Watering
Irrigation Calculator
Find out how much water your lawn or garden actually needs each week, in both inches and gallons, and how to split it across the week for your soil type.
Updated
Area to water
Add a section for each part of the space. Odd shapes are easiest as two or three simple ones.
Area = length × width
Watering target
1 in is standard for established cool-season turf; 1.5 in in real summer heat.
Rain counts toward the total, subtract it rather than watering on top.
The ideal. Deep, infrequent watering works well here.
Check your water bill. US residential rates commonly fall between $0.004 and $0.02 per gallon.
Enter your details to see water needs.
Build a watering schedule
Tell the assistant your climate, soil, and grass and it will turn these figures into a seasonal watering schedule.
Written by AI from the figures above. The quantities themselves are calculated deterministically and are never generated by AI. Check anything consequential, permits, utility locates, and chemical labels especially, before acting on it.
How this is calculated
Inches to gallons
Watering guidance is written in inches because that is how rainfall is measured, and rain and irrigation should be counted together. Converting to gallons takes one constant: one inch of water over one square foot is 0.6234 US gallons.
That works out to 623 gallons per 1,000 sq ft per inch, a figure worth knowing, because it makes the scale of lawn watering concrete. An inch a week on a 6,000 sq ft lawn is close to 3,750 gallons.
gallons = area in sq ft × inches of water × 0.6234
How much, and how often
About 1 inch per week is the standard target for established cool-season lawns, rising to 1.5 inches in genuine summer heat. Warm-season grasses generally need less. Count rainfall against the total rather than watering on top of it.
Frequency should follow soil, not habit. Sandy soil holds little water and needs smaller amounts more often. Clay holds plenty but accepts it slowly, so it wants fewer, longer, cycled sessions.
The goal in every case is deep, infrequent watering. Light daily sprinkling wets only the top inch, which is exactly where you do not want roots concentrated when the heat arrives.
| Soil | Intake rate | Holds per foot | Pattern for 1 in/week |
|---|---|---|---|
| Sand | 2.0 in/hr | 0.75 in | 3 sessions of ⅓ in |
| Sandy loam | 1.0 in/hr | 1.25 in | 2 sessions of ½ in |
| Loam | 0.5 in/hr | 1.75 in | 1–2 sessions |
| Clay loam | 0.3 in/hr | 2.0 in | 1–2 sessions, cycled |
| Clay | 0.15 in/hr | 2.25 in | 1 session, cycled 3× |
Drip is scheduled differently
Drip irrigation is not measured in inches, because it does not wet the whole area. It is measured in gallons per plant, and emitters are rated in gallons per hour.
Run time is simply the gallons a plant needs divided by the flow reaching it. A shrub wanting 5 gallons a week, fed by two 1 GPH emitters, needs 2.5 hours of run time per week, split into two sessions rather than delivered all at once.
Worked example
Weekly water for a suburban lawn
A 6,500 sq ft lawn in midsummer, targeting 1.25 inches per week, with 0.3 inches of rain forecast.
- Net irrigation needed
- 1.25 − 0.3 = 0.95 in
- Gallons per inch
- 6,500 × 0.6234 = 4,052 gal
- Weekly gallons
- 4,052 × 0.95 = 3,850 gal
- Split for loam
- 2 sessions of about 0.48 in each
About 3,850 gallons for the week, best delivered in two sessions. On a spray zone at 1.5 in/hr that is roughly 19 minutes per session.
Frequently asked questions
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