Tankless Water Heater Size Calculator

Who it's for: Plumbing contractors, service technicians, estimators, and homeowners planning tankless upgrades.

Calculator Inputs

Built for: Plumbing contractors, service technicians, estimators, and homeowners planning tankless upgrades.
Key outputs: Total simultaneous flow (GPM), Temperature rise (Delta T), and more.
Use this page for early planning estimates before final code and manufacturer verification.

Calculates total simultaneous flow from shower demand plus additional fixture flow.
Builds Delta T from desired outlet minus incoming water temperature.
Uses a planning constant (BTU/hr = 500 x GPM x Delta T) to estimate required input and map to common size classes.

Cross-check assumptions with Water Heater Size Calculator for a more complete planning workflow.

Use winter incoming water temperatures for conservative sizing in colder climates.
Include realistic simultaneous fixtures to avoid undersizing.
If your result is close to a class boundary, evaluate the next class with manufacturer flow-versus-rise charts.
Treat this as an estimate and verify venting, gas supply, and local code requirements.

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This calculator uses the common planning approximation BTU/hr = 500 x total GPM x Delta T to estimate heating demand.

Lower incoming temperatures increase Delta T, which raises required input capacity for the same target flow.

No. Final selection should be confirmed with manufacturer performance charts and local installation requirements.

Consider the next class and verify with real design conditions so peak-use performance remains acceptable.

Disclaimer: Use results for planning only. Final design and installation decisions must follow local code and manufacturer documentation.