Sprinkler Run Time Calculator

How run time is calculated

Sprinkler run time comes down to one division. You know how much water the lawn needs, and you know how fast your sprinkler puts water down. The run time is the first number divided by the second.

Minutes = inches needed divided by the precipitation rate, times 60. A zone that delivers 0.5 inch per hour needs 120 minutes to put down 1 inch. A zone that delivers 1.5 inches per hour needs 40 minutes for the same inch.

Two details change how you use that number. First, lawns do better with deep, infrequent waterings than with a light sprinkle every day, so the weekly total is usually split into 2 or 3 sessions rather than spread across 7. Second, most soils cannot absorb water as fast as a sprinkler puts it down, so a long run time wastes water to runoff. Splitting each session into cycles with a soak in between fixes that, and the calculator does the split for you.

Finding your sprinkler's precipitation rate

If you do not know the rate, there are two ways to get it.

The catch cup test. Scatter 4 to 6 straight-sided cans around the zone, run the sprinkler for 15 minutes, then measure the depth of water in each can with a ruler. Add the depths and divide by the number of cans to get the average, then multiply by 4 to turn 15 minutes into an hour. An average of 0.2 inch becomes a rate of 0.8 inch per hour.

The flow and area formula. If you know the flow at the zone and the area it covers, use rate = 96.25 x gallons per minute divided by square feet. A zone flowing 10 gallons per minute over 2,000 square feet runs at 96.25 x 10 / 2,000, which is 0.48 inch per hour. That is the figure the calculator uses when you pick the flow method.

For reference, typical rates run about 1.0 to 1.6 inches per hour for fixed spray heads, 0.4 to 0.6 for rotary nozzles and 0.2 to 0.5 for gear driven rotors. A zone with a mix of head types will not have one clean rate, which is another reason the catch cup test beats a number off a catalog page.

Worked example: a pop-up spray zone at 1.5 inches per hour

A pop-up spray zone with a measured rate of 1.5 inches per hour, a target of 1 inch a week, and 2 waterings a week.

• Depth per watering: 1 inch divided by 2 = 0.5 inch
• Run time per watering: 0.5 divided by 1.5, times 60 = 20 minutes
• Weekly run time: 20 x 2 = 40 minutes

If the zone covers 1,000 square feet, that 20-minute run puts down about 312 gallons. Half an inch over 1,000 square feet is 41.7 cubic feet of water, and a cubic foot holds 7.48 gallons.

Twenty minutes is a short run for a spray zone, and that is the point. Spray heads put water down fast, so the risk is runoff rather than underwatering. If water starts sheeting off the lawn after 12 minutes, split the 20 minutes into two cycles of 10 with a 30-minute soak in between.

Worked example: a rotor zone covering 2,000 square feet

Now a gear driven rotor zone, 2,000 square feet, flowing 10 gallons per minute.

• Precipitation rate: 96.25 x 10 / 2,000 = 0.48 inch per hour
• Target: 1 inch a week
• Weekly run time: 1 divided by 0.48, times 60 = 125 minutes
• Split into 2 waterings: 62 minutes each
• Water used: 2,000 x (1/12) x 7.48 = 1,247 gallons a week

At $6 per 1,000 gallons of water and sewer, that is $7.48 a week, or about $150 across a 20-week season. Rotors take a long time because they apply water slowly, which is also why they waste less to runoff than spray heads do.

Three zones like that come to about $450 a season and 3,741 gallons a week. That is the number worth putting next to a smart controller, which schedules around the weather and typically cuts outdoor water use by 15 to 30%.

Cycle and soak: how to stop water running off

How long you can run before water starts running off depends on the soil. These are practical starting points.

The clay row is the one that surprises people. Clay can absorb water at only 0.1 to 0.2 inch per hour, so a 120-minute run becomes seven short cycles with an hour of soak in between. It takes most of a morning, and it uses far less water than running the zone straight through, because the water ends up in the soil instead of the gutter.

Walk the zone during a run and watch for sheeting, puddling or water reaching the curb. That tells you more about your soil than any table does.

How much water a lawn needs by season

The standard figure is about 1 inch of water a week, counting rainfall. That number comes from decades of turf research and it is what most extension services and the EPA's WaterSense program recommend.

A lawn that is getting enough water springs back from a footprint within a minute or two. If the grass stays flattened, it is dry. If the soil is soggy an hour after you water, you are overdoing it.

Water early in the morning, between about 4 and 9 a.m. Evening watering leaves the blades wet overnight, which is how fungus gets started, and midday watering loses a share to evaporation before it reaches the roots.

What the water costs

Water is cheaper than most people assume, and the sewer charge is often the bigger half of the bill.

A typical US household pays somewhere between $4 and $12 per 1,000 gallons for water and sewer combined, depending on the city. The calculator takes whatever figure is on your bill. At $6 per 1,000 gallons, the 1,247 gallons a week from the rotor example costs $7.48.

Two ways to cut that without hurting the lawn. First, check the controller after every rain and skip the cycle, which is what a $30 rain sensor does automatically. Second, raise the mower deck. Taller grass shades the soil, so the same inch of water goes further, and most cool season lawns do better at 3 to 3.5 inches than at 2.

For the full guidance on how much to water and when, the EPA's WaterSense watering tips and its landscaping tips are the plainest sources. The University of Missouri Extension guide to lawn irrigation goes deeper on precipitation rates and runoff, and Penn State Extension covers the soil and turf basics underneath all of it.