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Generated on: July 20, 2026 at 09:33 PM

Lawn Watering Calculator

Inputs

Lawn area100 m²
Weekly water need25 mm
Rainfall this week0 mm
Application rate (per hour)15 mm
Watering days per week2

Estimates assume reasonably even sprinkler coverage and typical soil. Heavy clay sheds water if applied too fast (let it soak in stages), while sand drains quickly and needs more frequent watering. Follow any local watering restrictions, which may override these figures.

Generated with OneCalc · https://onecalc.app/en/lifestyle/lawn-watering

Lifestyle

Lawn Watering Calculator

Find how long to run your sprinklers. Enter lawn area, the weekly water your grass needs, recent rainfall, and your sprinkler’s application rate to get run time per session and total water used.

Inputs

Lawn & Sprinkler

≥ 0.1 mm
1 – 7

Results

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Watering Schedule

Details

Your lawn needs ... of water this week (... over the area). At your sprinkler’s rate that is ... of running — about ... per session.

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Watering by the Numbers, Not by Habit

Most lawns are watered on a fixed timer that ignores the weather — too much in spring, too little in a heatwave, and straight through the rain. The better approach is to start from how much water the grass actually needs, subtract what the sky already delivered, and run the sprinkler just long enough to make up the difference.

A 100 m² lawn needing 25 mm of water a week takes 2,500 L. At a sprinkler rate of 15 mm/hr that is 100 minutes of running — split into two 50-minute sessions.

A 1,076 sq ft lawn needing 1 inch of water a week takes about 660 gallons. At a sprinkler rate of 0.6 in/hr that is roughly 100 minutes of running — split into two 50-minute sessions.

The Idea: Depth, Not Time

Lawn water need is measured as a depth — like rainfall. "An inch a week" means that if you caught all the water on a flat tray, it would pool an inch deep, everywhere. Your sprinkler also applies water as a depth over time (its application rate, in mm or inches per hour). Knowing both lets you convert a depth into a run time.

Net depth=Weekly need−Rainfall\text{Net depth} = \text{Weekly need} - \text{Rainfall} Run time=Net depthApplication rate,Volume=Net depth×Area\text{Run time} = \frac{\text{Net depth}}{\text{Application rate}}, \qquad \text{Volume} = \text{Net depth} \times \text{Area}

Worked Example

  • Area: 100 m², Weekly need: 25 mm, Rainfall: 0 mm
  • Application rate: 15 mm/hr, over 2 days
Run time=2515=1.67 h=100 min  ⇒  50 min per session\text{Run time} = \frac{25}{15} = 1.67 \text{ h} = 100 \text{ min} \;\Rightarrow\; 50 \text{ min per session} Volume=0.025 m×100 m2=2.5 m3=2,500 L\text{Volume} = 0.025 \text{ m} \times 100 \text{ m}^2 = 2.5 \text{ m}^3 = 2{,}500 \text{ L}
  • Area: 1,076 sq ft, Weekly need: 1 inch, Rainfall: 0 inch
  • Application rate: 0.6 in/hr, over 2 days
Run time=10.6=1.67 h=100 min  ⇒  50 min per session\text{Run time} = \frac{1}{0.6} = 1.67 \text{ h} = 100 \text{ min} \;\Rightarrow\; 50 \text{ min per session}

Find Your Application Rate: The Can Test

You cannot look this up reliably — it depends on your heads, spacing, and water pressure. Measure it:

  1. Scatter several straight-sided cans (tuna or cat-food tins) across the zone.
  2. Run the sprinkler for 15 minutes.
  3. Measure the water depth in each can and average them.
  4. Multiply by four to get the depth per hour.

The cans also expose uneven coverage: if some fill twice as fast as others, dry patches are coming. Adjust head spacing or arcs until the catch is reasonably even.

Deep and Infrequent Wins

Splitting the weekly need into one to three long sessions sends roots downward chasing the moisture, which makes the lawn far more drought-tolerant. Daily sprinkles keep water at the surface, breeding shallow roots, fungus, and weeds. If your soil is heavy clay and water starts to run off before the session ends, water in two shorter bursts an hour apart so it has time to soak in.

Adjust With the Weather

Water need follows evapotranspiration — it climbs with heat, sun, and wind, and falls when it is cool, humid, or overcast. Raise the weekly figure in midsummer, cut it after good rain, and taper off in autumn. The point of starting from a number is that you can change it: re-run the schedule whenever the season turns instead of letting one setting run all year.

Frequently Asked Questions (FAQ)

How much water does a lawn need?

A common guideline for established cool-season grass is about 25 mm (1 inch) per week, including rainfall, delivered in one to three sessions. Warm-season grasses often need less. In a heatwave the figure can climb to 40 mm or more; in cool, overcast weeks it drops. Watch the grass: a bluish tint and footprints that linger mean it is thirsty.

How do I find my sprinkler’s application rate?

Use the “can test.” Place several straight-sided cans (tuna or cat-food tins work) around the zone, run the sprinkler for 15 minutes, then measure the water depth in each and average them. Multiply by four to get the depth per hour. This also reveals uneven coverage — cans that fill at very different rates mean heads need adjusting.

Why water deeply and less often?

Long, infrequent soakings pull roots downward, chasing the moisture, which makes the lawn more drought-tolerant and resilient. Brief daily sprinkles keep water near the surface, encouraging shallow roots and disease. Splitting the weekly need into one to three deep sessions is better than watering a little every day.

Should I change the schedule with the seasons?

Yes. Water need tracks evapotranspiration, which rises with heat, sun, and wind and falls in cool, humid, or cloudy weather. Increase the weekly figure in midsummer and cut it back — or skip watering entirely — after good rain or in autumn. Re-run the numbers whenever conditions shift rather than leaving the timer fixed all year.

Disclaimer

Estimates assume reasonably even sprinkler coverage and typical soil. Heavy clay sheds water if applied too fast (let it soak in stages), while sand drains quickly and needs more frequent watering. Follow any local watering restrictions, which may override these figures.

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