I water every day?
Not necessarily. The weekly figure is the amount; how to split it depends on soil. Sandy soils hold little water and need short, frequent cycles, while clay soils prefer fewer, deeper waterings.
Sources
- FAO Irrigation and Drainage Paper 56, Chapter 6 - ETc single crop coefficient (Kc) - Food and Agriculture Organization of the United Nations . ETc = Kc ETo (equation 58) and Table 12 single crop coefficients (e.g. maize grain mid 1.20, tomato 1.15, cool-season turf 0.95, citrus 0.65). Checked 2026-10-01.
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A2Z Tools Irrigation Calculator https://a2z.tools/embed/irrigation-calculator
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Irrigation Calculator by A2Z Tools - https://a2z.tools/embed/irrigation-calculator
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