Tankless Water Heater Sizing: How Many GPM Do You

Tankless water heaters don’t store hot water like traditional tanks. Instead, they heat water on demand, so the key sizing metric is how much hot water the system can deliver—measured in gallons per minute (GPM). If you pick too small a unit, you may notice lukewarm water or reduced flow when multiple fixtures run at once.

To size a tankless heater correctly, you need two things: your expected hot-water demand (how many fixtures are running and for how long) and the temperature rise (how much hotter the water must be heated compared with your incoming supply).