GPM
Gallons per minute — the flow rate that decides how big a tankless water heater you need.
GPM (gallons per minute) measures how fast water flows through a fixture or heater. It's the key sizing number for tankless water heaters, which have no tank to draw from and must heat water on demand: you add up the flow of every hot-water fixture you'd run at the same time — roughly 2.5 GPM for a shower and under 1 GPM for many faucets — and buy a unit that delivers that combined GPM at your local temperature rise. A gas tankless typically manages about 5 GPM at a 70°F rise; colder incoming water drops that figure, which is why [sizing a tankless heater](/guides/what-size-water-heater-do-i-need-calculator) depends on your climate as much as your household.