Water Heater Size Calculator
The number that matters is not how much hot water a house uses in a day. It is how much it uses in its busiest hour, because that is the hour someone gets a cold shower.
Peak hour demand
61gallons
- Minimum first-hour rating
- 61 gal
- Gas tank
- 40 gal
- Electric tank
- 50 gal
- Temperature rise
- 65 °F
- Tankless at 4 gpm
- 129,948 BTU/hr
- The number that matters on a tank is its first-hour rating, not its size. That is what it holds plus what it can reheat within the hour, and it is printed on the yellow EnergyGuide label.
- Gas recovers about twice as fast as electric, which is why a 40-gallon gas heater delivers as much in an hour as a 50-gallon electric one.
- A tankless unit is rated in gallons a minute at a given temperature rise. The same unit that gives four gallons a minute in a warm climate gives barely two in January further north, because the incoming water is that much colder.
About the water heater calculator
Two showers, a load of laundry and the dishwasher between seven and eight in the morning is the whole problem. A house that uses sixty gallons of hot water spread through the day needs a small heater; a house that uses the same sixty gallons in one hour needs a much larger one. Sizing on the daily total is how people end up with a tank that runs out every morning.
The figure to size against is the first-hour rating, and it is not the tank size. It is what the tank holds plus what the burner or element can reheat within that hour, which is why fuel matters so much: a gas burner recovers roughly twice as fast as an electric element, so a forty-gallon gas heater delivers about as much in an hour as a fifty-gallon electric one. The rating for any particular model is printed on its yellow EnergyGuide label.
A tankless unit sidesteps the hour entirely — it never runs out — but it is limited on flow instead. It is rated in gallons a minute at a given temperature rise, and the rise is the part people miss. The same unit that delivers four gallons a minute where the mains water arrives at 70°F delivers barely two where it arrives at 40, because it has half as far again to heat the water.
The per-use figures here are the Department of Energy's. They are averages, and a rain head running for fifteen minutes will beat the twenty gallons a shower is credited with comfortably.
What it works out
- Peak-hour hot water demand, fixture by fixture
- The minimum first-hour rating to look for
- Gas and electric tank sizes that meet it
- Tankless capacity for your incoming water temperature
The formula
Peak demand = Σ (Uses × Gallons per use) Tankless BTU = Flow × Rise × 500
The tank side is addition. Count what happens in the busiest hour and multiply each by what it draws: twenty gallons a shower, twenty a bath, six a dishwasher load, seven a laundry load, four for washing hands and face. Two showers, a dishwasher, a laundry load and two hand washes is sixty-one gallons.
Then find the smallest tank whose first-hour rating covers it. A forty-gallon gas heater rates around sixty-five and covers it; a forty-gallon electric rates around fifty and does not, so electric has to go up to fifty gallons for the same household.
The tankless side is a different question with a different formula. A US gallon of water weighs 8.33 pounds and a BTU raises one pound by one degree Fahrenheit, so a gallon a minute across a one-degree rise is 500 BTU an hour. Four gallons a minute across a 65-degree rise is just under 130,000 BTU an hour, which is a large domestic unit.
That flow figure is separate from the peak-hour demand and does not change with it. The tank question is how much water in an hour; the tankless question is how much at the same moment.
- Gallons per use
- The Department of Energy figures: 20 a shower or bath, 7 laundry, 6 dishwasher, 4 hand wash.
- First-hour rating
- What a tank delivers in an hour — its contents plus what it reheats. On the EnergyGuide label.
- Rise
- How far the heater has to lift the water: the setting less the incoming temperature.
- 500
- BTU an hour for one gallon a minute across one degree Fahrenheit.
A worked example
A morning with two showers, a dishwasher load, a load of laundry and two hand washes, with water arriving at 55°F and the heater set to 120.
That works out to 61 gallons.
- Minimum first-hour rating
- 61 gal
- Gas tank
- 40 gal
- Electric tank
- 50 gal
- Temperature rise
- 65 °F
- Tankless at 4 gpm
- 129,948 BTU/hr
Questions
What size water heater do I need for a family of four?
It depends on the busiest hour rather than the number of people. A typical morning of two showers, a dishwasher load, laundry and a couple of hand washes is 61 gallons, which a 40-gallon gas heater covers and a 40-gallon electric does not — electric would need 50.
What is a first-hour rating?
How much hot water a tank can deliver in one hour starting full: what it holds plus what the burner or element reheats during that hour. It is the number to size against, it is always larger than the tank, and it is printed on the yellow EnergyGuide label.
Why does a gas heater need a smaller tank than an electric one?
Because it recovers about twice as fast. A gas burner puts heat in quickly enough that a good part of the hour's supply is water heated during that hour, while an electric element is mostly delivering what was already in the tank.
How many gallons does a shower use?
Twenty is the standard figure, which assumes about ten minutes at a normal flow rate. A low-flow head uses noticeably less and a rain head or a long shower uses a great deal more, so adjust the count upward if that is your household.
Should I get a tankless water heater?
It never runs out, which solves the peak-hour problem entirely, and it takes up far less space. The trade-off is flow: it can only heat so much at once, so two showers running together may be its limit. It also costs considerably more to install, particularly the gas line and venting a gas unit needs.
What temperature should I set my water heater to?
120°F is the usual recommendation — hot enough to be useful, low enough to reduce the scald risk and to slow scale build-up. Some guidance suggests 140 to suppress legionella in larger systems, in which case a mixing valve at the outlet brings the delivered temperature back down.
How do I find my incoming water temperature?
Run the cold tap for a minute and measure it. It follows the ground temperature, so it is coldest in late winter — around 40°F across the northern US and closer to 70 in the south. Size on the winter figure, since that is when the heater works hardest.
What if my demand is more than any tank can supply?
Two tanks plumbed in series is the usual answer, and it is common in larger houses. A tankless unit or a heat pump water heater with a large tank are the other routes. Spreading the demand helps too: moving laundry out of the morning hour can drop the peak below what a single tank handles.
Does this account for a heat pump water heater?
Not directly. A heat pump water heater has a first-hour rating like any other, so compare that against the demand here — but they recover slowly in heat pump mode, and their published rating often assumes the backup element is running.