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 discharge:
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:
How to Determine Gallons Per Minute (Flow Rate Available, Pump
Capacity) on Private Systems with a Pressure Tank:
 Open any hose
bib or faucet until pump turns on.
 Close hose bib
or faucet and let pump fill up pressure tank until it turns off.
 Using a 1 or
5 gal. bucket, open faucet, collect and measure all water discharged
until pump turns on.
 When pump turns
on, immediately close faucet and start timing pump cycle.
 When pump turns
off, record pump cycle time to refill pressure tank in seconds.
 Divide the number
of gallons collected in Step 3 by the number of seconds in Step 5.
 Multiply the
answer from Step 6 by 60.
 The answer in
Step 7 is the average pumping capacity of the pump in gallons per
minute (GPM).
Example: Number
of Gallons collected during drawdown 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
