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How to Determine Flow Rate (Gallons per Minute)

If you need to compute flow rate in gallons per minute (or liters per minute) of a steady flow in to a holding tank, or open discharg
you can measure the flow into a bucket, (a five gallon bucket for instance) and time the flow with a watch or clock. For instance if it takes 1 minute to fill a five gallon bucket: 5 x 1 = 5 gallons per minute. If it were to take 45 seconds: 60 seconds divided by 45 times 5 gallons = 6.6 gallons per minute.

Alternatively, you could install a water meter on the line and record the flow rate as read from the water meter.

If you have a combination well pump and pressure tank, then you need to use this method:

to Determine Gallons Per Minute (Flow Rate Available, Pump Capacity) on Private Systems with a Pressure Tank:

  1. Open any hose bib or faucet until pump turns on.
  2. Close hose bib or faucet and let pump fill up pressure tank until it turns off.
  3. Using a 1 or 5 gal. bucket, open faucet, collect and measure all water discharged until pump turns on.
  4. When pump turns on, immediately close faucet and start timing pump cycle.
  5. When pump turns off, record pump cycle time to refill pressure tank in seconds.
  6. Divide the number of gallons collected in Step 3 by the number of seconds in Step 5.
  7. Multiply the answer from Step 6 by 60.
  8. The answer in Step 7 is the average pumping capacity of the pump in gallons per minute (GPM).

Example: Number of Gallons collected during draw-down of pressure tank (Step 3) = 20

Number of Seconds in pump cycle to refill pressure tank (Step 5) = 95

Gallons Per Minute (GPM) = (Gallons collected / seconds in cycle) x 60

GPM = (20 / 95) x 60    so: GPM = .21 x 60   so: GPM = 12.6

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