The flow rate of a faucet or shower head is measured in GPM and is a critical factor in sizing a tankless water heater, or for evaluating if an older fixture is wasting water.
The Federal Energy Policy Act of 1992 required all faucet and shower fixtures made the USA to have a flow rate of no more than 2.2 GPM at 60 PSI. Often you can get below 2.2 GPM with low flow aerators. However before 1992, older fixtures used much more water than 2.2 GPM, sometimes upward of 10 GPM. So before any calculations for water heater sizing can properly be done an accurate measurement of a fixture's flow rate should be done. If any fixture is much more than 2.2 GPM, it is a good candidate for replacement with a new code compliant water efficient fixture.
Here's how to quickly determine flow rate for a specific faucet or shower head: