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Flow rate math
How do you calculate the flow rate a pool needs?
Divide the pool volume in gallons by the required turnover time in minutes. Convert the turnover from hours to minutes first by multiplying by 60. A 90,000 gallon pool needing a 6 hour turnover requires 90,000 divided by 360 minutes, or 250 gallons per minute.
Flow rate is the pump output needed to hit a required turnover, and it is where a single skipped conversion sizes a pump sixty times wrong. Below is the formula, a worked example, the exact error class, and eight randomized problems. It builds directly on turnover rate.
The error class that fails people · Dividing by hours instead of minutes Flow rate is per minute, so the turnover hours have to become minutes first by multiplying by 60. Divide the volume by the hours directly and you skip the times-60 step, sizing a pump about sixty times too large. If your required gpm looks wildly high, you forgot to convert the turnover to minutes.
Drill it: 8 randomized problems
Eight flow-rate problems, including one where the turnover is already given in minutes so you must NOT multiply by 60. Every answer reveals the full worked steps, and the wrong choices are the hours-not-minutes slip. Free, no signup, progress saved locally.
Sample problems
A preview of the kind of problem in the set, authored from the underlying pool-hydraulics facts. Open the answer for the full worked solution.
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A 67,320 gallon pool must turn over in 6 hours. What flow rate in gpm is required?
- A 11,220 gpm
- B 187 gpm
- C 19 gpm
- D 1,870 gpm
Show the worked solution
Correct answer: B. 187 gpm
Convert the turnover to minutes first: 6 x 60 = 360 minutes. Then flow = volume / minutes = 67,320 / 360 = 187 gpm. Dividing by 6 hours instead gives 11,220 gpm, a pump 60 times too big, the signature flow-rate error.
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A 120,000 gallon pool needs an 8 hour turnover. What is the required flow rate?
- A 250 gpm
- B 15,000 gpm
- C 25 gpm
- D 2,500 gpm
Show the worked solution
Correct answer: A. 250 gpm
8 hours x 60 = 480 minutes. Flow = 120,000 / 480 = 250 gpm. The 15,000 gpm answer divided by hours and skipped the x 60.
Questions people ask
Why do I multiply the turnover hours by 60?
Flow rate is expressed in gallons per minute, so every term has to be in minutes. A turnover given in hours must be converted to minutes by multiplying by 60 before you divide the volume by it. Skip that step and the required flow comes out about sixty times too large.
How do I size a pump from the turnover requirement?
Divide the pool volume in gallons by the required turnover time in minutes. The result is the minimum gallons per minute the pump must deliver, before accounting for head loss through the plumbing and filter. Pick a pump rated at or above that flow, then verify it against the system's total dynamic head.
What if the turnover is already given in minutes?
Then divide straight away and do not multiply by 60. A spa required to turn over every 30 minutes at 3,000 gallons needs 3,000 divided by 30, or 100 gallons per minute. Multiplying an already-in-minutes turnover by 60 is the reverse of the usual error and gives a number far too small.