GPH to Nozzle Inches

If I have a 2 GPH emitter, what is the nozzle inches per hour?

Not enough information. And with emitters, the nozzle inches per hour almost doesn’t apply.

I’m not being a smart-xxx, nozzle inches per hour depends on the amount of water per unit time (which you have) divided by the area being watered (which you don’t). If a zone only has a single 2 GPH emitter (doubtful), and let’s say that emitter waters an area 2 feet in diameter (that’s where it gets sticky), then that would calculate out to about 1"/hour. If it’s watering a 3 foot circle, it would be 0.45"/hour.

Rachio’s Flex Daily isn’t designed for drip. With drip ins/hr don’t matter. What you need to know is 1) how much water you need to apply per watering; and 2) how frequently to water. These are different for different plant types, so you’ll need to figure out this yourself for your zones (and the more heterogeneous your plants are per zone the better, although you can still use different flow rates to compensate for differences between plants).

So, for drip the easiest thing to do is to use the fixed schedule and set up your own frequency and duration. One disadvantage you’ll have with Fixed, though, is that waterings won’t be adjusted to match the weather (precipitation and evapotranspiration). For this you’ll need shoehorn Flex Daily into working with drip.

So f\or Flex Daily you also need to start with your own gallons per watering and frequency. Do the best possible zone configuration using the non-Advanced parameters. Once you put the zone on a Flex Daily schedule Rachio will run your parameters through its normal equations and come up with some duration and frequency. Since you know how much water your emitters apply in an hour and how much water your plants need, you can calculate the desired application time (watering time).

Now you need to start modifying advanced zone parameters to match your desired time and frequency. For changing frequency you can try adjusting Allowed Depletion and for duration, in my opinion, fudging Nozzle Inches per Hour is the best approach.

I have a station that has 5 trees and there are two 2 GPH emitters per tree. What should I enter for nozzles per inch as a starting point?

  1. where do you live?
  2. what kind of trees are these?
  3. how much water do you need to apply per watering?
  4. how frequently do you need to water?

While it Fixed won’t adjust daily, it will adjust monthly if Seasonal Shift is turned on.

Irrigation is rated in gals per minute. A number 3 head is 3 GPM for the full cycle of the head. So if you go from a 360° to 180° you are doubling to amount of water to the area. Since it cover half the distance at twice the rate in the same amount of time. You want to know how much water use per zone. Read you meter at the end of each zone run.

There’s a lot of complication for drips, especially with different plants in the same zones. I think I actually have my Rachio tuned in pretty well now, maybe this will help you. I have two drip zones, both with trees, shrubs and some other stuff. I’m not as worried about the other stuff so I made a spreadsheet calculating only the estimate for trees and shrubs separately, then combined/averaged them together for the whole zone. That’s the part that you can’t ever get 100% correct since every plant is different.

I don’t use emitters on my lines, just wide open 1/4" to each plant. My flow rate per plant is about 15 GPH. However, I remade an example in my spreadsheet for use with emitters, here’s a pic:

For example, emitters at 5 GPH per tree and 2 per shrub, then add them up. My zone 7 with 7 trees (35 gph) and 16 shrubs (32 gph), total GPH comes to 67. I did the area in sq ft, I’m not sure yet if that’s the accurate way to do the area for this but it seems to calculate the watering amount and time pretty close to what I used to do before Rachio. For the area, I did 25 sq ft per each shrub and 100 sq ft for each tree then added them up. You could get a lot more specific with that per plant if you want.

So to get the nozzle inches per hour, ((GPH * 231) / area ) / # of drips or emitters. The 231 is how many cubic inches per hour there are per GPH. So for my zone 7 it would be: ((67 * 231) / 1100 ) / 23 which equals 0.6 nozzle inches per hour. Then when setting crop coefficient and root depth its just a educated guess based on your plants. Calculating how many days between watering and water per application can help with that guess. You can use a spreadsheet to do that or create a flex schedule and see what Rachio sets it as on the calendar as well as the recommended watering time.

My spreadsheet pic is probably over complicated looking lol :slight_smile:

Well, the actual formula for Nozzle Inches per Hour as a function of GPH and ACTUAL WATERED AREA is:

Nozzle In/hour = GPM / Area x 95.25 = GPH / 60 / Area x 95.25, or in the above case:
Nozzle In/hour = 67 / 60 / 1100 x 95.25 = 0.097 "/hr

I’m not saying your calculations are incorrect (haven’t gone though the spreadsheet, for example), except that using the Area you’ve stated (25 sq ft per shrub sounds very high), an average flow over that entire area is a fraction of what you’re specifying to Rachio. So, if you’re using Rachio to calculate the time, it would only come up with 0.097 / 0.6 x 100 = 16% or about 1/6 of what the 1100 square feet are expecting.

Using your calculations, If you start out wanting say 1" of water over the course of a week, inputting 0.6 "/hr into Rachio for Nozzle Inches per Hour would give about 100 minutes per week (not considering efficiency), which would give each tree with two 2 GPH emitters 100 / 60 x 0.6 x 2 x 2 = 4 gallons per week. IMHO not nearly enough for a tree, maybe about right for a small bush.

@rraisley The constant for precipitation rate is 96.3. That’s 231 in3/gal divided by 144 in2/ft2 = 1.604 inches deep for a gallon of water x 60 minutes = 96.3

Where does the 95.25 come from?

For my sprinkler heads I do GPM * 96.3 / Area = in/hr. I was trying to figure out something different for emitters though since precipitation is pinned to a single spot. I may be wrong in using the direct conversion of gph into in3/hr but I’m not convinced the same precipitation rate formula applies to emitters very well, maybe better suited to drip soaker hose. With my actual setup, each plant with its own .25" line (with no emitter) produces 15 GPH. So if you use your formula with my setup:

23 lines x 15 gph = 345 gph

345 / 60 / 1100 sq ft x 95.25 / 23 lines = .022 nozzle in/hr

The recommended watering time is ridiculous! Trees and shrubs typically should only need 10 gallons per inch of their trunk diameter every couple weeks. Here’s the comparison using my attempted new formula vs the precipitation rate formula you used:


Using the precipitation rate formula you used it’ll water enough to fill 1/10 of an olympic swimming pool in a month, 60000 gallons. It would drown my trees and shrubs, and my property.

Using my direct conversion method (345 x 231 / 1100 / 23 = 3.2 nozzle in/hr) may not be right, but 429 gallons per month is much more accurate. With 23 trees/shrubs, 429 / 23 = 18 gallons each plant gets per month.

My formula may water too little but using the precipitation rate formula waters WAY too much.


Edit: Forgot to explain why I did the area that way (25 sq ft per shrub, 100 sq ft per tree). I am trying to average the possible root needs per plant, like many shrubs are 5x5ish and trees 10x10ish (I have young and old plants so tried to average their individual areas). So instead of doing the depth for a gallon of water like the precipitation formula 96.3, I’m attempting to do the surface area over the estimated root needs per direct gallon of water that flows at the base of a plant. Not sure if that makes sense.

Edit: Interesting info. Here’s a way to estimate the individual water requirements of a tree, shrub, flower etc:

Gallons = ET x PF x Area x 0.623


  • ET is inches of historical average or real-time evapotranspiration for the period of interest.
  • PF is the Plant Factor for trees in tree-only plantings, or the PF for the plant type with the highest PF in a mixed planting (0.5 for trees, shrubs, vines, groundcovers, and herbaceous perennials; 0.8 for annual flowers and bedding plants).
  • Area is the surface area covered by plant canopy or the planted area, in square feet (the equation assumes plant canopy covers at least 80% of the landscape area).
  • 0.623 is the factor to convert inches of water to gallons.

So an individual 10x10 tree in my area right now needs .2 x .5 x 100 x .623 = 6 gallons of water per application. That 6 gallons is supposedly enough to saturate 50% of the soil in that 10x10 foot area with emitter at the base of the tree.

With that it should be easy to calculate a good average duration to water drip line plants. Now just need to incorporate that in how Rachio calculates nozzle inches per hour :slight_smile:


My error. It’s 96.25 (your 96.3 is slightly rounded). I was doing it all from memory, and got it wrong.

I agree with the tree and shrub requirement, but as soon as you’re using Area, the above calculation is still correct (except for the 96.25 vs 95.25 thing). Using Area along with Rachio’s other requirements is just like doing a lawn: assuming you have an area (in this case say 55 x 20) that has trees and shrubs in it). What you’re actually doing is concentrating water at the plants, watering a much smaller area.

In your spreadsheet, for calculating “Using precipitation rate formula”, I think you have an error in the Nozzle in/hr:

Nozzle in/hr = 345 gal/hr / 60 min/hr / 1100 sq ft x 96.25 = 0.50"/hr. You’ve apparently divided by 60 twice: once to get 0.50 (correct) and again to get 0.022 (incorrect). So it’s off by a factor of 60, meaning watering time would be 51.6 minutes (a reasonable run time), not 3,096 minutes.

Yeah, I don’t think you’ll be filling many olympic swimming pools using the formula.

IMHO, there are two ways of figuring out how long to water an area:

  1. Measuring the area actually being watered (evenly, more or less) using physical measurements and the flow rate of water to the zone, then calculating Nozzle in/hr by the standard formula. This works well for lawns and other areas being completely watered, regardless of crop, and is what Rachio is set up to do. You can also calculate Nozzle in/hr from head specs and catch cups, but I prefer the measurement method.

  2. For drip lines, where the unknown area is unknown, and is difficult to measure, for use with individual trees and shrubs for which water requirements are published, the above system does not work. You’re trying to develop a new formula which uses two contradictory values: Plant water requirements, and Plant Area. That will result in 2 different answers, because area, especially, is a WAG. But you can still calculate settings based on plant water requirements and measured flow rate:

If you are putting the 345 gallons per hour down as you mention, and you want to put a total of 429 gallons down during the course of a month, then you will simply need to water a total of 429/345 hours a month to water the plants. If you water every 2 weeks (fine for trees, shrubs might want it more often), then it would result in 429/345/2x60 = 37.3 minutes every 2 weeks. This is quite a bit less than normal drip systems IMHO because you’re applying it at a much higher rate than normal drip systems (345 / 23 = 15 gph per tree would take 7-8 2 gph emitters). So the number makes sense.

Now, to use that number with Rachio, if you use a Fixed schedule, you’re done. Set the minutes and that’s it. But one thing not taken into account with the water requirements is weather. Surely a shrub requires more water in 95 degree weather than in 70 degree weather. That’s where Flex Daily is helpful, and to use it you need to calculate Nozzle in/hr. Doing that in Flex Daily can be calculated out, but will depend on the values for Soil Available Water, Root Depth and Allowed Depletion, which calculate out to water applied at once. To that Efficiency and Nozzle in/hr result in Time, so knowing Time and Efficiency, you can calculate Nozzle in/hr. I was going to do that, but as no other factors have been shown, plus Flex Daily won’t just water every 2 weeks. You can estimate a value for Nozzle in/hr, then see how much and how often it will water, resulting in XX minutes per month, and re-adjust Nozzle in/hr until you have what you want. But it will vary based on temperature, of course.

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Nozzle in/hr = 345 gal/hr / 60 min/hr / 1100 sq ft x 96.25 = 0.50"/hr. You’ve apparently divided by 60 twice: once to get 0.50 (correct) and again to get 0.022 (incorrect). So it’s off by a factor of 60, meaning watering time would be 51.6 minutes (a reasonable run time), not 3,096 minutes.

Ah, I’ve been dividing that number by the number of nozzles, so in the case above it was 0.50" / 23 = 0.022". Just to verify I looked up the PR formula and you’re correct, not supposed to divide by the number of nozzles. I’m a little confused by that now though as for my lawn zones. For example zone 1 is 981 sq ft, 12 GPM, which would be 12 / 981 x 96.3 = 1.18" vs dividing by the 4 nozzles = 0.3". When I do catch cup tests the rate is more around 0.3" per hour, not anywhere close to 1" per hour.

I also see another thing I’ve done wrong though which will definitely change the calculation above. The area used in the PR formula isn’t supposed to be the entire area of the zone, only the area between sprinkler heads. I’ve been using the entire zone area. Rachio doesn’t explain that if you go to set up your yard map diagram for each zone. That’s going to make thing tricky.

I have a lot to redo in my calculations for all of my zones now, lol.

You can estimate a value for Nozzle in/hr, then see how much and how often it will water, resulting in XX minutes per month, and re-adjust Nozzle in/hr until you have what you want.

I think I will use that Gallons = ET x PF x Area x 0.623 formula to get the gallons needed per plant to figure out water duration, etc, for my drip zones. Just to get it as close as possible with an overall average water amount. That’s basically what I was trying to do with my surface area formula but understanding the PR formula now I agree with you. And for the drip zones it’ll be better to get with the water duration for the amount of water needed and adjust the nozzle rate per hour in Rachio to meet that time.

Thanks for the help!


I went back through my spreadsheet and updated my calculations. For my lawn zones I kept the same area as before (not using the between sprinkler heads area) but I adjusted the GPM to compensate. Like if a 360 degree head is 4gpm but it’s only doing 270 degrees then I set it to 3gpm, etc. After doing that and correcting my nozzle inch per hour formula mistake, everything looks great. Actually all zones recommended watering times are within 10 mins of what I historically was doing. Now using Flex Daily is going to water possibly more frequently than I historically did but the overall gallons per month should actually decrease. I’m curious to see how this does. Three of my six lawn zones I historically watered less than I should’ve to save money but did have dry patches. I’ll see how this Flex Daily schedule makes it look and if its too dry I’ll adjust my GPMs to correct nozzle inches per hour. For now I only estimated GPMs based roughly on what I described above like the 360 head only using 270 of its capability.

Now regarding my two drip zones, I went through and cataloged each plant canopy, calculated its estimated water gallon need per month and used the simple math you mentioned (total gallons per month / GPH / 2 x 60) to get the minutes to water every two weeks. For zone 7 I mentioned yesterday, I was estimating 1100 sq ft of area and after adding all of the canopy areas together came to 1074, lol. However in calculating the nozzle inch per hour rate to get the estimated water every two weeks close to that simple math, I had to divide the PR equation by the number of lines to plants. IMO that makes sense with direct drip lines (not soaker lines) since the nozzle rate is not spread over an entire area but pin pointed to a plant. So the simple math for water every two weeks is now almost identical to the recommended watering time Rachio calculates using water per application / nozzle inches per hour / (0.4 + 0.6 x efficiency) x 60 minutes for full sun. My next step may be to start using emitters to try and get the mixture of different plant a more correct amount of water each. For now though, its going to water almost identical to what I historically did :slight_smile:

Most of that probably doesn’t make sense since I’m not showing any of my data, if you want to see more info let me know! Thanks again for the help

You might check out this post if you haven’t already

Welcome back man. Your spreadsheet is very helpful.

I’m a fellow Arizonan and have had the Rachio 3 since early December 2020.

My plants are doing excellent, especially compared to the neighbors using flex daily. I’m surprised how easily they transitioned to the deep infrequent schedule. I did start in December, which helped them adjust before summer,.

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Thanks @ECOBEARD, that’s great to hear! Watch your yard this first summer or two. You know how crazy our temps get so sometimes summers dictate the settings that will work year round. The first summer or two I had to make a few tweaks but it’s been years now and I haven’t had to make any significant changes. Glad to hear it was useful to you!

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