Irrigation Water Calculator Formula

Understand the math behind the irrigation water calculator. Each variable explained with a worked example.

Use the Irrigation Water Calculator

Formulas Used

Weekly Irrigation Water

weekly_water = weekly_gallons

Monthly Irrigation Water

monthly_water = weekly_gallons * 4.33

Season Total (26 weeks)

season_water = weekly_gallons * 26

Sprinkler Run Time (1.5 GPM)

run_time = weekly_gallons / (1.5 * 60)

Variables

VariableDescriptionDefault
area_sqftIrrigated Area(sq ft)3000
weekly_need_inWeekly Water Need(inches)1
weekly_rain_inWeekly Rainfall(inches)0.5
irrigation_efficiencyIrrigation Efficiency(%)75
deficit_inDerived value= weekly_need_in - weekly_rain_incalculated
weekly_gallonsDerived value= area_sqft * (weekly_need_in - weekly_rain_in) * 0.623 / (irrigation_efficiency / 100)calculated

How It Works

How to Calculate Irrigation Water Needs

Lawns and gardens need about 1 inch of water per week, including rainfall.

Formula

Weekly Gallons = Area * (Need - Rainfall) * 0.623 / Efficiency

Irrigation Efficiency

  • Drip irrigation: 85-95%
  • Sprinkler: 60-75%
  • Flood/furrow: 40-60%

    • Overwatering is the most common irrigation mistake, wasting both water and money.

    Worked Example

    3,000 sq ft lawn needing 1 inch/week with 0.5 inches rain at 75% sprinkler efficiency.

    area_sqft = 3000weekly_need_in = 1weekly_rain_in = 0.5irrigation_efficiency = 75
    1. 01Water deficit: 1.0 - 0.5 = 0.5 inches
    2. 02Weekly gallons: 3,000 * 0.5 * 0.623 / 0.75 = 1,246 gallons
    3. 03Monthly: 1,246 * 4.33 = 5,395 gallons
    4. 04Season (26 weeks): 1,246 * 26 = 32,396 gallons

    Ready to run the numbers?

    Open Irrigation Water Calculator