Formula - Calculating Nozzle Inches per Hour

Please tell me if this is inaccurate, or even creating a problem.

I noticed that my measurements of water usage and what Rachio said I used in a cycle were vastly different. For instance, 20 minutes of watering in one zone, per Rachio, used 248 gallons, while my measurement from the water meter was 125 gallons. That’s 12.4 gallons a minute per Rachio, 6.25 per my measurement from the meter. Meanwhile, Rachio set the nozzle inches per hour to 1.5.

Would it be accurate to say that since the Rachio ratio is ~.12 , that number times 6.25 is about .75 (actually, .756), so that I should adjust the nozzle inches per hour on my zone program to .75?

I’m only trying to get a handle on how much water I use, so I don’t want to reset Rachio’s values to change how much I truly am watering and harm my plants.


Yep, you got it. Adjust the nozzle precipitation rate to bring the estimated water usage closer to reality. Expect the watering duration to go up.

I do my watering duration for fixed times, so, after double-checking, glad to say that didn’t happen. Here in CA, we gotta be careful not to waste!


See if this formula helps:

Thanks JB

The wsu calculator has a unit converter built in for square footage in different units, I have to assume their unit cancellation is accurate. The Rachio app zone mapping feature enables close calculation of zone area in sq ft.

Has the originator of the formula you shared ever showed his work disclosing how the units balance from one side of the formula to the other?

This isn’t my strong area so seeing units cancel to arrive at final conversion would boost my confidence in this formula.

By the way I ran the calculation using the formula you provided and came up with the same inches per hour as wsu’s calculator. It would still be nice to see units cancel.


You’re overthinking this calculation. It’s the historically correct and proven calculation of Irrigation Rate used since flow measurement was invented. It’s not something new - Rachio just called it something different because… something about nozzles.

Here you go…

1 Gal == 231 cu. in.
1 Hr == 60 min.
1 sq. ft. = 144 sq. in.
FLOW in gal/min == 231 cu. in. / min. == 60 x 231 cu. in. / hour
AREA in sq. ft. == 144 sq. in.

Irrigation Rate = FLOW / AREA == (60 x 231 cu. in. / hour) / (144 sq. in.) == (96.25) in. /hour

Happy watering.

John Bishop

thank you for humoring me and giving me this info so I could stay in practice verifying my units


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