Watering by the numbers
Most lawns need about 1 inch of water a week, from rain and irrigation combined. Sprinklers apply water at very different rates, so “20 minutes per zone” can mean a soaked lawn with one system and a parched one with another. This calculator turns your lawn’s need into minutes per watering day.
The calculation
- Net need = weekly need − rain received
- Gross need = net need ÷ irrigation efficiency
- Per watering day = gross need ÷ number of watering days
- Run time = depth per day ÷ precipitation rate × 60
Efficiency accounts for water lost to wind, evaporation and uneven coverage. Typical values are 65 to 75% for spray heads and 75 to 85% for rotors.
Worked example
A 2,000 sq ft lawn needs 1 inch a week, 0.25 inch of rain is expected, the zone uses spray heads (about 1.5 in/h) and you water 3 days a week at 75% efficiency:
| Result | Value |
|---|---|
| Run time per watering day | 13 minutes |
| Water applied per day | 0.33 in |
| Total run time per week | 40 minutes |
| Water used per week | 1,246 gallons |
Each inch of water on 1,000 sq ft is 623 gallons.
Typical precipitation rates
| Sprinkler type | Rate |
|---|---|
| Fixed spray heads | 1.0–2.0 in/h |
| Rotors (gear-driven) | 0.4–0.8 in/h |
| Rotary (multi-stream) nozzles | 0.4–0.6 in/h |
| Hose-end oscillating | 0.2–0.5 in/h |
For an accurate figure, do a catch-cup test: place six or more straight-sided containers across the zone, run it for 15 minutes, average the depths and multiply by 4.
Watering well
- Water deeply and less often. Two or three sessions a week build deeper roots than daily sprinkling.
- Avoid runoff. Clay soils absorb only about 0.2 to 0.4 inch per hour. If water puddles or runs off, split the time into two cycles an hour apart (cycle and soak).
- Water early. Between about 4 and 10 a.m., wind and evaporation are low and grass dries during the day.
- Adjust with the seasons. Lawns need roughly half as much water in spring and fall as in midsummer. A smart controller or rain sensor adjusts automatically.
Many cities set watering days and restrictions, especially during droughts; follow local rules.