Water Heater Size Calculator (Tank, Tankless, Heat Pump)

Pick your household size and which fixtures might run at the same time, and this gives you a recommended tank first-hour rating or tankless flow rate based on standard published sizing guidelines — not a one-size-fits-all rule of thumb.

Illustration of a tape measure wrapped around a storage tank water heater, with a notepad nearby.

Household size

Peak simultaneous use

Based on published household-size-to-first-hour-rating guidelines. A tankless unit is sized by flow rate (GPM) instead — use the tankless sizing calculator for that.

FAQ

Does this work for heat pump water heaters too?

Yes — heat pump water heaters are sized the same way as standard tanks (by first-hour rating), so the tank-sizing results apply to both.

Written by HotWater Decoded — water heater repair specialists. Technically reviewed by a licensed technician on our team.

Sources

Household-size-to-tank-size guidelines and typical fixture flow rates published across major manufacturers’ sizing documentation