Irrigation Water Calculator
Turn minutes on the timer into gallons on the lawn.
Irrigation Water Calculator
Depth per session = rate x minutes / 60; gallons = area x depth (ft) x 7.48052.
Calculated instantly
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What This Calculator Does
The Irrigation Water Calculator converts a sprinkler zone, application rate, and run time into gallons per session, per week, and per month. It also reports the weekly application depth in inches, which is the number to compare with local evapotranspiration and rainfall.
How It Works
Application rate in inches per hour multiplied by run time in minutes, divided by 60, gives the depth applied in one session. That depth is divided by 12 to convert to feet, multiplied by the irrigated area to get cubic feet, then by 7.48052 to get gallons. Multiplying by sessions per week gives the weekly total, and 4.33 weeks gives a monthly estimate.
Example
A 1,000 sq ft zone at 0.5 in per hour run for 20 minutes applies 0.167 in, or about 1,247 gallons per session. Three sessions a week uses about 3,740 gallons and applies 0.5 in of water per week. A typical month is about 16,195 gallons.
Formula
application rate x minutes / 60area x depth (ft) x 7.48052depth per session x sessions per weekweekly gallons x 4.33Reading the Results
Water per session
The volume applied to the stated area at the stated rate. It assumes even distribution; a windy site or mismatched nozzles will not deliver it uniformly.
Weekly application depth
Compare this with local evapotranspiration minus rainfall. If it is below the demand, the lawn is being deficit-irrigated; if it is far above, water is running off or leaching.
Water per month
Weekly use multiplied by 4.33. It is a planning average, not a bill: rates, season length, and controller adjustments change the real total.
Sources and References
US EPA WaterSense - Irrigation Controllers
Water-efficient irrigation scheduling
Explains how weather-based scheduling and cycle-and-soak reduce waste.
Alliance for Water Efficiency - Landscape Irrigation
Precipitation rate and distribution uniformity
Background on why matched nozzles and a measured application rate matter.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
What is a precipitation rate?
It is how fast a sprinkler zone applies water, usually in inches per hour. It depends on nozzle flow, spacing, and pressure, not on the timer. Without it, minutes are just a number and cannot be converted to gallons for a given area.
How do I measure my application rate?
Set several straight-sided catch cups across the zone, run the system for 15-20 minutes, and measure the average depth. Multiply by four to get inches per hour for a 15 minute run. This also reveals dry spots and wind distortion.
How often should I water?
Water deeply enough to wet the root zone, then let the surface dry before watering again. The exact interval depends on grass species, soil, weather, and slope. A rain sensor or weather-based controller adjusts automatically and usually saves more than a fixed schedule.
Why does the monthly figure use 4.33 weeks?
A month averages about 4.33 weeks across a year. It is an approximation for planning. For a real water bill, use the actual number of watering days in the billing period and the tiered rates from the utility.
Does this include rain or evapotranspiration?
No. It calculates what the system applies, not what the plants need. Subtract effective rainfall and compare with local evapotranspiration to decide whether the schedule is right. The calculator is a measuring tool, not a scheduling model.