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What size water heater do I need?
Count what happens in your busiest hour of hot water. The size comes from the U.S. Department of Energy's worksheet, not a guess, and you see real tank water heaters rated for it.
Going tankless instead? Those are sized by flow rate, not by the hour: use TanklessSizer.
Heaters for your hour
Closest rating first. Every number comes from the maker's spec sheet or ENERGY STAR's certified product data. If a number isn't published, the card says so instead of guessing.
The "check price" buttons are affiliate links.
Where the numbers come from
The method is the U.S. Department of Energy's, from its Energy Saver page "Sizing a New Water Heater." For a storage water heater, including a heat pump water heater with a tank, it says to use the first-hour rating: the number of gallons of hot water the heater can supply per hour, starting with a tank full of hot water. That rating depends on the tank size and on the burner or element and its size.
- Pick the one hour of the day (morning, noon or evening) your home uses the most hot water. Keep in mind how many people live there.
- Add up the hot water used in that hour with the worksheet. That total is your peak hour demand.
- Buy a heater whose first-hour rating at least matches it. In energy.gov's own words, a household with a 66 gallon peak hour "would need a water heater model with a first hour rating of 66 gallons or more."
Bigger isn't better here. On its storage water heater page, energy.gov says an oversized unit "carries a higher purchase price and increased energy costs due to higher standby energy losses." So the tool lists the closest rating first.
What the worksheet doesn't do: it doesn't estimate total daily hot water use, and it has no row for baths. energy.gov says its per-use figures are averages from a variety of published sources, and that some makers' calculators also factor in how long each use lasts.
One correction: energy.gov's worked example lists 3 showers (60 gallons), 1 shave (2) and 1 hand dishwashing (3), then gives a total of 66. Those rows add to 65. This tool adds the rows as the worksheet lists them.
Source: U.S. Department of Energy, Energy Saver, "Sizing a New Water Heater" (energy.gov/energysaver/sizing-new-water-heater). That address has shown "Page not found" since July 2026, so the text here is quoted from the last published copy, saved by the Internet Archive on July 2, 2026. Oversizing: energy.gov, Storage Water Heaters (archived copy). Heater specs: each maker's spec sheet or ENERGY STAR certified product data. All retrieved October 30, 2026.
Safety
Set the temperature. From energy.gov:
"Although some manufacturers set water heater thermostats at 140ºF, most households usually only require them to be set at 120ºF, which also slows mineral buildup and corrosion in your water heater and pipes. Water heated at 140ºF also poses a safety hazard—scalding."
energy.gov, Lower Water Heating Temperature (archived copy; the energy.gov address now shows "Page not found").
Gas water heaters and carbon monoxide. From the CDC:
"Have your heating system, water heater, and any other gas, oil, or coal burning appliances serviced by a qualified technician every year."
"Install battery-operated or battery back-up CO detectors near every sleeping area in your home."
CDC, Carbon Monoxide (CO) Poisoning Basics. Gas lines, venting and 240V circuits are work for a licensed plumber or electrician.
Questions
How do I know what size water heater I need?
Size a tank water heater by its first-hour rating, not its tank size. The Department of Energy's method: find the one hour of the day your home uses the most hot water, add up what happens in it (20 gallons per shower, 2 per shave, 3 for hand dishwashing or food prep, 7 per dishwasher load, 25 per top-loader wash, 15 per H-axis wash), then pick a heater with a first-hour rating at least that high.
Is a 50-gallon water heater enough for a family of four?
The tank size doesn't answer that by itself. Per the Department of Energy, the first-hour rating depends on the tank capacity, the heat source (burner or element) and its size. Two 50-gallon heaters on this page are rated 55 and 85 gallons. Count your busiest hour: four showers alone come to 80 gallons on the Department of Energy's worksheet.
Where do I find a water heater's first-hour rating?
On the EnergyGuide label, in the top left corner, as "Capacity (first hour rating)." The Federal Trade Commission requires that label on all new conventional storage water heaters but not on heat pump water heaters, so for a heat pump model check the maker's spec sheet.
Is a bigger water heater better?
Not past what you need. The Department of Energy says an oversized unit carries a higher purchase price and increased energy costs due to higher standby energy losses. Pick the closest first-hour rating at or above your peak hour.
What temperature should I set my water heater to?
The Department of Energy says most households only need 120°F, which also slows mineral buildup and corrosion. Water heated at 140°F poses a safety hazard: scalding.
What about a tankless water heater?
Tankless heaters are sized by flow rate and temperature rise, not by first-hour rating. TanklessSizer by Foundry NW covers that.
| Use | Gal per use | Times | Gal |
|---|---|---|---|
| Total Peak Hour Demand | 0 | ||