What size water heater do I need? Enter household size and usage to get the right tank or tankless capacity in gallons.
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The calculator estimates your household's peak-hour hot water demand using a worksheet-style model: 12 gallons of hot water per shower, plus 8 gallons if the dishwasher runs in that hour, 25 gallons for a laundry load, and 20 gallons for a soaking tub. That peak demand becomes the first-hour rating (FHR) your heater must meet or beat. It then maps the FHR to standard tank bands (30, 40, 50, 75 or 80 gallons), adjusted for fuel type since gas recovers faster than electric, and estimates the simultaneous flow (GPM) and temperature rise a tankless unit would need to serve the same peak hour.
The professional way to size a tank water heater is the peak-hour method, not the tank gallon number on the box. First you estimate your peak-hour demand: the total hot water your household draws during its busiest hour, usually the morning rush. This calculator uses a worksheet-style model based on published sizing guides: 12 gallons of hot water per shower, 8 gallons when the dishwasher runs in that hour, 25 gallons for a laundry load, and 20 gallons to fill a soaking tub. Your peak-hour demand becomes the first-hour rating (FHR) your heater must meet or beat. FHR is the gallons of hot water a heater can deliver in one busy hour, starting from a full tank: it is tank volume plus recovery, the speed at which the burner or heating elements reheat incoming cold water. Two 50 gallon tanks can have very different FHR numbers, which is why the FHR printed on the yellow EnergyGuide label matters more than the sticker gallons.
Enter the number of people in your household and how many showers happen during the busiest hour of the day. Tick yes or no for the dishwasher, laundry, and soaking tub if they run in that same hour. Pick your heater fuel type: gas, electric, or heat pump. Then enter your incoming water temperature and your target hot water temperature (120 F is the standard setting). The calculator shows your peak-hour demand in gallons, the first-hour rating a tank heater needs, the recommended tank size from the standard 30, 40, 50, 75 and 80 gallon bands, and the flow rate (GPM) plus temperature rise a tankless unit would need to serve the same peak hour.
Take a family of 4 with 2 showers in the morning hour and the dishwasher running: peak demand = 2 × 12 + 8 = 32 gallons. The first-hour rating needed is 32 gallons. For gas, the calculator divides by the recovery factor of 1.5, giving 22, so a 30 gallon tank is the pick: a typical 30 gallon gas unit delivers a 55 to 60 gallon FHR, comfortably above 32. For electric, it divides by 1.15, giving 28, so again a 30 gallon tank covers this mild morning, but a busier version of the same household (3 showers plus laundry, 69 gallon peak) lands on a 75 gallon electric tank. On the tankless side, the same family needs 2 × 2.0 + 1.0 = 5.0 GPM at a 70 F temperature rise (120 F target minus 50 F incoming). This is why the classic 40 vs 50 gallon question has no single answer: a 40 gallon gas heater with a 65 gallon FHR beats a 50 gallon electric with a 50 gallon FHR for the same family.
Standard tank bands are 30, 40, 50, 75 and 80 gallons, and the sizing chart every competitor publishes maps households to them: 1 to 2 people fit 30 to 40 gallons, 3 to 4 people fit 40 to 50 gallons, 4 to 5 people fit 50 to 60 gallons, and 5 or more people need 60 to 80 gallons. Fuel type shifts those bands because recovery rates differ. Typical first-hour ratings: a 30 gallon gas tank delivers 55 to 60 gallons in hour one versus 40 to 45 for electric; a 40 gallon gas tank delivers 65 to 75 versus 50 to 55 for electric; a 50 gallon gas tank delivers 75 to 90 versus 60 to 65 for electric. Gas heats faster and recovers quicker, so electric households should size up one tier. Heat pump water heaters are the most efficient but recover the slowest, so size them by peak demand alone, not by tank volume.
A tankless heater never runs out, so gallons do not apply: sizing a tankless water heater is a flow and temperature problem. Add up the GPM of every fixture that might run at once (a shower is about 2.0 GPM, a dishwasher about 1.0, a washing machine about 1.5). Then find your temperature rise: target temperature minus incoming water temperature. The catch installers know well is that a tankless unit's GPM rating falls as the required rise climbs. A unit rated 7 GPM in warm Florida groundwater might only manage 4 GPM in Minnesota, where incoming water is 30 degrees colder. So choose a unit rated for your simultaneous GPM at your specific temperature rise, using the manufacturer's chart, not the number on the box.
The most common mistake is shopping by tank gallons alone: FHR is the real spec, so always compare the first-hour rating. Oversizing for the guest bathroom wastes energy, because a too-big tank reheats water nobody uses every day. Undersizing gives you lukewarm showers at peak hour and wears the tank faster from constant full-reheat cycling. Size for the busiest hour, not the average day: you run out exactly on the busiest morning. Do not ignore the recovery rate: electric tanks recover much slower than gas at the same size. And do not assume tankless is always better: it costs more up front and usually needs a larger gas line or a heavy electrical circuit, up to 120 amps.
Cold groundwater raises your temperature rise and makes every heater work harder. Northern homes can see 40 F incoming water while warm states see 60 to 70 F, which changes both tank recovery time and the tankless GPM a unit can actually deliver. If your incoming temperature sits below 50 F, consider sizing up one tank tier or choosing a tankless unit with a bigger burner, and always size the tankless at your winter rise, not your summer one. Homes on off-peak electric tariffs have the opposite problem: the tank heats overnight and must last all day, so a bigger tank (often 60 to 80 gallons for a family) covers the long gap between heating cycles.
A family of 4 with two bathrooms usually needs a 50 gallon gas tank, or a 60 gallon electric tank, with a first-hour rating of at least 55 gallons. The real answer is your peak hour: two morning showers plus the dishwasher is about 32 to 48 gallons of demand, and the heater's FHR must meet or beat it.
Usually yes, if the model's first-hour rating is 55 gallons or higher. A 50 gallon gas tank typically delivers 75 to 90 gallons in the first hour, which covers a family of 4. Without a strong FHR, four back-to-back showers will run the tank cold.
Sometimes. Many 40 gallon gas models deliver a 65 to 75 gallon first-hour rating, which covers 4 people showering in sequence. But a 40 gallon electric with a 40 to 50 gallon FHR will likely run cold mid-shower. Check the FHR on the yellow EnergyGuide label, not just the tank size.
A 40 gallon tank suits 2 to 3 people or a gas unit with a strong FHR; choose 50 gallons for 4 or more people, high usage, or any electric unit. The price and size difference between 40 and 50 gallons is small, so when in doubt, electric households should take the 50.
Add up the hot water your household uses in its busiest hour: about 12 gallons per shower, 8 for the dishwasher, 25 for a laundry load, and 20 for a soaking tub. That total is your peak-hour demand. Pick a tank heater whose first-hour rating (FHR) meets or beats that number, or a tankless unit that covers your simultaneous GPM at your temperature rise.
First-hour rating is the gallons of hot water a heater can deliver in the first hour, starting with a full tank of hot water. It equals the stored tank volume plus how fast the unit reheats incoming cold water (recovery). You will find it on the yellow EnergyGuide label. FHR is the number your peak-hour demand must beat.
Tank heaters cost less up front and are simpler to install; they can run out during back-to-back use. Tankless heaters give endless hot water, save floor space and cut standby energy, but cost more, can struggle with several fixtures at once in cold climates, and often need a bigger gas line or a heavy electrical circuit. Pick based on your simultaneous demand and your home's fuel supply, not on trends.
Oversizing wastes energy: a too-big tank reheats gallons of water nobody uses, every single day. Size for your busiest hour, not for the guest bathroom that runs twice a year. Sizing up one tier from the bare minimum is cheap insurance; two tiers up is just a higher energy bill.
Add the GPM of the fixtures that might run at once: about 2.0 GPM per shower, 1.0 for a dishwasher, 1.5 for a washing machine. A family with two simultaneous showers plus a dishwasher needs about 5.0 GPM. Then find your temperature rise (target minus incoming water temperature) and pick a unit rated for that GPM at that rise, because output drops as the rise climbs.
Colder incoming water increases the temperature rise the heater must deliver, which slows recovery on tank heaters and cuts the effective GPM of tankless units. A unit rated 7 GPM in warm groundwater may only deliver 4 GPM where incoming water is 40 F. Northern households should size up one tier or pick a bigger tankless burner.
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