How it works
The formula
rain a week = the month's normal rain ÷ days × 7
water to add = need − rain a week
minutes = water to add ÷ sprinkler inches an hour × 60
water to add = need − rain a week
minutes = water to add ÷ sprinkler inches an hour × 60
- An inch a week: University of Nebraska Extension puts it at about an inch a week, rain included, and 1.5 to 2 in. in the hottest part of summer; Oklahoma State, UGA and Texas A&M extension say the same.
- Rain: NOAA's 1991-2020 normals for the station nearest your county's center of population. That's an average month: skip watering after a good rain, and water more in a drought.
- Tuna-can test: set a few flat cans around the lawn, run the sprinklers 15 minutes, average the depth. That's your output; sprinklers vary a lot, so measure rather than guess.
- Water deeply once or twice a week rather than a little every day, early in the morning. Split it if water runs off before it soaks in.
Sprinkler time in July, by city
| City | July rain a week | Sprinkler a week |
|---|---|---|
| Phoenix | 0.21 in. | 48 min |
| Las Vegas | 0.09 in. | 55 min |
| Los Angeles | 0.00 in. | 60 min |
| Denver | 0.51 in. | 30 min |
| Salt Lake City | 0.17 in. | 50 min |
| Dallas | 0.39 in. | 37 min |
| Atlanta | 1.25 in. | none |
| Chicago | 0.80 in. | 12 min |
| New York | 1.04 in. | none |
| Seattle | 0.13 in. | 52 min |
| Minneapolis | 1.02 in. | none |
| Miami | 1.66 in. | none |
An inch a week, sprinklers putting out 1 in. an hour (0.25 in. in 15 minutes).
Questions people ask
How long should I water my lawn?
Long enough to give it about an inch a week with the rain. With sprinklers that put out 1 in. an hour, that's an hour a week in a dry spell, best as two half-hour waterings. Your sprinklers may be faster or slower: do the tuna-can test.
How often should I water?
Once or twice a week, deeply, so the roots grow down. Daily light watering keeps roots shallow and the lawn less drought-tolerant.
What time of day is best?
Early morning: less water evaporates and the grass dries during the day, which helps keep disease down.