What size water heater do you need?

Buy on the first-hour rating, not on gallons

Tank size is the number on the box, and it is not the number that determines whether you run out of hot water. What matters is the first-hour rating: how many gallons the appliance can deliver in one hour starting full, which is stored volume plus whatever the burner or element manages to reheat while you are drawing. Two heaters with the same tank size can carry different first-hour ratings, so compare the rating printed on each label.

The Department of Energy publishes the sizing rule in a single sentence: find a model whose first-hour rating at least matches your peak hour demand. That is the entire algorithm, and this page implements it literally. The per-use gallons come from the same federal worksheet — a shower at 20 gallons^[1], an automatic dishwasher cycle at 7^[1], hand dishwashing at 3^[1], a shave at 2^[1].

One thing worth flagging honestly, because a careful reader will find it: the federal worksheet prints a worked example of three showers, one shave and one hand dishwashing, then totals it at 66 gallons. Sixty plus two plus three is sixty-five. The published total and its own line items disagree by a gallon. This calculator adds the line items and reports 65, because the per-use figures are the operative data and the total is a transcription of them. A gallon changes nothing about which appliance you buy; quietly printing a number our own inputs cannot produce would change quite a lot about whether anything else here can be trusted.

Why the usage bin matters when you compare efficiency

Efficiency ratings are not measured on a single universal test. The rating is taken against one of four standard draw patterns, and which pattern applies is decided by the appliance’s own first-hour rating: below 18 gallons is the very small usage pattern, up to 51 is low usage, up to 75 is medium, and above that is high usage^[2].

The consequence catches people out constantly. Two efficiency numbers are only comparable when both appliances sit in the same bin. A small unit rated in the low usage pattern and a large one rated in the high usage pattern are being graded on different exams, and reading their ratings side by side tells you almost nothing. That is why the tool above names your bin: it is the first thing to check before comparing any two yellow labels.

Sizing a tankless unit is a different question entirely

A storage heater buffers demand; a tankless one meets it in real time. So the rating that matters is not gallons per hour but gallons per minute, and it only means anything alongside a temperature rise. The published operating point is 5 gallons a minute through a gas unit and 2 through an electric one, both at a 70 degree rise^[1]. Federal guidance says tankless units typically deliver 2 to 5 gallons a minute^[3].

Flow and rise trade against each other because the heating capacity is fixed. Water arriving at seventy degrees and delivered at a hundred and twenty needs a fifty degree rise, and the same appliance that manages five gallons a minute at seventy degrees of rise would, by straight arithmetic, manage seven at fifty. The sources do not go above five, so the tool stops there. Turn it around: water arriving at forty degrees in February needs an eighty degree rise, and the flow falls. That is why a tankless unit that was fine in summer can deliver a cooler shower in winter, and why the tool asks for your incoming temperature. Its starting value is our assumption, not a published figure.

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