Ecology & Environment

Instant, private, and free

Water Demand Calculator.

Convert explicitly entered evapotranspiration, crop coefficient, area, days, and application efficiency into irrigation demand.

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Set your values

Results update as you type.

Results update automatically as you type.

How to Use

Scenario arithmetic only: every lifecycle or resource factor is entered explicitly; no regional defaults are used.

Use Cases

Agricultural irrigation planning

Farmers and agronomists can estimate the water needed for a specific crop over a growing period, helping to schedule irrigation and manage water resources.

Example: For a 2-hectare field (20,000 m²) with ET₀=5 mm/day, Kc=1.2, over 30 days, and 75% efficiency, demand is about 4,800 m³.

Landscape and garden watering

Homeowners and landscapers can determine how much water a lawn or garden requires, ensuring efficient watering without waste.

Example: A 500 m² lawn with ET₀=4 mm/day, Kc=0.8, for 7 days, at 90% efficiency needs about 12.4 m³.

Frequently Asked Questions

What does the Water Demand Calculator do?
It converts your inputs—irrigated area (m²), reference evapotranspiration (mm/day), crop coefficient (×), period (days), and application efficiency (fraction)—into an estimate of total irrigation water demand. It helps you plan how much water your crops or landscape need over a given time.
How is irrigation water demand calculated?
The calculator multiplies the reference evapotranspiration by the crop coefficient to get crop evapotranspiration, then multiplies by the irrigated area and period, and divides by the application efficiency to account for water losses. The result is the total water demand in liters (or m³).
What is application efficiency and why does it matter?
Application efficiency (as a fraction, e.g., 0.8 for 80%) represents how effectively your irrigation system delivers water to the crop. A lower efficiency means more water is lost to evaporation, runoff, or deep percolation, so the calculator increases the demand accordingly.

Tips & Common Mistakes

Tips

  • Use local reference evapotranspiration (ET₀) data from a weather station or online source for accurate results.
  • Choose the crop coefficient (Kc) that matches your crop's growth stage—values vary from planting to harvest.
  • Enter application efficiency as a fraction (e.g., 0.85 for 85%) to reflect your irrigation system's performance.
  • Double-check units: area in m², ET₀ in mm/day, period in days, and efficiency as a fraction.

Common Mistakes to Avoid

  • Using ET₀ in inches instead of mm—convert to mm/day first.
  • Forgetting to convert area from hectares to m² (1 ha = 10,000 m²).
  • Entering efficiency as a percentage (e.g., 85) instead of a fraction (0.85).

Last updated: August 13, 2026