What size water heater do I need for my household?

Tanks: size by first-hour rating, not gallons

Tank size in gallons tells you how much water is stored. What you care about is how much hot water you can actually use in your busiest hour. That is the first-hour rating (FHR): under federal test rules, an estimate of the most hot water a storage heater can supply in an hour that begins with the tank fully heated^[1]. It depends on both the tank volume and how fast the burner or elements reheat. You will find it on the yellow EnergyGuide label and on the maker's spec sheet.

Two 50-gallon tanks can have quite different FHRs. A gas tank usually recovers faster than an electric one of the same size.

Step 1: estimate your peak hour

Think of your busiest morning or evening hour and add up the hot water used. Use your own fixtures where you can. A WaterSense showerhead flows at no more than 2.0 gallons per minute, against a federal maximum of 2.5^[4], so a 10-minute shower uses up to about 20 to 25 gallons of mixed water, of which a large share is hot.

Peak-hour item	Where to find the number
Showers	Showerhead flow rate (gpm) times minutes, times the number of showers
Baths	Tub capacity, from the tub's spec sheet
Dishwasher	Water use per cycle, from the manual or spec sheet
Clothes washer on warm or hot	Water use per load, from the manual or spec sheet

Showers usually dominate. Count only the share of shower water that is hot, which depends on your setpoint and season.

Step 2: match the FHR

Pick a heater whose FHR is equal to or a little above your peak-hour estimate. Going far above it buys standby loss you will pay for every day.

Step 3: sanity-check against household size

DOE's Building America guide gives a way to estimate daily load by household size and climate^[2]:

People in the home	Load category DOE suggests (cold climates)	(Hot or mixed-dry climates)
1 to 2	Low to moderate	Low
3	Moderate to above average	Moderate
4	Above average to high	Moderate to above average
5+	High	Above average to high

The same guide cites average single-family use of about 50.6 gallons a day, or 17.0 gallons per person^[2]. It also notes that 58% of US households have one or two people^[2], which is why many homes have a bigger tank than they need.

Rules of thumb (industry, not DOE): HomeGuide pairs a 40-gallon tank with 1 to 2 people with low water use, and a 50-gallon tank with 3 to 4 people with moderate use^[5]. Use those to check your math, not to replace it.

Tankless: size by flow at your temperature rise

A tankless unit has no stored water, so FHR does not apply. DOE's guidance is to size for the maximum flow you will draw at once and the maximum temperature rise you will need, using the maker's capacity charts^[3].

Step 1: find your temperature rise

Temperature rise is the setpoint (often 120 F) minus your coldest incoming water temperature. Incoming water is colder in winter and colder in northern states, so a unit that is fine in Houston can fall short in Chicago. Your water utility can usually tell you the winter supply temperature.

Step 2: add up simultaneous flow

List the fixtures that realistically run at the same time and add their flow rates.

Step 3: read the chart at your rise

Worked example (our arithmetic): 50 F winter inlet and a 120 F setpoint is a 70 F rise. Two WaterSense showers at 2.0 gpm^[4] plus a 1.5 gpm kitchen faucet is 5.5 gpm. You need a model that delivers 5.5 gpm at a 70 F rise. The headline "maximum gpm" on the box is measured at a smaller rise and will overstate what you get.

For reference, an ENERGY STAR gas tankless must deliver at least 2.8 gpm over a 67 F rise^[6]. That is a floor for certification, not a whole-house size.

Common sizing mistakes

• Buying by tank gallons alone. Compare FHRs instead.
• Sizing a tankless by its maximum gpm. Use the flow at your winter rise.
• Ignoring a big tub or body sprays. One soaking tub can need more hot water than a tank holds. Size for it or plan around it.
• Oversizing "to be safe." Extra stored water means extra standby loss.
• Forgetting future changes. A baby on the way or teenagers soon can justify the next size up.

When to call a pro

• Large tubs, multi-head showers or more than two bathrooms in use at once.
• Converting from tank to tankless, where flow and gas supply both need checking.
• Choosing between a larger tank and a second heater for a distant bathroom.
• Any install that changes fuel type, venting or electrical service.