Sprinkler Run Time Calculator

How to use this calculator

1. Pick your grass type and season. The calculator suggests a weekly water target. You can type your own number instead.
2. Enter recent or expected rainfall for the week, in inches. Rain counts toward the target, so the calculator only schedules the difference.
3. Choose how many days a week you water. Two or three deep waterings suit most established lawns.
4. Choose your soil type. This decides whether a long run needs to be split into shorter cycles to avoid runoff.
5. Add each zone. Pick the sprinkler type, or enter a measured rate from a catch-cup test. Add the zone’s area if you want to see how many gallons you use.

The formula behind it

Every number in the schedule comes from three simple steps:

• Water needed per watering day = (weekly target − rainfall) ÷ watering days per week
• Minutes per watering day = water needed per day ÷ precipitation rate (inches per hour) × 60
• Gallons = area in square feet × inches applied × 0.623

The last figure comes from the fact that one inch of water spread over one square foot is about 0.623 gallons, which works out to roughly 623 gallons per 1,000 square feet.

Worked example

Say your cool-season lawn needs about 1 inch of water this week, the weather forecast expects 0.25 inch of rain, and you water twice a week on loam soil.

• Water needed per watering day: (1 − 0.25) ÷ 2 = 0.375 inch
• A zone of fixed spray heads applying 1.6 inches per hour: 0.375 ÷ 1.6 × 60 ≈ 14 minutes, split into 2 cycles of 7 minutes
• A zone of rotors applying 0.6 inch per hour: 0.375 ÷ 0.6 × 60 ≈ 38 minutes, split into 2 cycles of 19 minutes
• If the rotor zone covers 1,500 square feet, each watering day uses about 1,500 × 0.375 × 0.623 ≈ 350 gallons

Notice how different the two zones are. Spray heads and rotors apply water at very different speeds, which is why a single run time for every zone almost always over-waters some areas and under-waters others.

Typical sprinkler precipitation rates

These ranges are only a starting point. Pressure, nozzle size, head spacing and wear all change the real rate, so a catch-cup test is worth the 15 minutes it takes.

How to measure your real rate with a catch-cup test

1. Set 6 to 8 straight-sided containers around one zone: some near the heads, some in between.
2. Run that zone for exactly 15 minutes.
3. Measure the depth in each container with a ruler and average the readings.
4. Multiply the average by 4. That is the zone’s rate in inches per hour. Enter it in the calculator as a measured rate.

If the depths vary a lot between containers, your coverage is uneven. Check for blocked nozzles, tilted heads or low pressure before adjusting run times.

Why the calculator splits long runs into cycles

Soil can only absorb water so fast. Clay soaks it up slowly, sand quickly. When a sprinkler applies water faster than the soil can take it in, the extra runs off onto the sidewalk or pools in low spots. The calculator limits how much water each cycle applies based on your soil type, then splits the day’s watering into enough cycles to stay under that limit. Leave 30 to 60 minutes between cycles so the water can sink in. Most irrigation controllers let you set several start times for this.

These per-cycle limits are rules of thumb. If you still see runoff, add another cycle; if the soil stays dry an inch or two down after watering, lengthen the runs.

Tips for a healthier lawn with less water

• Check the soil, not just the clock. Push a screwdriver into the lawn after watering. It should slide in easily to a depth of about 6 inches.
• Adjust with the seasons. Lawns need far less water in spring and fall than in midsummer. Re-run the calculator whenever the weather changes.
• Raise your mowing height. Taller grass shades the soil and holds moisture longer.
• Follow local rules. Many towns limit watering days or hours, especially in summer. Set your watering days to match.

Frequently asked questions