Water Heater Sizing Calculator — Tank & Tankless

What size tankless water heater do I need?

Tankless units are sized by simultaneous flow (GPM) at your required temperature rise, not by gallons of storage. A single shower needs about 2.5 GPM; two showers plus a sink is roughly 6–7 GPM. In northern climates, incoming water near 40°F means a 70–80°F rise, which cuts a unit’s rated GPM roughly in half — so size from the cold-weather rating, not the spec-sheet maximum.

Related questions

• What size water heater do I need for a family of 4?
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