Loan Water Heater Sizing Calculator
Tank gallons alone do not size a water heater — the first-hour rating does: how much hot water it can deliver in the busiest hour, tank plus recovery. Enter people, bedrooms and tubs and get the FHR to shop for and the tank size that usually carries it.
The people × bedrooms table is the planning method popularized by the US DOE’s sizing guidance: first-hour rating targets by household shape, with +10 gallons per extra soaking tub beyond the first. Tank size is then the usual shelf size whose FHR clears the target with a typical burner/element — gas units recover faster than electric at the same gallons, so an electric household should favor the larger end of any borderline.
How it works
First-hour rating (FHR) = usable tank capacity + what the burner or elements recover during that hour; it is printed on every unit’s Energy Guide label. The sizing table maps household shape to a target FHR: 2 people/2 baths 49, up through 4 people/3 bedrooms 70, 5–6 people 80–90. Each soaking tub after the first adds 10. The recommended tank is the smallest standard size (40 / 50 / 65 / 75 / 100 gal) whose typical FHR meets the target — which is why a 50-gallon gas unit often out-serves a 65-gallon electric one.
Formula & worked example
Formula: Target first-hour rating from the DOE-style planning table by people × bedrooms (2p/2br 49 gal up to 6p/5br 90 gal), +10 gal per soaking tub beyond the first, + your extra peak gallons. Tank size = smallest standard tank (40/50/65/75/100 gal) whose typical FHR clears the target.
Worked example: 4 people, 3 bedrooms, one tub: FHR target 70 gal — buy the 50-gallon class.
When to use this
- The old heater died and the plumber is asking what size, today.
- Morning showers run cold and you suspect undersizing.
- You are adding a bathroom, a tub, or two teenagers.
- You are switching fuel types and the old size may not translate.
How to use it, step by step
- Enter people and bedrooms — the table’s two axes.
- Count soaking tubs; one is already in the table’s assumption.
- Add extra peak gallons only for a real, recurring pileup.
- Shop by the FHR target on the Energy Guide label, then confirm the tank size fits the space.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| People × bedrooms | The planning table’s household shape | 4 people / 3 bed → 70 gal FHR |
| Soaking tubs | Each beyond the first adds 10 gal of FHR target | 1 standard; 2+ for a primary-suite soaker |
| Extra peak | Gallons for a recurring simultaneous-demand event | 0 for most households |
Another worked example
Two people, two bedrooms, two soaking tubs: base FHR 49 + 10 for the extra tub = 59 — a 50-gallon tank still serves it, but the margin is gone: add house guests (use the extra-peak field) and the honest answer steps up to the 65-gallon class.
Frequently asked questions
What size water heater does a family of 4 in a 3-bedroom need?
The table target is a 70-gallon first-hour rating, which a standard 50-gallon tank (gas, typically) delivers. The mistake is shopping by tank gallons alone: a 40-gallon unit’s FHR around 55 runs out in the third back-to-back shower, while the 50 clears the morning peak.
What is first-hour rating, exactly?
How many gallons of hot water the heater can supply in one hour starting full: stored usable gallons plus recovery during that hour. It is the only sizing number that maps to real life (the 7 a.m. pileup), and it is printed on the yellow Energy Guide label of every tank sold in the US.
We have two soaking tubs. How much does that change?
Ten gallons of FHR per extra tub: the 2-person, 2-bedroom example rises from a 49 to a 59-gallon target — still served by a 50-gallon tank, but with no slack left for guests. A true 80-gallon soaking tub filled at once is a peak event: count it, and consider whether the second tub is a daily habit or a monthly one before upsizing for it.
Gas or electric at the same tank size?
Gas recovers roughly twice as fast, so its FHR at 50 gallons can match an electric 65. If you are switching fuel types, do not carry the old tank size across — re-derive it from the FHR target here. Heat-pump water heaters recover slowest of all and usually want the next tank size up (their efficiency pays you back for the patience).
Is a bigger tank always safer?
Up to a point. Oversizing costs standby losses every hour of every day, and a much-oversized tank in a small household is a Legionella-friendly lukewarm reservoir if the thermostat gets turned down to compensate. Size to the FHR target, keep storage at 120°F+ delivery-safe settings, and let recovery do the rest.
When should I go tankless instead?
When demand is spiky but modest in simultaneity, space is tight, or standby loss dominates (vacation homes). Tankless sizes by flow: add the GPM of everything that could run together (shower 2.0, bath 4+, washer 2, dishwasher 1.5) and the winter temperature rise for your climate. Two simultaneous showers plus a laundry cycle is where single tankless units start to struggle in cold climates.
The old heater was 40 gallons and fine. Why does the table say 50?
Fixture flow rates dropped (good), but household patterns concentrated: more bathrooms per house, everyone showering in the same 40 minutes. Also, "fine" often meant the last shower was lukewarm and nobody filed a report. If your 40 genuinely never failed your peak hour, your household is the counterexample that proves the table is a planning average — keep notes, trust them.