GPH nozzle to in/hr conversion for single point drip emitters

There are many threads regarding the confusion of converting from standard ‘gallon per hour’ (GPH) drip emitters to the precipitation rate of custom nozzles in the Rachio software. I’d like to propose a method that can simplify this in software. In the meantime, maybe it’s useful for folks to do it manually for Flex Daily users that have access to their moisture graphs. It’s particularly useful for those of us in the desert southwest, but others have expressed similar issues as well.

I think it’s important to state that there is no intent to change the total inches of water displacement calculated by the Rachio software, so the base Flex equations stay the same. This is just a way to take area out of the process of calculating the PR for a single point emitter, or a few single-point emitters on one plant, for those of us that have an idea of how many gallons we’d like to deliver to each plant. So here’s the procedure.

Step 1. Go through all normal setup procedures as they exist now, especially soil type, vegetation type and root depth. Set the efficiency to 100%. Don’t worry about area.

Step 2. Determine the number of gallons that you’d like to deliver to a plant (‘Watering Volume’). If it’s someone using Water Use It Wisely for a desert application, then reference step one from that website. Here’s a table that they provide for us in Phoenix.

Step 3. Look at your Flex Daily moisture graphs (found in your zone setup) and determine the total water displaced per watering, in inches of depth. Here is an example for my shrubs. In this case the the value for ‘Watering Depth’ = 1.19 in.

Step 4. Determine a Total GPH per plant. For my shrubs, I have a single 1 GPH emitter per plant, so Total GPH per plant = 1 gal/hr. For my trees, I have three 2 GPH heads per tree, so Total GPH per plant = 6 GPH (# of emitters x GPH/emitter).

Step 5. Calculate precipitation rate as PR(in/hr) = Total GPH per plant * Watering Depth (in) / Watering Volume (gal).

Step 6. Use your calculated PR(in/hr) value in a custom nozzle and attach it to your zone.


For those interested, the equation is determined by using the run time equation based on Rachio’s ‘Watering Depth’ value, and another for total gallons desired, then equating the two:

  • runtime Rachio (hr) = Watering Depth (in) / PR (in/hr)
  • runtime Gallons (hr) = Total Gallons Delivered (gal) / Total GPH per plant (gal/hr)

equate the runtimes:

  • runtime Rachio (hr) = runtime Gallons (hr)
  • Watering Depth (in) / PR (in/hr) = Total Gallons Delivered (gal) / Total GPH per plant (gal/hr)

solve for PR (in/hr)

  • PR(in/hr) = Total GPH per plant * Watering Depth (in) / Watering Volume (gal)


Step 1. All zone settings set properly except area.
Step 2. I looked at Water Use It Wisely and decided to target 7.5 gallons per plant, each plant having a single 1 GPH head.
Step 3. I looked at my moisture graph and found Rachio determining that 1.19in per watering was required.
Step 4. Determine a Total GPH per plant ( 1 emitter/plant * 1 GPH/emitter = 1 GPH per plant).
Step 5. pr (in/hr) = 1 GPH * 1.19in / 7.5 gal = 0.16 (in/hr)
Step 6. Enter 0.16 in/hr into a custom nozzle and attach the nozzle to my Shrub zone.
Step 7. Validate run time. The schedule shows me 7h 30min. 7.5 hrs * 1 GPH = 7.5 gallons, so it checks out.


Step 1. All zone settings set properly except area.
Step 2. I looked at Water Use It Wisely and decided to target 30 gallons per tree, each plant having a three 2 GPH heads, so I use a value of 6 GPH for each tree.
Step 3. I looked at my moisture graph and found Rachio determining that 1.95in per watering was required.

Step 4. Determine a Total GPH per plant ( 3 emitters/plant * 2 GPH/emitter = 6 GPH per plant).
Step 5. pr (in/hr) = 6 GPH * 1.95in / 30 gal = 0.39 (in/hr)
Step 6. Enter 0.39 in/hr into a custom nozzle and attach the nozzle to my Tree zone.
Step 7. Validate run time. The schedule shows me 5 hours. 5 hr * 6 GPH = 30 gallons, so it checks out.

If something like this is allowed for in the software, with caveats of course for proper emitter placement, the PR in in/hr calculation can be hidden from the user and avoid confusion. For that matter, they don’t even need to have an area estimate for proper water usage. The gallon delivery per plant is already entered, so you can just ask for the total numer of plants to calculate the zone total. Then the area can be hidden for this GPH emitter. All the fundamental Rachio equations remain the same.


So my concern with this formula is that pr is unstable. If I reduce your watering volume to 1 gallon your pr shoots through the roof. This tells the system that the smaller the applied quantities are the large the application velocity becomes.

If we take this formula and tweak the root depth sat 3 inches we get a pr of .6. It’s unstable, pr needs to be a fixed quantity so that iro knows how long to run a zone based on pr to fill it up. So the effect is that iro would run for less time with that deeper root because the pr almost doubled.

I’m a ssuming you are using the 6 foot diameter tree. So I,compute it by taking your ring of 6 inches pie r squared and compute .34. Which is,stable until you augment the ring.,now iro can make a stable computation for run time.

Your formula seems to be a compliment to flex.

I’ll kick this around some more.

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I’m not sure I understand. If I take my shrub example, with the current 7.5 gal requirement and a 1 GPH head,

PR (in/hr) = 1 GPH * 1.19 in / 7.5 gal = 0.16 in/hr

If I change it t 1 gallon instead of 7.5 gallons,

PR (in/hr) = 1 GPH * 1.19 in / 1 gal = 1.19 in/hr

That PR of 1.19 seems like a reasonable value. There have been suggestions around here that are clear up to 3.5 in/hr when the 1 sq. ft. assumption is used.

My formula doesn’t use root depth. That 1.95in you see is the inches of water that Flex puts down. I grabbed it from the moisture graphs (see the picture in step 3 for Example 2 - Trees). The root depth still needs to be accurately defined, as mentioned in step 1, and the Flex algorithm uses it to determine how much to water (in inches). If I change my root depth, I need to look at my moisture graph again to see what the new value is that Rachio wants to put down, in inches. The key here is that the Rachio Flex formulas don’t change. The existing Flex Daily algorithms calculate inches of watering, and use them to manage the AD. This is just a way to map a desired gallon displacement to in/hr. I and others have done this by trial and error. This formula just removes that trial-and-error. I’ve been working with these PR values from pleasant spring time temps to 117F, and flex has worked beautifully. I just didn’t use the 1 sq.ft. assumption since we have single point emitters. If I had, everything would be dead.

I followed everything fine until I got to this. What did you mean here?

In my example of the trees, I defined 30 gallons to be watered per tree. In order for the software to keep record of how many gallons my zone for trees consumes, I would just need to tell it how many trees there are. In my case I have 4 trees, so I use 120 gallons every time the zone runs.

No you are right, inches in rain, so your proposal is to have someone enter the number of gallons they want to apply per run, so that is a fixed quantity?

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Correct. Then Flex takes care of managing the intervals.

@franz, i think this is worth at least putting in a beta next year…i mean to really align flex with drip is borderline rocket science. i do feel that some ppl will never ever be please with anything less than a deep learning algorithm ripping through images of their yards, but i think this could handle ~60% of flex/drip users.

i still want to kick around the implications to et computations not having surface area explicitly defined. i mean, with turf its failry straight forward to handle et, but once you go 3 dimensional with a chaotic distribution of stomata, it starts to blow my mind.


@plainsane @azdavidr Thanks for laying all of this out, I need to consume all this :wink:

We are building a new watering platform for next year and I’ll be sure to determine if this fits into our overall architecture.


@plainsane @franz Thank you both !

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Ok, I think I get it now. It would be used instead of area for calculating gallons used. So my .2 in/hr nozzle for my 1GPH emitters for 75 plants, watering for one hour would be 75 gallons used, right? So not part of anything of the flex equations, just usage.

@Linn You’re correct for water usage calculations. Where Flex Daily comes in is just finding the number it is using for inches of watering then using it to calculate your PR, rather than having to play around with different PR values until you get the gallon output per plant that you’re looking for.

Good! That’s what I understood, and I like the way you’ve presented it.

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