Lawn Watering Calculator
Gallons and sprinkler minutes from lawn area, inches, flow, and days.
What it does
Sprinkler timers speak minutes while lawns think inches, and the bridge is one fixed constant. An inch of water over a square foot is 144 cubic inches, and dividing by 231 per gallon gives 0.623 gallons. The tool above multiplies that across any lawn with flow and days.
The imperial default waters 1000 ft2 with 1 inch at 10 gallons a minute across 2 days. One session needs 623 gal, runs 62 minutes, and the week totals 1246 gal. Metric mode tells the same story rounder: 100 m2 at 25 mm is 2500 liters per session.
Extension guides from Iowa State and Maryland agree on the targets behind the defaults. Lawns want about an inch a week including rain, delivered deep and infrequently so roots chase moisture downward, ideally early morning to cut disease and evaporation. Iowa State adds the runtime rule directly: gallons divided by gallons per minute.
Split the minutes the tool returns. Heavy clay and slopes shed a long soak as runoff, so two or three shorter passes water more than one marathon. Subtract rainfall first: half an inch from the sky means the system owes only the other half.
How to use
- Pick imperial (ft, gal) or metric (m, L).
- Type the lawn area being watered.
- Type inches or mm per session.
- Type the running zone flow rate.
- Set days per week and split the runtime.
Formula
- 0.623
- Gallons per ft2 per inch (144/231).
- flow
- Running zone delivery in GPM or L/min.
- days
- Watering sessions per week.
Guides round 623.4 gallons per 1000 ft2 to 624; this page keeps the exact 0.623 factor. Rain gauges and can tests beat label flow rates for real systems.
Worked example
Example: 1000 ft2, 1 inch, 10 GPM
Imperial defaults: 1000 ft2, 1 in, 10 GPM, 2 days.
- Gallons: 1000 x 1 x 0.623 = 623 gal.
- Minutes: 623 / 10 = 62 min.
- Weekly: 623 x 2 = 1246 gal/week.
- Sessions: split 62 min into two 31 min mornings.
- Tool shows 623 gal, 62 min, 1246 gal/week.
Reference table
| Lawn | Per session | Two days |
|---|---|---|
| 500 ft2 | 312 gal | 623 gal/week |
| 1000 ft2 | 623 gal | 1246 gal/week |
| 2000 ft2 | 1246 gal | 2492 gal/week |
| 5000 ft2 | 3115 gal | 6230 gal/week |
Limits
- Flat math; slopes and clay shed water as runoff.
- Label flow assumed; measure real GPM with a bucket.
- Rainfall must be subtracted by hand.
- One inch fits most lawns, not new seed or sod.
Common mistakes
- Watering daily shallow instead of deep twice.
- Running evenings that leave blades wet overnight.
- Trusting label GPM without a bucket test.
- Ignoring rainfall in the weekly total.
- One long soak on clay that runs to the street.
Try it yourself
Quick checks. Answers sit tucked under each question.
500 ft2, 1 in, 8 GPM, 2 days?
312 gal, 312 / 8 = 39 min, 623 gal/week.
100 m2, 25 mm, 20 L/min, 2 days?
2500 L, 125 min, 5000 L/week.
2000 ft2 at 1 inch per session?
2000 x 0.623 = 1247 gal.
Questions
How many gallons for 1000 ft2?
623 per inch of water.
Where does 0.623 come from?
144 cubic inches per ft2-inch divided by 231 per gallon.
How long to run sprinklers?
Gallons divided by GPM; 623 gal at 10 GPM is 62 min.
How often should I water?
Deep sessions twice a week beat daily sprinkles.
Best time of day?
Early morning, before 10 a.m.
Does rain count?
Yes. Subtract gauge inches from the weekly inch first.