How the calculation works

You set same-size, straight-sided cans on the lawn, run the sprinklers for a set time and measure the water in each can. From those readings the calculator works out:

Application rate (also called precipitation rate) = average depth in the cans ÷ test minutes × 60. This is how much water the zone puts down in an hour.
Distribution uniformity (DU) = average of the driest quarter of the cans ÷ average of all the cans. 100% would mean every can caught the same amount. This is the lower-quarter method in New Mexico State University's catch-can guide. When the number of cans doesn't divide by 4, the calculator rounds the driest quarter to the nearest whole can (3 of 10, for example).
Water per session = (weekly amount − rain so far this week) ÷ watering days.
Minutes per session = water per session ÷ application rate × 60.
Extra time for uneven coverage (optional) = minutes × scheduling multiplier, where the multiplier is 1 ÷ (0.4 + 0.6 × DU). This comes from UC Agriculture and Natural Resources' irrigation scheduling training, which calls the plain run time the lower boundary and the multiplied one the upper boundary.
Cycle-and-soak (optional): the EPA WaterSense method runs each session in cycles no longer than the time it takes water to start pooling. The calculator divides the minutes per session by that pooling time and rounds up to get the number of cycles, then makes the cycles equal.

Worked example

A spray-head zone runs for 15 minutes over 8 tuna cans, which catch 0.30, 0.25, 0.35, 0.40, 0.20, 0.30, 0.35 and 0.25 inches.

Average: 2.40 ÷ 8 = 0.30 inch in 15 minutes, so the rate is 0.30 ÷ 15 × 60 = 1.2 inches per hour.
The driest quarter is 2 of the 8 cans, 0.20 and 0.25 inch, which average 0.225 inch. DU = 0.225 ÷ 0.30 =75%: Excellent for spray heads (it would be Very good for rotors).
For 1 inch a week on 2 days, each session needs 0.5 inch: 0.5 ÷ 1.2 × 60 = 25 minutes, or 50 minutes a week.
Allowing for uneven coverage, the multiplier is 1 ÷ (0.4 + 0.6 × 0.75) = 1 ÷ 0.85 = 1.18, so each session becomes 29 minutes (58 a week).
If 0.4 inch of rain has already fallen this week, 0.6 inch is left: 0.3 inch a session, which takes 15 minutes (18 with the multiplier).
If water starts to puddle after 10 minutes, the 29-minute session (29.4 to be exact) splits into 3 cycles of about 10 minutes (9.8), with 30 minutes to an hour off between them.

In metric, 8 cans catch 7.5, 6, 9, 10, 5, 7.5, 9 and 6 mm in 15 minutes. That averages 60 ÷ 8 = 7.5 mm, a rate of 30 mm per hour. The driest quarter (5 and 6 mm) averages 5.5 mm, so DU is 5.5 ÷ 7.5 = 73%. For 25 mm a week on 2 days, each session needs 12.5 mm, which takes 25 minutes, or 30 minutes with the multiplier of 1 ÷ (0.4 + 0.44) = 1.19.

How to run the catch-can test

Cans: use straight-sided containers that are all the same size, such as tuna or cat-food cans (UF/IFAS, University of Minnesota). UF/IFAS's 2024 handout asks for 10 or more (its Gardening Solutions page says five to ten). New Mexico State University (NMSU) says at least 24 for reliable results, and notes that food cans need a few small stones inside to keep them level.
One zone at a time. For an in-ground system, spread the cans through a single zone. UF/IFAS scatters them at random; in odd-shaped areas NMSU lays them in a rough grid, about 5 to 8 ft apart for spray heads and 10 to 20 ft apart for rotors, and keeps them at least 12 to 24 inches in from the edge of the lawn. If two zones overlap, NMSU runs both before reading the cans.
Hose-end sprinkler: set the cans in a straight, evenly spaced line from the sprinkler to the edge of its spray (UF/IFAS). University of Minnesota Extension suggests a can every 1 or 2 feet.
Test at the time of day you usually water (UF/IFAS, NMSU), and on a calm day: NMSU says wind of 5 mph or less. UF/IFAS runs the test for 15 minutes; NMSU uses your normal run time.
Measure the depth in each can with a ruler and type each reading into the calculator. If some cans stay empty or catch far too much, UF/IFAS says to look more closely at the system to find out why.

How much water does a lawn need?

There is no single right amount. It depends on the grass, the season, the soil, the rain and how green you want the lawn. The calculator starts from 1 inch (25 mm) a week, and the sources put it like this:

US EPA WaterSense: a landscape typically needs about 1 inch a week, including rain. Its quick version of the can test is to time how long the sprinkler takes to put half an inch in the cans, then water for that long twice a week.
University of Minnesota Extension: lawns there use about 4 to 6 inches of water a month in June, July and August, more than the usual rainfall, so it suggests adding 1 to 1½ inches a week, minus any rain, in summer, and little watering in spring until June. It also notes that watering is optional in Minnesota: Kentucky bluegrass goes dormant and brown in a dry spell and usually greens up again when it cools.
University of Illinois Extension: cool-season lawns need about 1 to 1.5 inches a week to stay green and growing, depending on soil, weather and care.
UF/IFAS (Florida): ½ to ¾ inch per watering in most Florida soils. Soils with more clay or organic matter hold more water and need less, and it advises against more than ¾ inch (19 mm) at a time. It also reminds you to follow local watering rules.

These figures are for established lawns. University of Minnesota Extension notes that newly seeded lawns need light, frequent watering until the seedlings have roots.

How even is your coverage?

NMSU and the Irrigation Association rate lower-quarter DU as below. Rotors and impact sprinklers are held to a higher standard than fixed spray heads.

Rating	Spray heads	Rotors and impacts
Excellent	75%	80%
Very good	65%	70%
Good	55%	65%
Fair	50%	60%
Poor	40%	50%

The lowest-quarter DU at which each rating starts. Sources: NMSU Guide H-510, and the Irrigation Association table reproduced by Mecham (2004), which lists impact sprinklers with rotors. Both tables are for in-ground sprinkler zones, so for a hose-end sprinkler treat the rating as a rough guide.

NMSU suggests a zone should rate at least Good, and if it doesn't, to find out why. The Irrigation Association's 2004 best management practices, quoted by Mecham, set a slightly higher bar for rotors: at least 55% for spray heads and 70% for rotors. EPA WaterSense suggests checking the system every month for leaks and broken or clogged heads, and notes that high water pressure can make sprinklers spray faster than expected. Don't be surprised by a low score: across more than 6,800 audits summarized in that Irrigation Association paper, the average DU was about 50% for spray heads and rotors alike. Where the paper reports application rates, spray zones averaged roughly 1.3 to 1.6 inches per hour and rotor zones roughly 0.6 to 0.8, depending on the area.

Water deeply, less often

University of Minnesota Extension advises leaving as long as you can between waterings without the grass becoming stressed. Deep, infrequent watering grows deeper roots that can draw on more soil, while light, frequent watering keeps roots shallow; University of Illinois Extension adds that it also encourages weeds such as crabgrass. Signs that the lawn needs water are slight wilting, a greyish or bluish-green tinge, and footprints that stay in the grass after you walk on it (Minnesota).

Minnesota aims to wet the soil about 5 inches deep. When the soil is very dry, that takes about ½ inch of water on sandy soil and up to 1½ inches on clay, and anything more drains below the roots, so the calculator warns you if one session is more than 1½ inches (38 mm). When you need to cut back, UC Agriculture and Natural Resources advises leaving more days between waterings rather than shortening each run, because shorter runs don't wet the soil as deeply.

Water early in the morning

University of Minnesota Extension calls early morning, from 4 to 8 a.m., the most efficient time: it is cooler, with less sun and wind, so less water evaporates and the spray lands more evenly. UF/IFAS suggests 4 to 6 a.m. Minnesota and Illinois both advise against watering in the evening or at night, because grass that stays wet overnight can be more prone to disease, and EPA WaterSense says to avoid the middle of the day, when the sun evaporates much of the water.

If water runs off: cycle-and-soak

If your sprinklers put water down faster than the soil can take it in, it puddles and runs off. The EPA says this is common on clay soils and slopes. Its cycle-and-soak method: run the zone and note how many minutes pass before water starts to pool. That is the longest one cycle should be. Run as many cycles of up to that length as it takes to reach the full time, with 30 minutes to an hour off between them so the water can soak in. If water doesn't pool before the session ends, there's no need to split it.

University of Minnesota Extension gives a simpler version: when you see runoff, turn the water off for 15 minutes, then carry on until the full amount is down. University of Illinois Extension suggests splitting the water into two applications on heavy clay or slopes.

Common questions

How long should I run my sprinklers to put down 1 inch of water?: It depends on how fast your sprinklers apply water, which is why the can test matters. At 1.2 inches per hour, 1 inch takes 50 minutes a week, for example 25 minutes on each of two days. A slower zone that applies 0.6 inches per hour needs 100 minutes for the same inch. Test each zone separately, because spray heads and rotors on the same system can apply water at very different rates.
What is a good distribution uniformity (DU)?: DU compares the driest quarter of your cans with the average of all of them. If the driest quarter averages 0.15 inch and all the cans average 0.30 inch, DU is 0.15 ÷ 0.30 = 50%: the driest parts of the lawn get half the average. In the rating table used by New Mexico State University and the Irrigation Association, 50% is Fair for spray heads and Poor for rotors. Good starts at 55% for spray heads and 65% for rotors, and NMSU suggests a zone should rate at least Good.
Should I subtract rain from the weekly amount?: Yes. University of Minnesota Extension's summer guideline is a weekly amount minus any rainfall, and the EPA's 1 inch a week includes rain. With a 1 inch target and 0.4 inch of rain, 0.6 inch is left to apply: 0.3 inch on each of two days, which takes 15 minutes per session at 1.2 inches per hour. Type the rain into the "Rain so far this week" box.
Why add extra time for uneven coverage?: If you water for the average, the driest parts of the lawn get less than your target. UC Agriculture and Natural Resources multiplies the run time by a scheduling multiplier of 1 ÷ (0.4 + 0.6 × DU), treating the plain run time as the lower limit and the multiplied one as the upper limit. At a DU of 65% the multiplier is 1 ÷ 0.79 = 1.27, so UC's example of a 53-minute run becomes 67 minutes. At 75% it is 1.18, about 18% more time. Fixing broken or clogged heads raises DU and cuts the extra time.
Water puddles or runs off before the time is up. What should I do?: Split each session into shorter cycles with a rest between them, which the EPA calls cycle-and-soak. Time how long the zone runs before water starts to pool and make each cycle no longer than that. The EPA's example is a 15-minute zone that pools after 5 minutes, run as three 5-minute cycles with 30 minutes to 1 hour off between them. University of Minnesota Extension suggests turning the water off for 15 minutes before carrying on. Type the pooling time into the calculator and it works out the cycles for you.

Sources

Last checked and updated: October 31, 2026.