Water Heater Size Calculator
Size a water heater from peak-hour demand rather than guesswork.
How this calculator works
first hour rating = (showers × 20) + other peak hour use
First hour rating, not tank size, is the number that matters — it combines stored hot water with the recovery rate during that hour. A well-recovering 40 gallon gas heater can outperform a 50 gallon electric.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- US DOE 10 CFR 430 Subpart B, Appendix E 2023 — the first hour rating test procedure water heaters are labelled with
Standards are revised on a cycle. The edition above is the one this figure was checked against — confirm it against the edition your jurisdiction has adopted.
Worked example
With people in the household 4 , showers in the peak hour 2 , other peak hour use 15 gal, this calculator returns 55 gal.
First hour rating for households of common size
Click through any row to load those measurements. The address bar keeps them, which makes the result something you can paste into a message.
| Job | Size | First hour rating needed |
|---|---|---|
| One person | 1 × 1 × 10 gal | 30.00 gal |
| Couple | 2 × 1 × 12 gal | 32.00 gal |
| Family of four | 4 × 2 × 15 gal | 55.00 gal |
| Family of four, back-to-back showers | 4 × 3 × 15 gal | 75.00 gal |
| Large household | 6 × 4 × 25 gal | 105.0 gal |
Compare against the first hour rating on the yellow label, not against tank size — recovery rate is half the answer.
Frequently asked questions
What size water heater do I need for a family of four?
Work out the peak hour, not the daily total. Two showers plus a dishwasher run typically means a 55 gallon first hour rating, which a 40 gallon gas heater delivers comfortably.
What is first hour rating?
The gallons of hot water a heater can supply in one hour starting from full — stored water plus what it reheats during that hour. It is the figure to match against your peak demand.
Is gas or electric better for recovery?
Gas, substantially. A 40 gallon gas heater typically has a higher first hour rating than a 50 gallon electric because it reheats two to three times faster. Electric needs more storage to compensate.
Should I switch to tankless?
Tankless never runs out but is limited by flow rate rather than volume. Size it in gallons per minute at your winter incoming water temperature — a unit that handles two showers in Florida may manage only one in Minnesota.