Tankless Water Heater Size Calculator

A tankless heater does in seconds what a tank does in an hour: it must add a full temperature rise to however much water is flowing right now. Bigger flow or colder water means a bigger burner.

Tankless Water Heater Size Calculator converts your peak flow and rise into the BTU/h or kW capacity you need, and points at a unit class.

• Peak-flow math: BTU/h = GPM x 500 x rise.
• Gas or electric: BTU/h and kW both shown.
• Efficiency adjusted: net input vs rated input.
• Unit classes: common tankless inputs compared.
• Units needed: how many units to cover the peak.
• Export: Copy or download the report.

1. Estimate your simultaneous peak flow (two showers? add them).
2. Enter incoming and desired temperatures.
3. Pick the fuel - gas or electric.
4. Read the required input and nearest unit class.

Example - Two showers in winter. 5 GPM from 40 F to 120 F needs 5 x 500 x 80 = 200,000 BTU/h. That lands on a 199,900 BTU/h condensing unit - exactly the kind of calculation the sticker rarely shows.

• Units-sized reality: catches the undersized-buyer trap.
• Gas and electric: compare fuels on the same page.
• Rise-aware: winter-cold numbers included.
• Free and private: no uploads, no logging.