Terms of use

Use the calculators freely. They produce planning estimates from published worksheet data and stated assumptions, and by using them you accept that they are exactly that.

Estimates, not specs

HeaterMath provides sizing estimates for information and planning. It is not an engineering spec, not a code determination, and not a substitute for the label on the unit or a licensed plumber's assessment of your venting, gas supply, and local requirements. Our first hour rating figures are a stated approximation; the EnergyGuide label on the unit you are buying is the authoritative number. When our table and a professional in your house disagree, the professional wins.

Not financial advice

Nothing on this site estimates or advises on purchase price, operating cost, financing, or rebates. Sizes are hardware arithmetic, and we keep it that way deliberately. Decisions about spending money belong with your own judgment.

The usual paragraph

Content is provided as-is without warranties of any kind; to the maximum extent permitted by law we are not liable for damages arising from use of this site. Outbound links don't imply endorsement. These terms can change with the site, and the effective date below tracks the current revision. If part of this page is unenforceable where you live, the rest still applies.

Effective 2026-07-16. Questions: contact.