Drip Emitter Calculator for Precipitation Rate & Area

I put together a calculator for precipitation rate (PR) and area of emitter systems that do not have continuos, evenly spaced emitters. The calculator is complete, and includes the link to the Web Soil Survey instructions as well. The calculator is easy to use, but please read the notes above the calculator to be sure you give it the right data. The information below the calculator is just a description of the equations involved, so ignore that part of you don’t care for the details.

Please let me know if you have any issues. I’m hoping this cuts down on the setup issues that people have for these types of systems.

When you download the spreadsheet, be sure to enable editing or you won’t be able to use the calculator.

Drip Emitter Calculator for Precipitation Rate & Area (Download)

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I’m on mobile right now so can’t download it yet but can’t wait to take a look at it later. I’ve set up all of my drip zones on flex using your recommendations and they are all doing great so I’m sure this will be a great help to everyone


Love, love, love what you put together!!!

I had one minor problem with it, and can’t figure out why it’s not working for me — in the area calculation, the trees one works, but the shrubs and groundcover cells don’t change the area amount. The nozzle portion of the spreadsheet worked great! I’m still on Windows 7 and using MSOffice 2007 (yes, I know that I am WAY overdue for an update, on my list of things to do). Being so far out of date might be my problem.

Glad that you like it!

Hmmm, that’s strange. I just downloaded it and it seems to be working OK for me. You should see a change in the ‘ZONE AREA’ if you change any of the green numbers, except for ‘GPH per plant’. Is there a chance that you were only changing ‘GPH per plant’ for the zones that you had a problem with ?

@azdavidr, I finally got around to redownloading your spreadsheet. This time, when I made the changes to the number of plants, the trees, shrubs, and groundcover cells ALL work! I’m thinking it means I’m going to have to finally bite the bullet and upgrade my windows system to 10 and upgrade microsoft office. I’ve been having some other odd problems with excel as well. And of course, everything I’m running for windows is all unsupported now. (And I should know better – I used to be an IT Professional dealing with this stuff every day). Well … at least I’m making sure I’m taking multiple backups of everything . Sigh …

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Good to know @Linn, thanks for your post. I upgraded two PC’s to Windows 10 last year from 7. It didn’t go nearly as bad for me as previous migrations. Upgrading Office takes some getting used to, but otherwise it wasn’t a big deal. Good luck with it!

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I was running Office 2008 up until a couple weeks ago on my Mac. I tried to use the Mac products (pages, numbers), but Office is just too engrained into the culture. Fortunately got the upgrade for $10 through my company.

@azdavidr’s spreadsheet is awesome, isn’t it!!

FYI, I am running Win 7 on my desktop with Office 2010 and everything worked as expected on the spreadsheet. Maybe it was just a setting?

I upgraded to Win10 on my Surface Pro, because I really disliked Win 8 that it shipped with. My thoughts on 10? Meh.


Thank you, azdavidr. This spreadsheet is excellent. Wish the calculator included a zone for perennials (I have many roses, but they’re only a few years old, not mature bushes/shrubs).

How would I go about figuring out the “Inches of Water applied in Flex Daily schedule” for perennials?


@DroughtSaver You can use any of the calculator entries for perennials . Trees, shrubs and pots can just be considered arbitrary labels. Realize that the “Inches of Water applied” are meant to be replaced by your specific flex moisture graph tables. Don’t use the ones already in the calculator as they are specific to my yard. Does this make sense?

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Thanks, yes, that makes sense. I believe something strange may have happened after I setup the daily flex schedule. When I first installed the device two days ago, I tested all my drip zones and according to the Moisture Level statistics, I applied 0.15 in of water to each zone. How does it know this if I had yet to input custom nozzle PR using your helpful calculator? Or is it calculated irrespective of PR rate? When calculating custom Nozzle PR, for perennials, the flex daily inches of water number was 0.75in for perennials (3 zones), and for trees (1 zone), it was 1.88in. Last night, after setting up the schedule, I recalibrated each zone’s moisture balance by emptying it so it could have more accurate data going forward. That seemed to work fine, and Rachio gave me a schedule where it would water all zones tomorrow in the evening, and then ten days later, it would run a zone of perennials, and then the following day my other three perennial zones, irrigation of the tree zone wasn’t in the schedule (I guess it only shows what will be watered 10-14 days in the future?).

This morning, I check the watering schedule, and it seems to have changed. Right now it’s set to water at the scheduled time this evening, but there no scheduled watering events in the coming 10-14 days. When I try to look for the flexible daily schedule ‘Inches of Water’ number under moisture stats, in each zone, it’s at 0 in. Do you know what happened here? I’m thoroughly confused. Do I have to wait for Rachio to complete a full irrigation cycle again?

btw, we had a light drizzle in the early morning but it was recorded as only 0.02in (according to moisture stats), I doubt this made any sort of effect at all in the schedule. I should note that I have clay soil, which is moisture retentive but I do live in a pretty warm climate (inland Southern California, closer to the desert) and we still get days in the high 80s in October, so I’m expecting the zones to be watered more frequently.

Thank you in advance

Any thoughts of what happened or how explanations as to how I screwed this up? @franz or anyone else?

There’s a lot of information to take in, though I wish setting up drip zones could be a little easier. Btw, my custom PR values calculated through the spreadsheet for my drip emitters in the perennial zone (0.6in/hr) are not far from the default 0.5in/hr.

Thanks in advance

@azdavidr btw, why set efficiency to 100%? Drip has very efficient distribution uniformity? Doesn’t that depend on a lot of factors?

Every time you perform an ‘empty’ or ‘fill’ of the moisture tables it takes about a day for the moisture graphs to make sense again. If you still see this problem tomorrow then include some snapshots of your graphs, but be sure to expand the table detail underneath.

The goal of the calculator is to put out a set amount of volume, in gallons. At 100% you get exactly what you would expect based on the equations. You could dial the efficiency down from there and the watering duration will get longer. For drip systems I’m not sure that the ‘efficiency’ is terribly clear, at least to me. I know that some of my emitters put out a lower rate than others, but it would take an eternity to measure each one and figure out what the efficiency really is. That’s why I left it at 100% and left it as an ideal target. Again, if you want to lower it after you’re done with the calculator you can do that, and your times will increase accordingly to make up for the specified drop in efficiency.


You’re right, it took 12 hours or so to make sense again. Why is that interval? I noticed this behavior for other things when adjusting various settings.

it would take an eternity to measure each one and figure out what the efficiency really is.

I just measured a sample of 10 rainbird emitters in every 4 of my drip zones for the heck of it using paper cups and a stopwatch. They all give consistent flow and are calibrated correctly. I feel safe setting the efficiency to 100%

You must have a new system to have such a uniform distribution! Over time mineral deposits can plug emitters and leaks can occur. Just curious–are you using pressure compensating emitters?

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Yes, they’re a few months old and they’re pressure compensating. They can be completely taken apart for cleaning. I also have 200 mesh stainless steel screen filter on each drip zone. I flush out the drip lines once a month as preventative maintenance (we have hard water here).

These are the emitters I use: https://www.dripdepot.com/item/ceta-cleanable-pressure-compensating-dripper-flow-rate-half-gph

Thanks for the link! I may have to try some of those emitters.

Exploring the site led me to a gravity feed timer that might come in handy for the fruit trees that need roughly a gallon of water /month during the winter. Since we winterize the rest of the garden and put the Iro in standby, the Hubs built a passive solar water storage system. The problem was finding a timer that could deliver that amount of water during the winter.


@azdavidr do you know why flexible daily schedule ‘Inches of Water’ values change or just switch to 0.00 in? I haven’t touched the “empty” or “fill” functions in moisture graph since my last post here. I was used to consistently seeing the values 0.75 in. for perennial zones and 1.88 in. for tree zones since I got my Rachio in October, just before the start of the rainy season in southern California. I changed some drip nozzle values with your helpful calculator and ‘allowed depletion’ settings to adjust watering amount and frequency, but everything else stayed the same (soil type, slope, root depth, crop coefficient, sun exposure, etc).

the flex daily schedule inches of numbers have changed for all zones. Does this maybe have to do with fact that there hasn’t been scheduled irrigation in 2+ weeks for all but one zone (because of rains and cooler weather)? I’m seeing 0.00 in. on moisture graphs for all but one zone (the zones displaying 0.00 in all have same settings - vegetation type, soil, root depth, crop coff, etc). I did manually run some zones for a little bit when we had extremely windy weather last week. For another perennia zone, I see 0.6 in. instead of 0.75 in (this is a drip zone for lilies that need more frequent watering than the rest of my xeriscape/perennials). Aren’t these numbers supposed to be constant irrespective of weather?


This Flex Daily FAQ talks about the watering duration:


I noticed that they talk about managed allowed depletion (MAD) changing the duration, so that would be my guess as to why the ‘inches of water’ changes.

You might play with it to see if that’s indeed the case. Note that changing MAD can be a bit tricky and end up changing both duration and frequency, whereas crop coefficient only changes frequency and the nozzle precipitation rate only changes watering duration. I think most people keep MAD at 50%?