WSU Irrigation Calculator: How to Use It

The WSU Irrigation Calculator is a free online tool from Washington State University that estimates how much water your lawn or landscape needs each week. You enter your location, plant type, soil type, and sprinkler system, and it returns a recommended run time in minutes per watering day. It is based on decades of local evapotranspiration (ET) data, so the numbers are tailored to your specific climate rather than a generic national average.

Formula

The WSU calculator itself uses a more complex formula than the simple volume calculator above, but the core principle is the same: water needed equals the area times the depth of water applied. For the volume calculator shown here, the formula is:

Volume (ft³) = Length (ft) × Width (ft) × Depth (in) / 12

To convert to cubic yards, divide by 27 (since 1 yd³ = 27 ft³): Volume (yd³) = Length × Width × Depth / 324 (because 12 × 27 = 324).

• Length – the length of the lawn area in feet (or meters, if you convert).
• Width – the width of the area in feet.
• Depth – the depth of water or soil in inches.

The WSU tool goes further: it multiplies your area by an ET rate (in inches per week) and divides by your sprinkler application rate (in inches per hour) to get run time. That’s the real “irrigation calculator� part.

Worked Example

Using the default values from the calculator above: a lawn 40 ft long, 25 ft wide, and a watering depth of 3 inches.

1. Calculate area: 40 ft × 25 ft = 1,000 ft².
2. Convert depth to feet: 3 in ÷ 12 = 0.25 ft.
3. Volume in cubic feet: 1,000 ft² × 0.25 ft = 250 ft³.
4. Convert to cubic yards: 250 ÷ 27 = 9.26 yd³.

The calculator shows 9.26 yd³ and 250 ft³, matching the formula exactly.

Background: Why WSU Built This Tool

Washington State University Extension developed the irrigation calculator to help homeowners and landscapers in the Pacific Northwest reduce water waste. The region has distinct wet and dry seasons, and many people overwater their lawns by 30–50% without realizing it. The tool uses data from the U.S. Bureau of Reclamation’s AgriMet system and local weather stations to estimate how much water evaporates from the soil and transpires from plants (together called evapotranspiration, or ET).

The calculator is free and online, and it has been widely adopted beyond Washington because the underlying ET data covers much of the western U.S. It is not an official government tool, but it is backed by a public university’s agricultural research, which gives it credibility.

How the WSU Calculator Works

The tool asks for four key pieces of information:

• Location – you pick your nearest weather station from a dropdown list.
• Plant type – turf grass, shrubs, trees, or a mix. Each has a different water use coefficient.
• Soil type – sandy, loamy, or clay. This affects how fast water infiltrates and how often you should water.
• Sprinkler type – spray, rotor, or drip. Each puts out water at a different rate (inches per hour).

Once you enter these, the calculator looks up the current week’s ET rate for your area, multiplies it by a crop coefficient (e.g., 0.8 for cool-season turf), and subtracts any effective rainfall. The result is the net irrigation requirement in inches per week. Then it divides that by your sprinkler’s application rate to give you the run time in minutes per watering day.

It also tells you how many days to water per week, based on your soil’s available water holding capacity. Sandy soil holds less water, so you water more often for shorter periods; clay holds more, so you water less often but longer.

How to Choose the Right Inputs

Getting accurate results depends on entering realistic values. Here’s what to look for:

• Location – choose the closest weather station, not the nearest city if it’s far away. The ET data can vary significantly over even 20 miles.
• Plant type – if you have a mix, use the dominant type. For a typical lawn, “cool-season turfâ€� is a safe bet.
• Soil type – do a simple jar test or check your local soil survey. If you’re unsure, choose “loamâ€� as a middle ground.
• Sprinkler type – look at your sprinkler head. Spray heads put out about 1.5 inches per hour, rotors about 0.5 inches per hour, and drip systems about 0.2–0.5 inches per hour. The calculator has typical defaults, but you can adjust.

If you don’t know your sprinkler’s exact rate, you can measure it by placing several empty tuna cans on the lawn, running the system for 15 minutes, and measuring the average depth of water collected.

Common Mistakes and Misconceptions

One big mistake is using the calculator during the rainy season. The tool is designed for the growing season (April through September in the PNW). In winter, ET is low and rainfall usually covers the need, so the calculator will often show zero irrigation required—that’s correct.

Another misconception is that the calculator’s run time is a one-time number. It changes weekly because ET changes with weather. The WSU site encourages you to check it every week and adjust your controller accordingly. Many modern smart controllers can do this automatically if you connect them to local ET data.

Also, the calculator assumes your sprinkler system is uniform and well-maintained. If you have broken heads, clogged nozzles, or poor coverage, the actual run time will be different. Fix leaks and adjust head spacing before relying on the numbers.

Comparing to Other Irrigation Calculators

Other free tools exist, such as the California Department of Water Resources’ calculator and the EPA’s WaterSense tool. The WSU calculator is unique because it uses a very localized ET dataset and is specifically tuned for the Pacific Northwest. The EPA tool is more generic and uses regional averages, while the WSU one gives a weekly number based on actual weather station readings.

For homeowners outside the Northwest, the WSU tool may still work if your area is covered by an AgriMet station, but the data density is lower. In that case, a local extension service calculator might be more accurate.

The simple volume calculator on this page (above) is a different beast—it just tells you how much water or soil you need for a given area and depth. It doesn’t account for ET or sprinkler efficiency. Use it for planning a new lawn or ordering soil, not for scheduling irrigation.

Costs and Safety Considerations

The WSU irrigation calculator is completely free—no registration, no subscription. The only cost is your time to enter the data. There is no safety risk in using the calculator itself, but following its recommendations can prevent overwatering, which reduces runoff of fertilizers and pesticides into storm drains.

When you do water, follow local water restrictions. Some municipalities limit watering to certain days or times. The calculator doesn’t know those rules, so you may need to adjust the schedule.

Also, be careful with the run time if you have a steep slope or compacted soil. The calculator assumes normal infiltration; if water runs off, you need to cycle and soak—run for a few minutes, pause, then run again.

Alternatives and When to Use Them

If you want a simpler approach, you can use the “1 inch per week� rule of thumb: most lawns need about 1 inch of water per week during summer. But that’s a rough average and doesn’t account for your local climate or soil. The WSU calculator is more precise.

For a very small garden, you might just use a rain gauge and water by hand. For large agricultural fields, you’d use a professional tool like the FAO Penman-Monteith equation. The WSU calculator sits in the middle—perfect for a homeowner with a standard sprinkler system.

If you have a smart irrigation controller, many models have built-in ET adjustment. You can compare its schedule with the WSU calculator’s recommendation to see if they agree.

Frequently Asked Questions

Is the WSU irrigation calculator free?

Yes, it is completely free to use. You just go to the WSU Extension website, enter your data, and get a schedule. No account or payment is required.

Can I use it outside Washington state?

Yes, if your area has a weather station in the AgriMet network. The tool covers much of the western U.S., including Idaho, Oregon, Montana, and parts of California. If your location isn’t listed, you can choose the nearest station, but results may be less accurate.

How often should I check the calculator?

Weekly is best during the growing season. ET changes with temperature, wind, and cloud cover, so a schedule that works in June may be too much in September. Checking every Monday takes two minutes and can save a lot of water.

What if I have a mix of grass and shrubs?

Choose the plant type that covers the largest area. Or, if you have separate zones, run the calculator twice—once for the lawn and once for the shrubs—and program each zone accordingly. Shrubs typically need about half the water of turf.

Why does the calculator sometimes say zero minutes?

That means rainfall has met your plants’ water needs for the week. You should skip watering that week. If you have a smart controller, it may automatically skip cycles based on rain sensors.

Does the calculator account for my sprinkler system’s efficiency?

Not directly. It assumes a typical efficiency of about 75% for spray systems. If you have old, misting heads or high wind, you may need to increase the run time by 10–20%. The calculator does let you adjust the sprinkler type, which changes the application rate.

Related

• SA Water Irrigation Management Calculator Guide
• Rain Bird Irrigation Calculator: How to Use It
• Rain Bird Drip Irrigation Calculator Guide

in l | Lawn length | ft | 40 | 0 | 100000
in w | Lawn width | ft | 25 | 0 | 100000
in d | Depth | in | 3 | 0 | 120
out yd | Volume | yd³ | l * w * d / 324 | 2
out ft3 | Volume | ft³ | l * w * d / 12 | 1