Sprinkler run-time calculator

“Water half an inch” is useless until you know how long that takes on your system. A catch-can test tells you, once, and then the answer holds all season.

The test takes fifteen minutes

Put several straight-sided containers — tuna tins are the classic — around the zone, run it for a set time, then measure the depth in each and average them. That average, over that time, is your precipitation rate in inches per hour. Every watering instruction you will ever read is given in inches, so this is the number that converts advice into minutes on the timer.

Test each zone separately. Rotors and spray heads differ by a factor of three or more, and a single run time across mismatched zones is how one half of a lawn drowns while the other bakes.