Do the tuna-can test once — get your sprinkler minutes per session and a weekly schedule that builds deep roots.
Run sprinklers 15 min, measure the can. No test yet? 0.25 is a typical oscillating-sprinkler answer.
Run each session
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Your sprinkler's output
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Still needed after rain
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Weekly total run time
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Water early morning. If runoff starts before the session ends (slopes, clay), split the session into two back-to-back shorter ones — "cycle and soak."
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The watering math
output (in/hr) = can depth in 15 min × 4 minutes per session = (target − rain) ÷ sessions ÷ output × 60
Every "how long should I water" answer that skips the can test is a guess — sprinkler output varies 5× between an oscillating sprinkler on low pressure and in-ground rotors. Fifteen minutes with tuna cans converts your actual system into actual minutes, once, forever (re-test only if you change sprinklers).
A worked example
Cans catch a quarter inch in 15 minutes (1 in/hr — a common answer). Target 1 inch, no rain, 3 sessions: 20 minutes per session, an hour a week total. A half inch of Tuesday rain cuts it to two 15-minute sessions. Sandier soil in a heat wave at 1.5 inches: 30 minutes × 3.
Reading the lawn itself
The grass votes too: a blue-gray cast, footprints that stay pressed, and folded blades all say thirsty — water that day. Screwdriver test: if it slides 6 inches into the soil easily, moisture is fine. Pair the schedule with the rightmowing height — tall grass shades soil and cuts water needs measurably — and feed on schedule with thefertilizer calculator.
Frequently asked questions
How long should I run my sprinklers?
Until the lawn has received about 1 to 1.5 inches for the week, counting rain — and that translates to wildly different minutes depending on your sprinkler. The tuna-can test (below) measures YOUR system's output; this calculator turns that into a weekly schedule. Typical answers land between 20 and 60 minutes per session, 2–3 sessions a week.
What is the tuna can test?
Set a few empty tuna cans (or any straight-sided containers) around the lawn, run the sprinklers for exactly 15 minutes, and measure the water depth with a ruler. Average the cans — that's your system's output per 15 minutes. It's the ten-minute measurement that makes every watering question answerable.
Is it better to water deeply or frequently?
Deep and infrequent, always: 2–3 sessions a week that each soak several inches down train roots to grow deep, which is what survives August. Daily light sprinkles keep roots at the surface and invite fungus. The exception is new seed, which needs the surface kept moist with short daily waterings until germination.
When should I water — morning or evening?
Early morning (roughly 4–9am): winds are calm, evaporation is low, and blades dry off during the day. Evening watering leaves grass wet all night — the classic fungus recipe. Midday isn't harmful, just wasteful (20–30% can evaporate before it lands).