Calculate sprinkler precipitation rate and run time for target water depth.
The sprinkler run time calculator works out how long your zone must run to deliver a chosen depth of water — for example the inch per week that established cool-season lawns commonly receive. From the per-head flow rate in gallons per minute (GPM, on the nozzle spec sheet) and the head and row spacing of your layout in feet, it first derives the precipitation rate in inches per hour, then converts your target depth into minutes of run time. This replaces guesswork timer settings with numbers tied to your actual hardware and spacing.
Precipitation rate (in/hr) = 96.25 × GPM ÷ (head spacing × row spacing). The constant 96.25 converts gallons per minute over square feet into inches per hour: 60 min/hr × 231 in³/gal ÷ 144 in²/ft². Run time (minutes) = target depth ÷ precipitation rate × 60. Closer spacing or higher-flow nozzles raise the precipitation rate and shorten the run; the math assumes head-to-head coverage where each sprinkler throws to its neighbors.
Rotors at 3 GPM spaced 15 × 15 ft give 96.25 × 3 ÷ 225 = 1.28 in/hr. Delivering 1 inch takes about 47 minutes — far longer than the 15-minute default many controllers ship with.
Fixed sprays at 2 GPM on 12 × 12 ft spacing produce 1.34 in/hr, so 1 inch needs about 45 minutes. Split it into two 22-minute cycles an hour apart if water pools or runs off a slope.
Keeping the 15×15 ft rotor zone above, a 0.5 inch target takes about 23 minutes per session. Two such sessions a week deliver the same weekly inch with deeper soaking than daily light sprinkling.
It is 60 min/hr × 231 in³ per gallon ÷ 144 in² per ft² — the exact unit conversion from GPM-per-square-foot to inches per hour. Some irrigation texts round it to 96.3; this tool uses the exact value.
Check the nozzle chart from the manufacturer for your model, nozzle size, and operating pressure. Typical fixed spray heads run 1–3 GPM and residential rotors roughly 2–5 GPM depending on the arc and nozzle.
It is a widely used starting point for established lawns in the growing season, counting rainfall. Sandy soils favor splitting it into more frequent smaller doses; clay soils absorb slowly, so use cycle-and-soak run times.