Size it before you buy it
Every water heater decision comes down to two numbers: how much hot water you need in your busiest hour, and how far the water has to be lifted in temperature. Get those right and the rest follows. Get them wrong and you buy the wrong appliance, which is what most people do.
This site holds 16 calculators and 392 pages built on the DOE test procedures, the NFPA 54 gas pipe capacity tables and the NEC ampacity and appliance rules. Every figure is labeled as a Published standard or a Convention, because a test procedure figure and a plumber's rule of thumb are not the same kind of number. Where the arithmetic runs out, the calculators return no answer and explain why rather than extrapolating.
Quick answer
Size a storage water heater on its first hour rating, not its gallons: a four person, two bathroom home uses about 54 gallons in its busiest hour, which a 50 gallon tank with an ordinary recovery rate delivers. Size a tankless unit on flow and temperature rise together: two showers at a 70 F rise need roughly 4 gallons per minute, which takes about 205,000 BTU of gas input or 58 kilowatts of electric.
Sizing
First hour rating is what decides whether you run out: the usable storage plus everything the burner or element reheats during that hour. Two 50 gallon tanks can differ by twenty gallons of first hour rating. It is printed on the yellow label.
Tankless
Flow and temperature rise trade against each other directly. The unit advertised at 8.4 gallons per minute makes roughly half that at double the rise, which is why the same heater behaves completely differently in Florida and in Minnesota.
Running cost
Annual cost is delivered energy divided by UEF, times the fuel price. A heat pump water heater shows a UEF above 1 because it moves heat rather than making it, which is roughly a quarter of the running cost of a resistance element.
A whole house tankless unit, a heat pump tank, the expansion tank almost every closed system needs, and the flush kit every tankless owner uses once a year. Prices come from verified catalog data and move often.
A condensing indoor unit at 157,000 BTU. This is the size most two to three bathroom houses land on once the inlet temperature is taken seriously.
Best for: The common two to three bathroom whole house replacement
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The lowest price of entry into a heat pump water heater in this catalog, and Energy Star listed, which means its efficiency figure was measured against the DOE test procedure rather than claimed.
Best for: A first heat pump water heater in a warm garage or basement
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A 2 gallon potable expansion tank with a 3/4 inch MIP connection, which is the size and the fitting that suits almost every 40 to 50 gallon residential tank.
Best for: The standard 40 or 50 gallon tank on a closed system
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Pump, hoses, bucket and an NSF certified descaling solution in one box, so nothing is missing on the day.
Best for: A first descale with nothing already on hand
Check price on AmazonEach one shows its formula and renders a worked example into the page, so it reads correctly with no scripts running at all.
269 pages, each one working a single combination of values all the way through with the tables that go with it.
We do not test water heaters in person and never claim to. Everything here is compiled from the DOE test procedures, the NFPA 54 and NEC code tables, manufacturer specifications and installation manuals, and the consensus of verified owner reviews. Products are checked against the live Amazon catalog: 208 of them, each read against what the page claims about it.
Where a manufacturer does not publish a figure, this site says so rather than estimating one. That is why no page here asserts a flow rate at a temperature rise the manufacturer never published, and why the gas pipe calculator refuses propane outright: NFPA 54 uses a separate table for it, and sizing propane pipe off the natural gas table understates what the run needs.
Two adjacent subjects belong to sister sites rather than this one. HouseProjectCalc covers what an installation costs, and HomeWaterCalc covers softening, filtration and water quality. What scale does to a heat exchanger is ours; whether to treat the water is theirs.
Researched guidance, not professional advice. A water heater is a pressure vessel: the temperature and pressure relief valve is never capped, plugged or removed, and a valve that will not open when the test lever is lifted has failed. Gas line, flue and vent work belongs to a licensed plumber or gas fitter, because improper venting produces carbon monoxide and backdrafting on an atmospheric vent is invisible. Water at 140 F scalds a child in seconds. Turn the breaker off before touching an element, and never energise an element that is not submerged.