The flow rate listed on all new faucet brochures is the gallons per minute (GPM) of water from a faucet. In the case of a kitchen faucet the flow rate is limited by the Energy Policy Act of 1992 (EPAct 92) and subsequent EPAct actions to 2.2 GPM for kitchen faucets. The maximum flow rate of lavatory faucets and faucet accessories at 1.5 gallons per minute.
All the above maximums are based on an average pressure of 60 psi. The pressure in your home may impact the flow rate. Most faucets are made to operate at a maximum of 80 psi pressure.
Using a lav or kitchen faucet as a tub faucet would likely produce too low a water flow as it would take too long to fill a tub at 1.5 GPM. Tub faucets are normal max of 2.5 GPM.
Older faucets may have a much higher flow rate – as high and 10 GPM – and to save water should be replaced.