Water Heater Sizing Calculator

Tank water heaters

Build the busiest hour of hot-water use from showers, faucets, tubs and appliances. The app shows the minimum First Hour Rating (FHR) the heater should meet, because FHR accounts for recovery and storage together, not tank size alone.

Tankless water heaters

Choose the fixtures that may run at the same time and enter your incoming and outlet temperatures. The app shows the flow you need in gallons per minute at your temperature rise and the heat load in BTU/hr, so you can check it against the manufacturer's flow curve instead of the advertised maximum.

Free, with a one-time upgrade

Basic tank and tankless sizing is free, with no calculation limit, account or subscription.

Lifetime Pro is a single $4.99 purchase (U.S. price) that adds:

• Tank vs. tankless comparison
• Custom fixtures and adjustable assumptions
• Candidate equipment checks
• The full step-by-step calculation
• Saved jobs and PDF reports

Private by design

Calculations run on your iPhone and work offline. There are no ads, no tracking and no account. See the privacy policy.

Important

Water Heater Sizing Calculator is an estimating and planning tool. Always verify final equipment selection against manufacturer specifications, installation instructions, applicable codes and local requirements.