Four questions. The size your home actually needs.
Installers size by rule of thumb; this calculator sizes by first-hour demand — the measure the DOE rates tank heaters on — and by temperature-rise flow for tankless. Tell it your household size, bathrooms, tub size and laundry habits, and it returns the tank capacity or tankless flow rate that covers your busiest hour, with the reasoning shown.
High-demand fixtures
Running in the same busy hour
Fuel for the new heater
Look for a First Hour Rating of at least 32 gallons.
The math — showers, sinks and shaving for 2 people: 32 gal.
Even the smallest tankless units made deliver more than that at a 70°F rise, so any unit covers a house this size.
Take the First Hour Rating to the installer, not just the gallon size. And if a quote pushes you past a 50-gallon tank for this house, ask what problem it is solving — your busiest hour does not need it.
Tank picks that match your number
Most homes that run this calculator land on a 50-gallon gas tank — enough first-hour rating for the morning rush without paying to keep water hot that nobody uses. The Rheem Professional Classic PROG50-42N RH67 PV is that class of heater: 50 gallons, 42,000 BTU, and power-vented, so it can also replace a tank in a finished basement or closet where a standard flue will not work.
Browse the Tank Water Heaters hub →See this Rheem 50-gal on Amazon →
Going tankless?
If your answer came back in GPM instead, the unit class to price is a condensing gas tankless around 9 GPM. The Rheem RTGH-90DVLN-3 delivers 9.0 GPM at a 0.93 UEF and heats only the water you actually run — no tank reheating itself all day. Size it against the flow rate your calculator result gave you, not against your square footage.
How the calculator works
Tank heaters are not really rated by how much water they hold. They are rated by first-hour rating: how many gallons of hot water the heater can deliver in its busiest hour, starting with a full tank. A tank gives up roughly 70% of what it holds before the outflow turns lukewarm, then adds whatever it can reheat during that same hour — its recovery rate. Add the two together and you get the first-hour rating printed on the yellow EnergyGuide label.
This calculator works your house backwards through that math. It estimates your busiest hour — usually the morning rush — with the same per-use figures the U.S. Department of Energy publishes for its sizing worksheet:
| Use | Hot water per use |
|---|---|
| Shower | 10 gal |
| Bath | 20 gal |
| Washing hands and face | 4 gal |
| Shaving | 2 gal |
| Automatic dishwasher | 6 gal |
| Clothes washer | 7 gal |
So everyone in the house counts for 16 gallons in the peak hour — a shower, the sink, and shaving — and the extras stack on top: 20 for a soaking tub filled in that hour, 10 more for a large or multi-head shower that runs longer and flows harder, and the laundry and dishwasher only if they actually run in that same hour. Bathrooms do not change the tank math — they change how much of that water can run at the same time, which is what sizes the tankless answer.
Recovery is where fuel shows up. A typical gas burner reheats about 40 gallons an hour; a standard electric element setup manages about 20. Same house, same peak hour — the electric tank often has to be a size larger, because more of the hour has to come out of storage. High-recovery models beat these figures, which is why the calculator says so plainly when a standard tank runs out of road: at that point the honest options are a high-recovery 80-gallon, two units, or tankless.
Tankless heaters have no storage to lean on, so they are sized on flow at the worst moment instead: the showers that can run at once — capped by your bathrooms — at 2 gallons a minute each, a tub filling at 4, plus anything else running at the time. The answer is quoted at a 70°F temperature rise — 50° groundwater in, 120° water out — because that is the winter number for most of the country, and a unit that covers January covers July for free.
These are planning numbers, built from the DOE worksheet — the same figures an honest installer is working from. If your household runs two showers and a tub every morning with guests in the spare room, size for the house you actually live in, and hold any quote up against the first-hour rating this page gives you.
The $30 part that doubles a tank’s life
Inside every tank heater, a magnesium anode rod is quietly corroding so the steel tank does not have to. Once it is spent — usually in three to five years — the tank starts rusting itself, and that is what actually kills most water heaters. A fresh 44-inch flexible rod bends into tanks with low overhead clearance and fits the standard 3/4-inch port on Rheem, GE, Reliance, AO Smith and most other major brands; the Eau kit includes the socket and Teflon tape, so the swap takes about twenty minutes.
Inside this hub
- Sized on first-hour rating and peak-hour demand, not gallons-per-person folklore
- Tankless answers figured at a 70°F temperature rise, so winter groundwater is already in the number
- Every result shows the math, so you can check a quote against it
- No email required. The answer is on this page, in full
Sized your home? The Sizing & Cost hub works the installed-cost side of the same job — what the unit, the labor and the permit should actually add up to.