Water Heater Size Calculator — First Hour Rating
Tank size is the wrong number to shop by. What matters is the first hour rating — how much hot water the heater can actually deliver in the busiest hour of your day. Count that hour and size to it.
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We'll email the quantities above, what you typed in, and a link that reopens this calculator already filled in — so you can adjust it on your phone in the aisle.
How to work it out
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Find your busiest hour
Not your daily total — the single worst hour. For most households that is a weekday morning: two or three showers close together, someone shaving, the dishwasher started on the way out. That hour is what the heater has to survive.
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Add up what it uses
The Department of Energy publishes the figures: a shower is 20 gallons of hot water, a bath 25, shaving 2, hand dishwashing 3, a dishwasher 7, a top-loading clothes washer 25. Count each event in that hour and total them.
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Match it to a first hour rating
First hour rating is how much hot water a heater delivers in one hour starting full — the stored water plus whatever it can reheat while you are drawing. It is on the yellow EnergyGuide label. Your peak hour demand should be at or below it.
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Do not just buy a bigger tank
A tank you never empty is money spent keeping water hot for nothing. Size to the peak hour with a little headroom, not to the worst day you can imagine. If your peak is genuinely extreme, a tankless or a second heater may be the better answer than a 100 gallon tank.
Hot water per use
US Department of Energy figures.
| Use | Gallons of hot water |
|---|---|
| Shower | 20 |
| Bath | 25 |
| Shaving | 2 |
| Hand dishwashing or food prep | 3 |
| Automatic dishwasher | 7 |
| Clothes washer, top-loading | 25 |
| Clothes washer, front-loading | 15 |
These are hot water only, not total water at the tap.
Typical tank sizes
First hour rating is what to shop by. Gas recovers faster than electric, so a smaller gas tank delivers as much in an hour as a larger electric one.
| Tank | Gas FHR | Electric FHR | Suits |
|---|---|---|---|
| 30 gal | ~51 gal | ~38 gal | 1–2 people, no bath |
| 40 gal | ~68 gal | ~50 gal | 2–3 people |
| 50 gal | ~85 gal | ~63 gal | 3–4 people |
| 66 gal | ~112 gal | ~83 gal | 4–5 people |
| 80 gal | ~136 gal | ~100 gal | 5+ or a big tub |
Approximate. The real figure is on the EnergyGuide label of the specific model.
What is a first hour rating?
The number of gallons of hot water a heater can supply in one hour, starting with a full tank. It combines what is already stored with what the burner or element can reheat while you are drawing water off.
It is the only figure that answers the question people actually have, which is "will the last person get a cold shower". It is printed on the yellow EnergyGuide label, and it is what you should compare between models — not the tank size on the front.
Why gas and electric of the same size are not the same
A gas burner puts heat in fast. An electric element does not. A 50 gallon gas heater might deliver 85 gallons in its first hour; a 50 gallon electric closer to 63 — the same storage, but far less recovery while you are using it.
So if you are switching fuel, the equivalent is not the same number of gallons. Going gas to electric typically means going up a tank size, sometimes two.
Peak hour, not daily total
A household might use 200 gallons of hot water a day and never run out, because it is spread across eighteen hours. Another might use 120 and run out constantly, because 80 of it happens between seven and eight in the morning.
Sizing on the daily figure gets this exactly backwards. Count the worst hour. If your household has two very different bad hours — a morning rush and a bath-and-laundry evening — count both and size to the larger.
Is tankless the answer?
Sometimes. A tankless heater never runs out, which solves the peak hour problem completely, but it is sized on flow rate rather than storage — how many gallons per minute it can raise to temperature at once.
The catch is temperature rise. The same unit that delivers 6 GPM in Florida might manage 3.5 in Minnesota, because the incoming water is far colder. Two simultaneous showers is about 4 to 5 GPM, so a cold climate needs a substantially bigger unit for the same result. Gas tankless also needs a much larger gas line than a tank, which is often the real cost of switching.
Where the tank goes matters too
A tank in an unheated garage or crawl space loses heat constantly and recovers more slowly in winter, which quietly eats into the first hour rating you paid for. An insulating blanket helps on older tanks; most modern ones are already well enough insulated that adding one does little.
Long uninsulated runs to a distant bathroom waste more than the tank does. Insulating the first few feet of hot pipe out of the heater is cheap and genuinely effective.
How this is worked out
No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.
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peak hour demand = sum of each use × its gallonsDOE figures: shower 20, bath 25, shave 2, hand wash 3, dishwasher 7, laundry 25 or 15. -
first hour rating ≈ tank × (1 + recovery fraction)Recovery is about 70% for gas and 25% for electric within the first hour. -
tank size = smallest standard size whose FHR meets peak demandShop by first hour rating, not by the number on the front. -
tankless GPM = simultaneous fixtures × their flow ratesA shower is about 2.1 GPM. Tankless is sized by flow, not storage.
Common questions
What size water heater do I need for a family of four?
Usually a 50 gallon gas or a 66 to 80 gallon electric, but it depends entirely on whether your four people shower at the same time or across the morning. Count the peak hour rather than the heads.
What is the difference between tank size and first hour rating?
Tank size is storage. First hour rating is storage plus what the heater can reheat while you are drawing — so it is always larger, and it is the figure that tells you whether you will run out. Two 50 gallon heaters can have very different ratings.
How many gallons does a shower use?
About 20 gallons of hot water for a standard shower, per the DOE. A low-flow head brings it nearer 14; a long shower or a rainfall head can push it past 30. Total water at the tap is higher — this is the hot portion only.
Is a bigger water heater always better?
No. An oversized tank keeps water hot around the clock that you never use, which costs money every day of the year. Size to your peak hour with modest headroom.
Should I switch to tankless?
It never runs out and it saves standby losses, but it is sized on flow rather than storage and its capacity drops sharply with colder incoming water. Gas models usually need a larger gas line and a new vent, which is often the bulk of the cost. Worth it for a household that genuinely runs out; less compelling otherwise.
How long does a water heater last?
Eight to twelve years for a tank, longer if the anode rod is replaced before it is used up. Tankless units often run twenty. If yours is over ten and you are reading this, plan the replacement rather than waiting for the leak.
What is recovery rate?
How many gallons the heater can raise by a given temperature in an hour. It is why gas and electric of the same tank size perform so differently, and it is the second half of the first hour rating.
Does incoming water temperature matter?
Enormously for tankless, quite a lot for tanks. Groundwater comes in around 70°F in the south and can be under 40°F in the north in winter, so a northern heater is doing far more work for the same output. This is the main reason a unit that is fine in one state is undersized in another.
Where these figures come from
Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.
- US Department of Energy, Energy Saver — first hour rating and peak hour demand worksheet
- EnergyGuide labels — published first hour ratings by model
- International Residential Code, Chapter 28 — water heater installation requirements
Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 15, 2026.