How do I find the ET₀ value for my area?

Use our ET₀ Calculator with your local weather data, or check your regional agricultural weather network (such as CIMIS in California, CoAgMet in Colorado, or FAWN in Florida). Many university extension services also publish daily ET₀ data during the growing season.

What Kc value should I use for my crop?

Select your crop from the dropdown above to auto-fill the mid-season Kc from FAO-56 data. For more precise scheduling, use the initial Kc during early growth and the end-season Kc as the crop matures. Your local extension service may have region-specific Kc values.

How do I convert the result to gallons per acre?

1 mm of water over 1 hectare = 10 m³ = 2,642 gallons. To convert mm/day to gallons per acre per day, multiply by 27,154 / 25.4 = about 1,069 gallons per acre per mm.

Why is efficiency important?

Irrigation efficiency accounts for water lost to wind drift, evaporation, deep percolation, and runoff. Without accounting for efficiency, you would under-irrigate your crop. A drip system at 90% efficiency wastes only 10%, while flood irrigation at 60% efficiency loses 40% of applied water.

Irrigation Water Requirement Calculator — Crop ET from ET₀ and Kc

Calculate how much water your crop actually needs by combining reference evapotranspiration (ET₀) with a crop-specific coefficient (Kc) and your irrigation system efficiency. This calculator converts atmospheric demand into a practical application depth in mm or inches per day, helping you avoid both under- and over-irrigation.

Keep Crop Need, Collected Supply and Stock Mixing Separate

This result is the crop-side application depth. Reduce it with Effective Rainfall only for rain that reached the field root zone. A roof-catchment estimate belongs in the Rainwater Harvesting Calculator as a separately stored and delivered supply, not as extra field rainfall.

If you inject fertilizer with that irrigation water, use the Fertigation Injector & Stock Tank Calculator to calculate one selected nutrient concentration from the final stock volume and calibrated injector ratio. That concentration calculation neither changes crop water requirement nor chooses the irrigation depth.

Inputs Explained

Reference ET (ET₀): The baseline evapotranspiration rate for a grass reference surface. Obtain from our ET₀ Calculator or your local ag weather network.
Crop Coefficient (Kc): A multiplier that adjusts ET₀ for your specific crop and growth stage. Select your crop from the dropdown to auto-fill the FAO-56 mid-season Kc.
Irrigation Efficiency: The fraction of applied water that actually reaches the root zone. Drip: 85–95%, sprinkler: 70–85%, flood: 50–70%.
Growth Stage: Crop growth stage determines which Kc value applies — initial, mid-season, or late-season — each with different water demand.

How This Calculator Works

Based on: FAO-56 single crop coefficient method: IWR = (ET₀ × Kc) / Efficiency

Best for: Daily or weekly irrigation scheduling for field crops, orchards, and vegetable production

Check locally: Verify Kc values with your local extension service, as regional varieties and climates may shift coefficients slightly.

Units supported: Metric (mm/day, L/m²), Imperial (in/day, gal/acre)

Worked Example

Mid-season tomato crop under drip irrigation in a semi-arid region

1. Enter ET₀
5.5 mm/day from local weather station data.
2. Select crop and stage
Tomato, mid-season — Kc auto-fills as 1.15.
3. Set efficiency
Drip irrigation at 90% efficiency.
4. Calculate
IWR = (5.5 × 1.15) / 0.90 = 7.03 mm/day.

Apply approximately 7.0 mm/day through drip irrigation to meet full crop water demand without deficit stress.

How to Interpret Your Results

Condition	What It Means
IWR < 3 mm/day	Low water requirement — early growth stage or cool weather. Short irrigation sets are sufficient.
IWR 3–6 mm/day	Moderate requirement — typical spring/autumn demand for most crops.
IWR 6–10 mm/day	High requirement — peak season for high-Kc crops. Ensure system capacity can deliver this depth daily.
IWR > 10 mm/day	Very high — hot climate with low-efficiency system. Consider upgrading to drip or improving efficiency.

Common Mistakes to Avoid

Using mid-season Kc for young seedlings

Select the initial-stage Kc (typically 0.3–0.5) until canopy covers about 10% of the ground.

Ignoring irrigation efficiency

Always divide by efficiency. A 60% efficient system must apply nearly double the net crop requirement.

Applying the full IWR every day regardless of rainfall

Subtract effective rainfall from the gross requirement. Use the Effective Rainfall calculator to determine usable rain.

Related Calculators

ET₀ Calculator

Calculate reference evapotranspiration using the FAO-56 Penman-Monteith equation for irrigation planning

Irrigation Runtime Calculator

Calculate how long to run your irrigation system based on flow rate, area, and target application depth

Soil Moisture Depletion Calculator

Calculate irrigation scheduling from soil properties, root depth, and daily crop water use

Effective Rainfall Calculator

Calculate effective rainfall using the USDA-SCS method to determine irrigation reduction

Rainwater Harvesting Calculator

Estimate potential roof runoff from supplied rainfall, catchment area and collection efficiency, with optional monthly demand coverage and coarse tank balance.

Fertigation Injector & Stock Tank Calculator

Calculate soluble fertilizer mass, stock concentration and selected-nutrient outlet ppm from your product analysis, source-water contribution and stock-to-final-solution injector ratio.

Pipe Friction Loss Calculator

Calculate friction head loss in irrigation pipes using the Hazen-Williams equation

Irrigation Pump Sizing Calculator

Calculate required pump power from flow rate and total dynamic head for irrigation systems

Drip Emitter Spacing Calculator

Calculate emitter count, total flow, and lateral length for drip irrigation layout

Water Cost Calculator

Calculate seasonal irrigation water and energy costs per hectare or acre

Related Guides

How to Calculate Crop Water Requirements

Learn how to use ET₀ and crop coefficients to determine exactly how much water your crops need throughout the growing season.

Rainwater Harvesting for Farm Irrigation

Estimate roof-water supply from measured rainfall and collection losses, compare matching demand periods, and plan storage without confusing harvested roof water with effective field rainfall.

Greenhouse Fertigation Stock Tank Mixing

Calculate a selected nutrient concentration through a stock injector, document source-water assumptions, and verify the real mixture against labels and calibration.