How This Calculator Works

A simple, research‑based way to figure out exactly how many minutes to run each of your sprinkler zones—so you save water, money, and keep your lawn green.

In short: We look at how much water your grass actually needs based on the weather (FAO‑56 Penman–Monteith ET), subtract effective rainfall, and translate what’s left into minutes per zone using your system’s application rate.

1) Weather Tells Us How Thirsty Your Grass Is

Grass loses water each day through heat, sun, and wind. Scientists call this ET (short for evapotranspiration). Public weather networks publish ET values so we know how much water grass would use if water were not limiting.

Studies show you don’t need to replace 100% of that water to keep a healthy lawn. Most home lawns look great with about 70–80% of ET replaced in summer, and 60–70% in spring/fall (Romero & Dukes, 2009; Carrow et al., 2022).

2) Giving Credit for Rainfall

When it rains, your sprinklers can take a break. But not all rain soaks into the soil—some runs off or evaporates. That’s why we only count the portion that actually benefits the roots, called effective rainfall.

3) How Fast Your Sprinklers Apply Water

Every sprinkler system puts out water at a certain speed. The calculator asks for your minutes per inch value (Rate). You can figure this out by:

Cup test (best way): Place a few cups on the lawn, run sprinklers for 15–20 minutes, and measure how much water collected. From this, calculate how many minutes it takes your system to apply 1 inch.
Specs/math: You can use gallons per minute and area formulas, but real‑world testing is more reliable (especially with mixed heads and pressure differences).

4) Putting It All Together

The calculator uses the following formula to determine runtime per irrigation event:

Runtime (minutes) = (W ÷ 7) × (ET × 0.75 − R) × Rate

W = watering frequency in days (e.g., water every 3 days)
ET = weekly evapotranspiration in inches for your location
R = weekly total rainfall in inches for your location
Rate = minutes it takes your sprinklers to apply 1 inch of water

Following this formula, the result is the number of minutes per zone you should run your sprinklers. If that time is long, split it into two or three shorter “cycle‑and‑soak” sessions to reduce runoff and improve infiltration.

Why This Works

This approach is backed by decades of turfgrass and irrigation research. Universities and organizations like the Irrigation Association and EPA WaterSense recommend weather‑based irrigation and water budgeting that accounts for ET and rainfall.

References (Research & Guidelines)

Want to see this with your own lawn’s numbers? Go back to the calculator, enter your location and sprinkler rates, and it will show you exactly how many minutes to water each zone.