Water Heater Size Calculator (Tank & Tankless)

Water heater sizing is the single most common spec error we see in residential plumbing — homeowners replace a failed unit with whatever size the previous one was, without ever asking whether the previous one was correctly sized in the first place. A 40-gallon tank serving a four-bedroom household that gained two teenagers since the original install is going to produce cold showers on weekend mornings regardless of how new it is. A 75-gallon tank serving a two-person condo is wasting energy 24/7 keeping water hot that no one is going to use. The calculator on this page exists to make that diagnostic 30 seconds of work instead of an expensive surprise after install day.

Tankless sizing follows a different math entirely. Tank heaters store hot water and replenish it; tankless heaters produce hot water on demand at a maximum gallons-per-minute rate determined by the unit's BTU output and your incoming water temperature. South Florida's relatively warm incoming water (mid-70s most of the year) is genuinely favorable for tankless performance — a smaller unit can deliver the same effective flow here as a larger unit would need to deliver in a northern climate. The calculator accounts for this and gives you a GPM figure you can actually use against manufacturer spec sheets.

Once you have a recommended size, talk to a licensed plumber before you order anything. Tankless installations in particular have requirements that are not obvious from the spec sheet: gas units need a dedicated gas line of sufficient capacity (3/4-inch is typical), proper venting (concentric or conventional depending on the install location), and adequate combustion air. Electric tankless units have substantial amperage requirements that frequently exceed available panel capacity in older homes. We verify all of this on site before quoting a tankless install — the calculator gives you the right size, not the right install plan.