You can get a fairly close idea of your water flow rate by simply running water at full open position through either an outside garden hose faucet or with your bathtub faucet. Example: Turn the faucet on to the full open position… then quickly put the gallon container under the full flow of water. Immediately start timing how many seconds it takes to fill the container all the way up. If it fills the container up in 15 seconds, you simply divide 60 seconds (1 minute)…by 15 seconds (the amount of time it took to fill the container up). The answer is 4, so your flow rate would be very close to 4 GPM! We recommend that you order a unit that would handle at least 4 GPM. It would be over size the unit to ensure you are getting a unit with plenty of GPM flow capacity.