Lawn Watering Calculator
Calibrate your sprinkler with the tuna-can test, then get minutes per session to hit your weekly watering target.
Step 1: the tuna-can test
Set a few flat-bottomed cans (a tuna can works well) around the lawn, run the sprinkler for a set time, then measure the depth collected.
Step 2: your watering plan
Lawns generally want about 1 inch of water per week including rain. Fewer, deeper sessions (2 per week is a common starting point) grow deeper roots than watering a little every day.
How it works
Every sprinkler and every water pressure setup puts down water at a different rate, so "water for 20 minutes" means something different in every yard. The tuna-can test fixes that: run the sprinkler for a known time, measure how much water landed in a flat-bottomed can, and you get your system's actual rate in inches per hour. From there, the calculator works backward from a weekly watering target, minus any rain that already fell, split across however many sessions you water per week.
Worked example: the can collects a quarter inch in 15 minutes, so the rate is 1 inch per hour. Aiming for the standard 1 inch per week, with no rain and 2 sessions per week, each session needs to deliver half an inch. At 1 inch per hour, half an inch takes 30 minutes. If it rained half an inch that week, the remaining half inch splits into two 15-minute sessions instead.
FAQ
Why deep and infrequent instead of a little every day?
Light daily watering only wets the top inch or two of soil, so roots stay shallow and the lawn struggles the moment you skip a day or two. Watering deeply but less often pushes moisture further down, which encourages roots to follow it, and a deep-rooted lawn handles heat and short dry spells much better.
What if my cans collect different amounts in different spots?
That's common and usually means uneven sprinkler coverage. Average the readings for a rough overall rate, and consider adjusting sprinkler heads or overlap in the driest spots so the whole lawn gets closer to the same depth.
Should I water in the morning or evening?
Early morning is generally best. It gives the grass blades time to dry before nightfall, which cuts down on the damp conditions that fungal lawn disease needs to take hold, and it loses less water to evaporation than watering in the heat of the day.
Do I need to water on rainy weeks at all?
If rainfall alone meets or beats your weekly target, the calculator will show 0 minutes per session for that week, since watering on top of a soaked lawn just wastes water and can encourage disease.
For more on watering rhythm and setup, see how often and how deep to water your lawn and how to set up and adjust a lawn sprinkler.