Catch-Can Test & Measure Sprinkler Uniformity
Evaluates sprinklers
Type the depths your catch cans caught into a live grid to get the Christiansen CU, low-quarter DU, mean and low-quarter depth, a uniformity rating, and exactly how much extra run-time the driest quarter needs.
Catch-can field
Type each can's caught depth (mm, mL or in — any unit). Driest-quarter cells are ringed.
Next: uniformity is acceptable — schedule a normal set but add about 14.4% run-time so the driest quarter still reaches 13 mm. Re-test after any nozzle change.
CU = 100·(1 − Σ|xᵢ−m|/Σxᵢ) · DU = 100·(low-quarter mean ÷ mean) · scheduling coef = mean ÷ low-quarter mean.
Catch-can uniformity — key facts
- CU formula
- 100 × (1 − Σ|xᵢ − m| ÷ Σxᵢ)
- DU formula
- 100 × (low-quarter mean ÷ mean)
- Scheduling coef.
- mean ÷ low-quarter mean
- CU bands
- ≥90 excellent · 84–90 good · 75–84 fair
- DU bands
- ≥84 excellent · 75–84 good · 65–75 fair
- Cans needed
- ≥16–24 on an even grid
- Test in
- calm, low-wind conditions
- Privacy
- Runs in your browser; nothing uploaded
Interpretation bands and system benchmarks
Your result falls into the highest band whose minimum it meets. The benchmark table shows the uniformity a well-maintained system of each type typically achieves — judge your result against the row that matches your equipment.
| Rating | CU minimum (%) | DU minimum (%) | What it means |
|---|---|---|---|
| Excellent | ≥ 90 | ≥ 84 | Near-ideal; little correction needed |
| Good | ≥ 84 | ≥ 75 | Acceptable; minor scheduling correction |
| Fair | ≥ 75 | ≥ 65 | Investigate; meaningful water waste |
| Poor | < 75 | < 65 | Find and fix the cause before irrigating |
Typical uniformity by system type
| System | Typical CU (%) | Typical DU (%) |
|---|---|---|
| Center pivot (LEPA / low-pressure spray) | 92 | 88 |
| Center pivot (impact) | 87 | 80 |
| Solid-set / hand-move sprinkler | 84 | 75 |
| Travelling gun | 75 | 65 |
| Micro-sprinkler | 88 | 82 |
Sources: Christiansen (1942); Merriam & Keller (1978); ASABE / extension irrigation-evaluation guides. Benchmarks are typical well-maintained values and vary with pressure, spacing and wear.
Your system is only as good as its driest quarter
A catch-can test is the cheapest, fastest way to know whether your sprinkler or pivot is doing its job. You set out identical containers, run the system for a timed period, and measure the depth each can caught. From those depths the Christiansen Uniformity Coefficient describes how evenly water lands overall, while the low-quarter distribution uniformity zooms in on the driest 25% of the field — the part that wilts first and therefore sets your irrigation schedule.
The catch is that you have to irrigate to satisfy the dry quarter, not the average. The scheduling coefficient turns that into a number: run the system that many times longer so the dry corner reaches its target depth. But every extra minute also over-waters everywhere else, so poor uniformity quietly costs water, energy and leached nutrients. This tool plots your grid, flags the dry quarter, rates the system and tells you the run-time correction — and whether you should fix the cause instead.
How to run the test in five steps
- 1Lay out the cans
Place at least 16–24 identical containers on an even grid across the wetted area (a radial line for a pivot).
- 2Run for a timed period
Run the system for a fixed time on a calm day, then measure the depth in each can with a ruler or graduated cylinder.
- 3Type the depths in
Enter each can's depth into the matching cell of the live grid above; resize the grid to your layout.
- 4Read CU, DU and the rating
The tool returns the Christiansen CU%, low-quarter DU% and a good/fair/poor rating, with the dry quarter ringed.
- 5Correct or fix
Add the extra run-time the scheduling coefficient calls for — or, if the rating is fair/poor, find and fix the cause first.
Frequently Asked Questions
What is the Christiansen Uniformity Coefficient (CU)?+
CU is the classic measure of how evenly a sprinkler system applies water. It is calculated as CU = 100 × (1 − Σ|xᵢ − mean| ÷ Σxᵢ), where xᵢ are the depths caught in each can. A CU of 100% means every can caught exactly the same depth; lower values mean more variation. The standard interpretation is 90%+ excellent, 84–90% good, 75–84% fair, and below 75% poor.
How is distribution uniformity (DU) different from CU?+
DU, the low-quarter distribution uniformity, focuses on the driest part of the field: DU = 100 × (mean of the lowest 25% of cans ÷ overall mean). It is the figure that drives scheduling, because the dry quarter is what suffers first. DU is always a little lower than CU for the same data and is the more conservative, decision-relevant number. Bands are 84%+ excellent, 75–84% good, 65–75% fair, below 65% poor.
How many catch cans do I need and how should I space them?+
Use identical containers on an even grid across the wetted area — a minimum of about 16 to 24 cans gives a stable result, more for a pivot. Space them evenly (for example every 3 metres on a solid-set, or along a radial line for a pivot) and run the system for a fixed, timed period. Then type each can's caught depth into the grid above; the tool sizes the grid to match.
What does the scheduling coefficient tell me?+
The scheduling coefficient is mean depth ÷ low-quarter mean depth. It is always 1 or greater and tells you how much longer you must run the system so the driest quarter still receives the target depth. A scheduling coefficient of 1.3, for example, means you must run 30% longer — and the rest of the field is over-watered by that amount, which is wasted water and a runoff risk.
What is a good uniformity for my system?+
It depends on the system. A well-set-up center pivot with low-pressure spray often reaches CU 92% / DU 88%; impact pivots run a little lower; solid-set sprinklers around CU 84% / DU 75%; and a travelling gun is inherently less uniform at roughly CU 75% / DU 65%. The tool shows the benchmark your result sits closest to so you can judge whether the system is performing as it should.
Why does poor uniformity waste water?+
Because a sprinkler system can only be as good as its driest quarter. If you irrigate to make sure the dry spots get enough, every other part of the field gets too much — that surplus drains away or runs off, carrying nutrients with it. Improving uniformity lets you cut total run-time while still satisfying the dry quarter, which is the cheapest water you will ever save.
What causes low uniformity in a catch-can test?+
The usual culprits are worn or mismatched nozzles, incorrect or failed pressure regulators, wrong sprinkler spacing, wind during the test, leaning risers, and clogged screens. A pattern in the grid is a clue: a consistently dry corner points to pressure loss at the end of a line, while scattered dry cans suggest nozzle wear or wind. Fix the cause before you simply run the system longer.
Does wind affect the test?+
Yes — wind is the single biggest source of error in a sprinkler catch-can test. It pushes droplets downwind, creating apparent dry and wet zones that are really a weather artefact. Run the test in calm conditions (ideally under about 1.5 m/s wind) and repeat on a still day if results look unusually poor. The formulas themselves are unaffected; only the depths you feed them matter.
Can I use this for a center pivot as well as solid-set sprinklers?+
Yes. The CU and DU formulas are method-agnostic — they work on any set of caught depths. For a pivot, lay cans along a radial line out from the pivot point and enter those depths; for solid-set or hand-move, use a grid between sprinklers. The interpretation bands and the system benchmark list cover pivots, solid-set, travelling guns and micro-sprinklers alike.
What units should I enter the depths in?+
Any consistent unit. CU and DU are ratios, so millimetres, millilitres caught, or inches all give the same percentages as long as every can uses the same unit. The mean and low-quarter figures will be in whatever unit you typed. For the run-time correction, enter the target application depth in the same unit you used for the cans.
Should I re-test after fixing a problem?+
Always. A catch-can test is cheap and fast, so re-run it after any nozzle change, pressure adjustment or regulator replacement to confirm the fix worked. Uniformity drifts over a season as nozzles wear, so an annual test — or one each time performance seems off — is good practice and quickly pays for itself in water saved.
Is my data uploaded anywhere?+
No. The entire calculation runs locally in your browser using the Christiansen and low-quarter formulas. Nothing you type is sent to a server or stored, so your field data stays on your device.