What a catch-can test tells you
A nozzle's box or spec sheet gives a catalog precipitation rate, calculated from its rated flow and the spacing it's designed for. What actually lands on your lawn depends on your real pressure, your real spacing, nozzle wear and how level your heads are, so the only way to know your zone's true output is to measure it. A catch-can test does exactly that: set out open containers, run the zone, and use what the cans collect to work out a measured precipitation rate and how evenly the water is spread.
The test is the same one landscape irrigation auditors use, and it's simple enough to do at home with tuna cans or a set of calibrated catch cans. New Mexico State University (NMSU) Cooperative Extension's step-by-step guide for homeowners recommends a minimum of 24 catch cans laid out in a grid across the zone for reliable results, though a smaller test with fewer cans still gives a useful check.
This calculator handles both common ways of reading the cans afterward: measuring the depth of water with a ruler, or measuring the volume with a graduated cylinder and converting it using the can's opening size. It reports the precipitation rate and a low-quarter distribution uniformity (DUlq) summary. For the full picture, including the coefficient of uniformity and a scheduling coefficient, use the distribution uniformity calculator with the same can data.
How this calculator works
The calculator averages your catch can readings, converts that average to a water depth if you entered volumes, then turns the depth into a rate per hour. Separately, it ranks your readings from low to high to work out how the driest quarter of the cans compares with the overall average.
1. Can opening area (volume readings only)
Area of a round catch can
Area = π × (Diameter ÷ 2)²
Needed to convert a volume reading into a depth. Skipped entirely if you read depth directly with a ruler.
2. Average catch and depth
Depth from volume
US: Depth (in) = Volume (fl oz) × 1.804687 ÷ Area (in²)
Metric: Depth (mm) = Volume (mL) ÷ Area (cm²) × 10
1.804687 converts fluid ounces to cubic inches (231 in³ per gallon ÷ 128 fl oz per gallon), the same relationship used elsewhere on this site. In metric units, 1 mL over 1 cm² is exactly 1 cm of depth, so the result only needs converting from centimeters to millimeters.
3. Precipitation rate
Measured precipitation rate
Precipitation rate = Average catch depth ÷ Test time × 60
This is the standard catch-can precipitation rate calculation, expressed per hour from a test run in minutes. Source: Center for Irrigation Technology, California State University, Fresno, "Evaluating Sprinkler Irrigation Uniformity."
4. Low-quarter distribution uniformity (DUlq)
DUlq
Low-quarter count = round(Number of cans ÷ 4), minimum 1
DUlq = Average of the lowest-catch cans ÷ Average of all cans × 100
Readings are ranked directly, whether they're depths or volumes; the ratio is the same either way since every can shares the same opening size. This is the low-quarter method described by NMSU Extension Guide H-510 and by CATI's sprinkler uniformity guide.
Rounding policy
Precipitation rate and depths display to 2 decimal places in US units and 1 decimal place in metric units. DUlq displays as a whole percentage. Internally, every step keeps full precision; only the final display is rounded, so the numbers you see are exact conversions rather than "about" estimates.
When this calculator isn't the right tool
It's built to turn field can readings into a precipitation rate and a quick uniformity check. Use a different tool for:
- Detailed uniformity metrics, including the Christiansen coefficient of uniformity and a scheduling coefficient. See the distribution uniformity calculator.
- Designing run times from a catalog precipitation rate rather than a measured one. See the sprinkler precipitation rate calculator.
- Fixing a zone where water runs off before the target depth is applied. See the cycle and soak calculator.
- Drip irrigation and micro-spray, which aren't measured with open catch cans. See the emission uniformity calculator.
Catch-can test calculation example
A homeowner sets out 8 catch cans across a spray zone, runs it for 15 minutes, and reads the depth in each can with a ruler: 0.28, 0.30, 0.32, 0.34, 0.27, 0.31, 0.29 and 0.33 inches.
- Average catch: (0.28 + 0.30 + 0.32 + 0.34 + 0.27 + 0.31 + 0.29 + 0.33) ÷ 8 = 0.305 in.
- Precipitation rate: 0.305 ÷ 15 × 60 = 1.22 in/hr.
- Ranked readings: 0.27, 0.28, 0.29, 0.30, 0.31, 0.32, 0.33, 0.34.
- Low-quarter count: round(8 ÷ 4) = 2 cans, so the average of 0.27 and 0.28 = 0.275 in.
- DUlq: 0.275 ÷ 0.305 × 100 = about 90%, which NMSU Extension's scale rates "excellent" for fixed spray heads (75% or higher).
The same calculation with a volume reading in metric
A rotor zone is tested with 6 catch cans, each with a 10 cm opening diameter, run for 20 minutes. The volumes collected are 120, 135, 128, 140, 118 and 130 mL.
- Can area: π × (10 ÷ 2)² = 78.54 cm².
- Average volume: (120 + 135 + 128 + 140 + 118 + 130) ÷ 6 = 128.5 mL.
- Average depth: 128.5 ÷ 78.54 × 10 = 16.4 mm.
- Precipitation rate: 16.4 ÷ 20 × 60 ≈ 49.1 mm/hr.
- DUlq: ranked volumes are 118, 120, 128, 130, 135, 140; the lowest 2 average 119 mL, so DUlq = 119 ÷ 128.5 × 100 ≈ 93%, rated "excellent" for rotors on NMSU's scale (80% or higher).
Running the test well
NMSU Extension's guide lays out a grid pattern for placing cans: 5 to 8 feet apart for fixed spray heads on an irregular residential lawn, wider for rotors, and always at least 12 to 24 inches in from the edge of the lawn so cans near the border don't under-catch. Run the system on a calm day, since both NMSU and the Center for Irrigation Technology (CATI) at California State University, Fresno recommend testing when wind is 5 mph or less, as wind pushes spray off target and skews every can differently.
CATI's guide suggests running spray zones for 3 to 10 minutes and rotor zones for 10 to 30 minutes, long enough to collect a measurable amount without overflowing any can. Whatever time you choose, record the exact minutes rather than rounding, since the precipitation rate is calculated directly from that number.
Reading the cans
A ruler works for straight-sided cans read to the nearest 0.1 inch or 1 mm. A graduated cylinder is more precise for small volumes and is the method NMSU's guide walks through in detail. Whichever you choose, use the same method for every can in the test, and enter the can's opening diameter only if you're reading volume, since the depth method skips that conversion entirely.
Applying the result
Once you have a measured precipitation rate, it's the number to use for scheduling that zone rather than a manufacturer's catalog figure, since it reflects your actual pressure and equipment. Feed it into the lawn watering calculator to get a run time for a weekly target, or the sprinkler run time calculator for a single session. If the rate is high enough that water runs off before your target depth is applied, split the run time using the cycle and soak calculator, and compare it with how fast your soil can absorb water on the soil infiltration rate chart.
A low DUlq points to a fixable problem, not just a scheduling one. Before extending run time to cover the driest cans, check for clogged or worn nozzles, mismatched heads, incorrect pressure and wind. Longer run times just waste water on the wetter spots without a design or maintenance fix.
What changes your catch-can test results
The math above is exact once you have good readings. Most bad results trace back to how the test itself was set up or run.
| Factor | Effect | What to adjust |
|---|---|---|
| Wind | Drifts spray off target and skews individual cans unevenly. | Retest on a calm day; CATI recommends 5 mph or less. |
| Can size and shape | A different opening size changes the volume-to-depth conversion. | Use identical, straight-sided cans and measure the diameter carefully. |
| Number of cans | Too few cans make the low-quarter average unstable. | Use at least 24 cans where practical (NMSU Extension); treat results from fewer cans as a rough check. |
| Test duration | A very short run leaves too little water to measure accurately. | Run 3–10 minutes for spray, 10–30 minutes for rotors (CATI). |
| Nozzle wear or clogging | Lowers catch at affected heads and drags down DUlq. | Inspect nozzles and screens; clean or replace as needed. |
| Operating pressure | Too high atomizes water into mist that drifts; too low shortens throw. | Check pressure with a gauge against the nozzle's rated pressure. |
| Zone overlap | Overspray from an adjacent zone inflates catch in shared cans. | Test overlapping zones together and run each for the same time, per NMSU's method. |
Frequently asked questions
What does a catch-can test actually measure?
A catch-can test measures how your sprinklers actually perform in the field, not what a catalog says they should do. You place open containers across a zone, run the system for a set time, then use the water each can collects to calculate the zone's real precipitation rate and how evenly that water is spread.
How many catch cans do I need?
New Mexico State University Cooperative Extension recommends a minimum of 24 catch cans for reliable results. Fewer cans, such as 8 to 12, still give a usable estimate for a quick check, but the distribution uniformity number becomes less certain as the count drops. This calculator needs at least 4 readings to compute a low-quarter average.
Should I measure the water depth or the volume in each can?
Either works, and the calculator accepts both. Reading the depth with a ruler is faster and needs no extra tools. Reading the volume with a graduated cylinder is more precise, especially for straight-sided calibrated cans, but you'll also need the can's opening diameter so the calculator can convert volume to depth.
What counts as a good distribution uniformity score?
It depends on the sprinkler type. NMSU Extension rates a low-quarter DU of 75% or higher as excellent for fixed spray heads and 80% or higher as excellent for rotors, with "good" starting around 55% for spray and 65% for rotors. Separately, EPA's WaterSense program requires a low-quarter DU of at least 65% for irrigation systems in WaterSense labeled new homes.
How is DUlq different from just averaging the catch cans?
The average catch tells you the overall precipitation rate, but it can hide dry spots. Low-quarter distribution uniformity (DUlq) looks specifically at the driest 25% of your catch cans and compares that to the overall average, so it flags the areas most likely to go short of water even when the zone average looks fine.
Can I use this calculator for drip irrigation or micro-spray?
Not directly. Catch cans measure water falling through the air, which works for spray heads, rotors and rotary nozzles but not for drip emitters, which deliver water at the soil surface at individual points. For drip systems, use the emission uniformity calculator instead.
See the emission uniformity calculator.
How long should I run the test?
Run long enough to collect a measurable amount in each can without letting any overflow, commonly 15 to 30 minutes for a home system. Whatever time you choose, keep every zone's test time the same and enter the exact minutes you ran, since the precipitation rate is calculated directly from that number.
What if some of my catch cans collected a lot more or less water than the others?
A wide spread between cans is exactly what distribution uniformity is meant to catch, and it usually points to a real problem: a clogged or worn nozzle, incorrect pressure, mismatched heads, or wind during the test. Check for those causes before assuming the whole zone needs a longer run time.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Precipitation rate and DUlq formulas verified by hand against both worked examples.
- Unit conversions checked in both directions between US and metric.
- DU ratings and can-placement guidance checked against the NMSU and CATI sources below.
Sources
- New Mexico State University Cooperative Extension Service. Kent, C. & Leinauer, B. How to Perform a Catch Can Irrigation Audit on a Home Lawn Sprinkler System (Guide H-510).
- Center for Irrigation Technology, California State University, Fresno. Wilson, T.P. & Zoldoske, D.F. Evaluating Sprinkler Irrigation Uniformity.
- U.S. EPA WaterSense. Guidelines for Irrigation Audits on WaterSense Labeled New Homes (v1.1).