Determine Spa Plumbing System Flow Rate

To accurately determine the flow rate of any spa or swimming pool plumbing system you will need to install measurement meters into the plumbing, such as a flow meter or pressure & vacuum gauges.

It's very important to know actual flow rate in the spa/pool plumbing system in order to correctly size the associated safety suction fittings to not only ensure the pump receives sufficient flow, but to ensure bather safety against entrapment.

Using Vacuum and pressure gauge method - install a pressure gauge into the pump discharge plumbing, and vacuum gauge into the pump inlet plumbing. With a completed plumbing system, run the pump and record the gauge readings.
Add the two gauge readings in KPa, and then divide by 9.804 - this equals the installation in metres of head pressure. You now know the actual head pressure the spa or pool plumbing installation is operating at. Use pump manufacturer flow rate data / curve to determine flow rate at that given head pressure. 
Select appropriate suction fittings compliant with the determined flow rate - you may require multiple suction fittings to ensure combined total equals or exceeds double the spa / pool plumbing system flow rate. 

Using the Flow meter method - Install a flow meter in the plumbing from the pump ensuring installation is as per meter instructions. Correct installation often requires minimum lengths of pipe before and after the meter without fittings such as valves or elbows which can affect even flow and cause meter inaccuracy. Some flow meters available will only provide an accurate reading if they remain permanently plumbed into the system as they themselves affect head pressure and therefore flow. On removal flow would increase compared to that recorded. The better flow meter design utilises a small paddle or post rather than a spring loaded flap. Record the actual flow rate from the flow meter. 
Select appropriate suction fittings compliant with the determined flow rate - you may require multiple suction fittings to ensure combined total equals or exceeds double the spa / pool plumbing system flow rate.