Water Heater Sizing Calculator

1. Choose tank or tankless. Tank mode calculates the First Hour Rating and recommends a storage tank size. Tankless mode calculates required GPM and BTU input for on-demand heating.

2. Enter your household details. Bathrooms, bedrooms, and occupants determine peak-hour hot water demand. More people means more simultaneous showers and faucet use.

3. Select appliances. Dishwashers add about 7 gallons and clothes washers add about 8 gallons to peak-hour demand for tank sizing, or 1.5 and 2.0 GPM for tankless sizing.

4. Enter your inlet water temperature. Colder inlet water (northern states, 40-50 deg F) requires more BTU to heat. Warmer groundwater (southern states, 60-70 deg F) reduces heating demand significantly.

5. Read the recommendation. For tanks, match the recommended size to available models. For tankless, ensure the unit meets both the GPM and BTU requirements at your temperature rise.

This calculator uses two different methods depending on heater type.

The tank method sizes based on the worst-case hour of hot water use — typically morning showers plus dishes. The tankless method sizes for simultaneous flow since there is no stored hot water to draw from.

Replacement sizing. When replacing an existing water heater, verify the old unit was properly sized before installing the same capacity. Many homes have undersized units that never delivered adequate hot water.

New construction. Size the water heater during the plumbing rough-in phase so gas line, venting, and electrical are spec'd correctly from the start.

Tank-to-tankless conversion quotes. Show the customer exactly what GPM and BTU they need, and why the upgrade makes sense for their usage pattern.