Irrigation Calculator
Proper irrigation is one of the most important factors in lawn and garden health. Too little water causes drought stress; too much wastes resources and can lead to disease. The USDA recommends that most lawns receive about 1 inch of water per week from rain and irrigation combined. This calculator determines how many minutes per zone you need to run your sprinklers to achieve your target water depth, and how many gallons you will use each week. You enter your zone area in square feet, your sprinkler system's precipitation rate (in inches per hour), and your desired weekly water depth. Precipitation rates vary by head type: rotary heads typically apply 0.4 to 0.8 in/hr; fixed spray heads apply 1.0 to 1.5 in/hr. You can measure your actual rate using the can test (place tuna cans in the zone, run sprinklers for 15 minutes, and measure the average depth). The gallons calculation uses 0.623 gallons per square foot per inch of water applied (the standard conversion). Run sprinklers in early morning for best efficiency.
Irrigation run time formula
Run time (min/week) = (Target depth (in) / Precipitation rate (in/hr)) x 60
Minutes per session = Run time / Run days per week
Gallons per week = Area (sq ft) x Target depth (in) x 0.623
Worked example
Zone: 2,000 sq ft. Precipitation rate: 0.75 in/hr. Target: 1 in/week. 3 run days.
- Run time = (1 / 0.75) x 60 = 80 min/week
- Minutes per session = 80 / 3 = 26.67 min
- Gallons per week = 2,000 x 1 x 0.623 = 1,246 gal
- Gallons per month = 1,246 x 4.33 = 5,395 gal
Frequently asked questions
How long should I run my sprinklers each week?
Most lawns need about 1 inch of water per week from rain plus irrigation. To apply 1 inch of water, divide 1 by your sprinkler's precipitation rate (inches per hour) and multiply by 60 to get run time in minutes. A sprinkler with a 0.5 in/hr precipitation rate needs 120 minutes (2 hours) to apply 1 inch.
What is a sprinkler precipitation rate?
The precipitation rate is the depth of water a sprinkler system applies per hour, measured in inches per hour. Pop-up rotary heads typically apply 0.4 to 0.8 in/hr. Fixed spray heads apply 1.0 to 1.5 in/hr. Impact sprinklers vary widely. The USDA recommends matching run times to your specific head type.
How many gallons per week does an average lawn need?
One inch of water over 1,000 sq ft is approximately 623 gallons. A typical 5,000 sq ft lawn applying 1 inch per week uses about 3,115 gallons. Water in early morning to reduce evaporation and allow foliage to dry before evening.
How do I measure my sprinkler precipitation rate?
Place several straight-sided cans (tuna cans work well) around the irrigated zone. Run the sprinklers for a set time, then measure the water depth in each can. Divide the average depth by the run time in hours to get the precipitation rate in inches per hour.
Should I adjust irrigation for rain?
Yes. Smart irrigation controllers with rain sensors or weather-based scheduling automatically reduce or suspend irrigation after rainfall. Even without a smart controller, subtract recent rainfall from your target depth before running the system. The USDA and EPA WaterSense program recommend weather-based irrigation scheduling.
Sources
- USDA: Water and Irrigation.
- EPA WaterSense: Irrigation efficiency.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. Adjust for local climate and soil conditions.