Catch Can Irrigation Test Calculator
Can catches to precipitation rate, DU, and rating.
What it does
Two zones can spray the same inches and grow different lawns. Precipitation rate says how fast water lands; distribution uniformity says how evenly. Catch cans measure both in twenty minutes.
The tool above divides average catch by runtime for the rate and the driest quarter by the average for DU, then rates it from excellent to poor. Imperial mode reads inches; metric mode reads millimeters.
Extension irrigation audits and EPA outdoor-water guidance run this same can test with the same low-quarter math. Sprays near 1.5 in/hr with DU past 65 water well; DU under 55 means spacing, pressure, or nozzles need work first.
Retest after fixes, not before. Changing heads, pressure, or spacing moves both numbers, and one audit per zone per season keeps schedules honest as landscapes grow.
How to use
- Pick imperial (in) or metric (mm).
- Set tuna cans on a grid and run the zone.
- Enter average catch and low-quarter average.
- Enter runtime in minutes.
- Read precipitation rate, DU, and rating.
- Fix spacing, pressure, and nozzles, then retest.
Formula
- average
- Mean catch across all cans.
- low quarter
- Mean of the driest quarter of cans.
- minutes
- Zone runtime in minutes.
Average and low-quarter depths share units (in or mm) with runtime in minutes. DU bands follow spray-head audit convention: 75 excellent, 65 good, 55 fair. Rate converts directly: 1 in/hr equals 25.4 mm/hr.
Worked example
Example: 0.5 in average, 68 % DU
0.5 in average, 0.34 low quarter, 20 minute run.
- Cans average 0.5 in, low quarter 0.34 in, 20 min run.
- Rate: 0.5 / 20 x 60 = 1.5 in/hr.
- DU: 0.34 / 0.5 x 100 = 68 %.
- Rating reads Good (65 to 75).
- Fix spacing, pressure, and clogged heads to climb higher.
Reference table
| DU | Rating | Move |
|---|---|---|
| 75 % and up | Excellent | Schedule by rate |
| 65 to 75 % | Good | Minor tweaks |
| 55 to 65 % | Fair | Fix worst heads |
| Under 55 % | Poor | Redesign spacing |
Limits
- Cans only. Wind and slope skew readings.
- Spray bands; rotors read kinder.
- One zone at a time; mixed zones mislead.
- Rate needs the full runtime, not a guess.
Common mistakes
- Too few cans for the zone.
- Mixing nozzle types on one zone.
- Reading before the breakthrough puddles.
- Auditing in heavy wind.
- Scheduling by rate while ignoring DU.
Try it yourself
Quick checks. Answers sit tucked under each question.
0.4 in average, 0.22 low quarter, 15 min?
Rate is 0.4 / 15 x 60 = 1.6 in/hr. DU is 0.22 / 0.4 x 100 = 55 %, rated Fair.
10 mm average, 5.5 low quarter, 15 min?
Rate is 10 / 15 x 60 = 40 mm/hr. DU is 5.5 / 10 x 100 = 55 %, rated Fair.
Questions
How do I run a catch can test?
Straight-sided cans on a grid, run the zone, then read every depth. Average all cans for the rate; average the driest quarter against it for DU. Twelve or more cans beat six.
What is DU?
Low-quarter distribution uniformity: driest quarter over the average, in percent. High DU waters evenly; low DU drowns spots to green the dry ones.
What DU counts as good?
75-plus excellent, 65 to 75 good, 55 to 65 fair, under 55 poor for sprays. Rotors grade a touch lower by nature. This page bands sprays; rotors read one step kindly.
Does metric change anything?
Millimeters work the same: 12.7 mm average with 8.6 low quarter over 20 min gives 38.1 mm/hr at 68 %. Units convert, uniformity does not care.
What wrecks uniformity most?
Head spacing past matched precipitation, mixed nozzles on one zone, tilted heads, low pressure, and wind. Audit first, then change the worst third.
How do I turn rate into minutes?
Multiply rate by runtimes: 1.5 in/hr for 20 min lays 0.5 in. Lawns mostly want about 1 in weekly from rain plus irrigation; the schedule page turns depth into minutes.