Catch Cups

Outstanding PDF! Thank you for sharing that with the community.

A great discourse regarding catch cups. @Ed3120 feel free to browse the document provided by @JPedrego. Please let the community know what you think. In addition, if it provides you with the answers you are looking for let us know as well. We are always looking for solid documentation to pass along to the community.

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Ha! I did some searching last night and found the same PDF. It was very helpful. It’s pretty easy in the end and I don’t want to scare everyone away with a long math lesson, so the short story is this:

  1.   Buy some catch cups that capture 16 square inches.  (That is a standard catch cup size and that’s what the Orbit catch cups that I linked to are.)
  2.   Set them out, run your sprinklers for 10 minutes.
  3.   Find the average mL reading across your catch cups.  (If you put out 10 cups, add the readings from them together and divide by 10.)
  4.   Multiply your average mL reading by 0.0225. The answer will be your nozzle’s inches per hour for that zone. (The answer that you get should be somewhere between 0 and 2.5.  If it isn’t, you probably did something wrong.)
  5.   Edit your zone and set a custom nozzle for that amount. 
  6.   You’re done.  Move on to the next zone and repeat the process.  (Remember to recreate your watering schedules, as a change to the custom nozzle setting does not impact previously created watering schedules.)


            My average mL after a 10 minute run was 53 mL. 
            53 * 0.0225 = 1.1925

            I rounded it off and set my custom nozzle to 1.2.

For those that want the long story, here you go: (If anyone finds an error in my assumptions or math, please let me know.)

It doesn’t matter how many sprinkler heads you have. It doesn’t matter the square footage of your zone. The only thing that matters is the amount of “inches per hour” of water that you put down. You need to figure this out, and set your nozzle for that zone to that rate.

Now for some math:

My average mL collected by my catch cups was 53mL over 10 minutes. I need to multiply by 6 to get that number to represent an hour.

            53 mL * 6 = 318mL per hour

But that 318 mL per hour is what was collected across a 16 square inch catch cup. I need to know how much fell in a single square inch. This is referred to in the PDF as the “Irrigation Amount.”

            Irrigation Amount = 318 mL / 16 square inches = 19.875 ml/sq inches

But the Rachio software doesn’t want to know this data in mL’s…it wants it in terms of (vertical) inches of water. According to this article, we can convert from mL to inches by multiplying by 0.06.

            53 mL * 6 = 318mL per hour

1.2 inches sounds like a reasonable answer, as I see the preset Rachio nozzles tend to range between 1 and 2 inches. If I got an answer like 45, I would have to assume that I did something wrong. Now, I go back into my zone and change the nozzle to a custom nozzle with the value of 1.2 inches. After that, I create a new watering schedule (as changing the nozzle amount won’t impact previously created watering schedules), and the Rachio software will automatically set the correct runtime.

Now that it’s all explained, let’s combine the variable into a simple equation that can be used for the other zones. Assuming a 10 minute catch test and standard size 16 square inch catch cups:

            The overall multiplier = 6 /16 * 0.06 = 0.0225

Now, I know that I can take the average mL picked up in 10 minutes in my catch cups, and multiple that number by 0.0225 to get my inches per hour. Let’s try:

            53 mL * 0.0225 = 1.1925

And the math checks out!


I bought these:

The “Pro” cups are $4/ea.

FYI I got the Orbit cups and used the Hydrorain calculator. Before my tests all zones looked relatively uniform so I was using the default 80% efficiency and the default .70 in/hr rotary nozzle (I have MP1000 rotators). Technically according to the data sheets these nozzles should be putting out .43 in/hr based on my pressure but that had seemed too low.

My catch cup testing found my uniformity/efficiency is roughly 68% on 4 zones, 58% and 78% on 2 others. I tried tweaking the sprinkler settings for the 58% one but it didn’t make that much difference. My sprinklers were originally all fixed spray head where it was easier to pick different distances so I doubt I can do much without physically moving sprinklers. As for the nozzle output I got from .56 in/hr to .76 in/hr with the average being .66 in/hr so at least that part was pretty close. I was watering all of these zones with roughly the same amount but according to Hydrorain I should be running my .56 in/hr zone 24 min longer than my .76 in/hr zone in the peak of summer. As for the suggested sprinkler runtime Hydrorain is suggesting about 15% more than Iro was with the same settings in a Flex schedule. Compared to a Fixed schedule which doesn’t seem to take much of the advanced settings into account Hydrorain suggested 60% longer runtimes for my 2 days a week schedule. When I switch to an As Needed schedule the difference was only 10%.

Overall the testing was useful to show me which areas needed adjustments without having to wait for brown spots in the summer that don’t end up recovering until fall and were likely being over corrected.


Great right up @Ed3120! Curious, are you setting your efficiency to 100% after changing your PR or are you leaving it at the default?

Since most sprinkler heads have a default PR at a given PSI, it seems like you could add one more step to your methodology to calculate efficiency per zone by dividing the calculated PR (using your formula) by the Nozzle Published PR (0.4 @ 40PSI for MP Rotators, for example) and get an zone efficiency rate.

So if your calculation results in 0.3 in/hr in Zone 1 and I use all MP Rotators in that zone with published PR of 0.4 in/hr, I could leave the zone nozzle type to MP Rotators (0.4 in/hr) but set the efficiency to 75% (0.3/0.4).

Would keep you from cluttering up your custom nozzles at least :wink:


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Please check my answer here Don't have a flow sensor yet..but.
I went through same learning process and put together a google doc excel sheet to calculate precipitation rates.

@GregS I’m curious as to why you used orbit curious but the hydrorain calculator, instead of the one that orbit supplies, or are they equivalent? Do the hydrorain cups have the same diameter at the opening?

Orbit doesn’t show the precipitation rate in the results which I wanted to plug into the Iro zone settings. Both calculators take the same information and assume the same size catch cup though the latter allows you to change that if you used non-standard 16.25 sq in catch cups. Hydrorain allows the catch cup data in delimited form so I could paste it in directly from an excel sheet while Orbit had individual boxes per cup.

@GregS The info. is there, but it’s a bit hidden. Sounds like a Hydrorain has it’s advantages though. Here’s how to find the additional Orbit data, including precipitation rate:


@GregS I was performing a test with Orbit catch cups today, so I went ahead and ran through Hydrorain for comparison. It looks like they use the exact same engine. Results were identical, and the GUI seems pretty much the same.

Both of those look like Hydrorain. When I use orbit the results look like this:

@GregS Click on where it says ‘Station 1’

Lol, nice, I never noticed that. Both sites resolve to the same IP so I’d say they are the same :smile:

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@GregS Mystery solved! :ok_hand:

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Doh! I totally did all the math involved after my test to figure out my in/hr and i didn’t even realize that info was available on the orbit site.

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But it’s really cool watching you run with this…your pics from earlier shows your hard work is paying off.

I’m a bit of a johnny come-lately here, but you can get a 12-pack of these from Home Depot for about $16. That amazon price is ungodly.