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

    Frequently Asked Questions

    How much water does a lawn need?

    Most lawns need about 1 inch of water per week during the growing season. Cool-season grasses may need slightly more in heat. Warm-season grasses are generally more drought tolerant.

    When is the best time to water?

    Water early morning (4-8 AM) to minimize evaporation and fungal disease. Avoid midday watering (high evaporation) and evening watering (promotes fungus).

    How does drip irrigation compare to sprinklers?

    Drip irrigation delivers water directly to roots at 85-95% efficiency versus 60-75% for sprinklers. Drip uses 30-50% less water for the same plant health.

    Ready to run the numbers?

    Open Irrigation Water Calculator