Sprinkler Catch Can & Runtime Calculator
Use eight catch-cup readings from one sprinkler zone to estimate lower-quarter distribution uniformity, precipitation rate, and a runoff-aware runtime schedule.
Uniformity-adjusted total runtime39.98
- Average catch volume
- 20.5
- Lower-quarter average
- 17
- Lower-quarter distribution uniformity
- 82.93%
- Average precipitation rate
- 0.9
- Lower-quarter precipitation rate
- 0.75
- Recommended number of cycles
- 3
- Runtime per cycle
- 13.33
- Total elapsed schedule time
- 99.98
Cycle-and-soak schedule
| Cycle | Run time | Soak after cycle |
|---|---|---|
| 1 | 13.33 | 30 |
| 2 | 13.33 | 30 |
| 3 | 13.33 | done |
Assumes eight identical circular catch cups, level placement, and steady pressure during the test.
Cycle-and-soak timing includes one soak interval between each watering cycle.
How to use this calculator
- Set out eight identical catch cups across one sprinkler zone and run the zone for a measured time.
- Measure the water in each cup in milliliters and enter the cup opening diameter.
- Enter the net watering depth you want the driest quarter of the zone to receive.
- Add the longest runtime you can run before runoff starts and the soak time you plan to use.
How the catch-can runtime is calculated
A catch-can test measures what the sprinklers actually put on the lawn, including overlap problems and low-output areas that are easy to miss by eye. This calculator uses eight equal cups, converts the collected volume to water depth, and then adjusts the runtime so the driest quarter of the test area receives the target depth.
The cup opening matters because the same water volume is a different depth in a narrow cup than in a wide cup. For a circular cup with inside diameter d, the opening area is:
A = pi x d^2 / 4
With eight cup volumes V1 through V8, the average catch is:
Vavg = (V1 + V2 + ... + V8) / 8
The lower-quarter average is the mean of the two smallest readings. Lower-quarter distribution uniformity is:
DUlq = VLQ / Vavg
The calculator converts milliliters to cubic inches using 16.387 mL/in^3, then scales the test to an hourly precipitation rate:
PR = [Vavg / (16.387 x A)] x (60 / test minutes)
The lower-quarter precipitation rate is PRlq = PR x DUlq. The total runtime for a target net depth D is:
runtime minutes = 60 x D / PRlq
What changes the answer most
The two lowest cups have a strong effect because they represent the driest quarter of an eight-cup test. A single very low reading can make the adjusted runtime longer, and two empty cups can make the schedule impossible until the coverage problem is fixed. Cup diameter and test duration also matter: a wrong diameter changes the depth conversion, and a very short test can exaggerate startup, wind, and reading errors.
The cycle-and-soak part does not change the total watering depth. It only splits the required runtime into shorter cycles using the maximum continuous runtime you enter. If the total runtime is shorter than that limit, the calculator recommends one cycle. If it is longer, it rounds the cycle count up and divides the runtime evenly.
What this calculator leaves out
This is a homeowner screening audit, not a certified irrigation audit. It does not model evapotranspiration, rainfall, changing pressure, wind drift, slope, soil intake rate, plant stress, or controller rounding. Use local watering guidance to choose the target depth, and repair obviously poor coverage before relying on extra runtime to compensate.
Worked example
Suppose eight cups collect 16, 18, 19, 20, 21, 22, 23, and 25 mL during a 10-minute test, and each cup has a 3.25-inch opening. The average catch is 20.5 mL, the lower-quarter average is 17 mL, and lower-quarter distribution uniformity is 82.93%.
The cup area is 8.296 square inches, so the average precipitation rate is about 0.90 in/hr. Adjusting by lower-quarter uniformity gives a lower-quarter precipitation rate of about 0.75 in/hr. To put 0.5 inch on the driest quarter, the zone needs 39.98 minutes of runtime. With a 15-minute runoff-free limit and 30-minute soaks, that becomes 3 cycles of about 13.33 minutes each and 99.98 minutes elapsed from first start to final stop.
Common questions
How do I perform a sprinkler catch-can test?
Place eight identical, straight-sided catch cups across one sprinkler zone, including areas that look dry and wet. Run only that zone for a measured time, then pour or read each cup in milliliters. Test spray zones and rotor zones separately because they usually apply water at different rates.
What is lower-quarter distribution uniformity?
Lower-quarter distribution uniformity compares the average of the lowest quarter of catch readings with the overall average. With eight cups, the lowest quarter is the two smallest readings. A lower number means the dry spots receive much less water than the average area.
What is a good distribution-uniformity result for lawn sprinklers?
Higher is better, and many home systems have room for improvement. A result below about 65% is a warning sign that spacing, pressure, clogged nozzles, sunken heads, or blocked spray patterns may need attention. Adding runtime can help dry spots, but it also adds water to already wet spots.
Why does cup diameter affect the precipitation rate?
The calculator turns collected volume into water depth by dividing the volume by the cup opening area. A wide cup needs more water to represent the same depth than a narrow cup. Use the inside opening diameter, not the outside rim, because that is the area catching water.
How do I determine the maximum runtime before runoff?
Run the zone and watch for puddling, water flowing down the slope, or water reaching pavement. The longest runtime before that begins is a practical cycle limit for this schedule. Soil, slope, thatch, and compaction can change the limit through the season.
When should I repair the system instead of increasing runtime?
Repair is usually the better first step when cups are empty, heads are blocked or tilted, pressure looks uneven, or one area is far wetter than another. Runtime compensation is useful for modest unevenness, but severe unevenness wastes water and can stress the wettest part of the lawn.