How long should I run my sprinklers?
Long enough to put down what the lawn needs this week, in as few sessions as the soil allows. The first half of that is arithmetic: inches per week ÷ sessions per week ÷ your sprinklers' inches per hour × 60. The second half is where most schedules go wrong — running every day for ten minutes, which wets the top inch and grows shallow roots, instead of twice a week for long enough to soak six inches down.
One inch a week from rotors at 0.5 in/hr, two days a week: 0.5 in per session ÷ 0.5 × 60 = 60 minutes, twice. The same inch from spray heads at 1.5 in/hr is 20 minutes twice — but on clay that has to be four 5-minute cycles with soaks between, or it runs down the sidewalk.
Minutes per session by sprinkler rate
| Rate | Minutes per inch | 1 in/week, 2 days | 1 in/week, 3 days |
|---|---|---|---|
| 0.4 in/hr | 150 | 75 min × 2 | 50 min × 3 |
| 0.6 in/hr | 100 | 50 min × 2 | 33 min × 3 |
| 1 in/hr | 60 | 30 min × 2 | 20 min × 3 |
| 1.5 in/hr | 40 | 20 min × 2 | 13 min × 3 |
| 2 in/hr | 30 | 15 min × 2 | 10 min × 3 |
Before rain is subtracted and before cycle-and-soak splitting. Your rate comes from the can test.
How much water does a lawn need each week?
About an inch a week in the growing season is the standard rule, and it's a reasonable average: more in July, less in April and October, none when the grass is dormant. It includes rain — a half-inch storm is half the week's watering, which is why the calculator asks. The planner's monthly targets:
| Month | Cool-season | Warm-season |
|---|---|---|
| April | 0.5 in | 0.4 in |
| May | 1 in | 0.8 in |
| June | 1 in | 0.8 in |
| July | 1.25 in | 1 in |
| August | 1 in | 0.8 in |
| September | 1 in | 0.8 in |
| October | 0.5 in | 0.4 in |
Rules of thumb from the planner template. A controller that adjusts to local evapotranspiration (ET) does this properly, day by day; these are the right numbers to set a dumb timer to.
Roots grow where the water is. Ten minutes every morning keeps the top inch wet and the roots in the top inch, which means the lawn wilts on the first hot afternoon the sprinklers miss. The same weekly inch in two sessions wets the soil six inches down and roots follow it — and a lawn with six-inch roots shrugs off a week of heat. Sandy soil is the exception: it can't hold a half-inch at once, so three lighter sessions beat two.
Cycle and soak: when the soil can't keep up
Spray heads put water down at 1.5 in/hr; clay soil absorbs 0.2. After a few minutes the surface is full and the rest runs off — onto the driveway, into the street, anywhere but the roots. The fix is cycle and soak: split each session into short cycles with 30–60 minutes between them for the water to sink in. The calculator sizes the cycles from the soil's intake rate (sandy 1, loam 0.5, clay 0.2 in/hr, reduced 25% on moderate slopes and 50% on steep ones) and your sprinkler rate; most controllers can run the same zone multiple times in a program, which is all cycle-and-soak is.
| Soil | Heads | Session | Max per cycle | Run it as |
|---|---|---|---|---|
| sandy | Rotors, 0.5 in/hr | 60 min | no limit | one cycle |
| sandy | Sprays, 1.5 in/hr | 20 min | 20 min | one cycle |
| loam | Rotors, 0.5 in/hr | 60 min | no limit | one cycle |
| loam | Sprays, 1.5 in/hr | 20 min | 10 min | 2 × 10 min, 45 min soaks |
| clay | Rotors, 0.5 in/hr | 60 min | 12 min | 5 × 12 min, 45 min soaks |
| clay | Sprays, 1.5 in/hr | 20 min | 5 min | 4 × 5 min, 45 min soaks |
½ in per session (1 in/week over 2 days), flat ground. Rotors on loam and sand don't out-run the soil; sprays on clay always do.
What the week costs
An inch of water on a lawn is 623 gallons per 1,000 sq ft, so a 5,000 sq ft lawn drinks about 3,115 gallons a week in summer — at $6 per thousand, roughly $81 a month. Your water bill states the rate per 1,000 gallons (or per CCF — one CCF is 748 gallons); enter it and the calculator prices each week, which is also the honest baseline for judging a smart controller: it can only save what you were over-applying.
| Lawn | Gallons per inch | Per month at 1 in/week |
|---|---|---|
| 2,500 sq ft | 1,558 | 6,744 gal |
| 5,000 sq ft | 3,115 | 13,488 gal |
| 7,500 sq ft | 4,673 | 20,232 gal |
| 10,000 sq ft | 6,230 | 26,976 gal |
0.623 gal per sq ft per inch; 4.33 weeks per month. Subtract the rain.
When to run it
Early morning — between about 4 and 8 a.m. — when wind is low, evaporation is low, and the grass dries quickly once the sun is up. Evening watering leaves the blades wet all night, which is how fungal disease starts; midday loses a third to evaporation and wind. Put the two sessions a few days apart (Tuesday and Friday, say) rather than back to back, and skip one when the rain gauge says so.
Sources: USDA NRCS, Irrigation Guide (soil intake rates); Irrigation Association, Landscape Irrigation Best Management Practices (cycle-and-soak, run-time math); Colorado State Extension, Watering Established Lawns; Texas A&M AgriLife, Lawn Watering.