About Water Heater Sizing Calculator

Water Heater Sizing Calculator started as an iPhone app for sizing tank and tankless water heaters from real hot-water use instead of rules of thumb. This site puts the same calculations on the web, free, with the reasoning and sources behind them.

What we do

We calculate performance requirements: the First Hour Rating a tank needs, or the flow and heat a tankless unit must deliver at your temperature rise. We do not sell water heaters, recommend brands or take commissions, and the site carries no advertising.

How the calculations are built and checked

• The formulas follow the U.S. federal water heater test procedure (10 CFR 430 Appendix E), federal fixture flow limits and published engineering references. Every formula, default and source is on the methodology page.
• Where a default is an engineering assumption rather than a published figure, we label it as one.
• The web calculators are a direct port of the app’s sizing engine, and an automated test runs both on hundreds of cases to confirm they produce identical results.
• Tables on guide pages are generated by the same engine at build time, so text and tools can’t drift apart.

What we are not

We are not a plumbing or engineering firm, and this site is not a substitute for a licensed professional. Results are estimates for planning. Confirm any equipment choice against the manufacturer’s specifications, installation instructions, applicable codes and your local building department.

How the site is funded

The website is free. The iPhone app is free too, with an optional one-time Lifetime Pro purchase for professional features. That purchase is what supports the project.

Corrections

If you spot an error in a number, a source or an explanation, please tell us at gpmediacontact@gmail.com. We review every report and update the page and the app engine if needed.