Water Heater Size Calculator

A water heater size calculator answers a different question for each technology. A tank is sized by how much hot water it can deliver in your busiest hour, its first hour rating. A tankless heater has no reserve, so it is sized by the flow of every fixture running at once and by how much it has to warm the incoming water. Three showers, a dishwasher load and some food prep need a 70-gallon first hour rating; one shower and one faucet on a tankless unit need 3.25 gpm through a 70°F rise.

The fuel decides the running cost as much as the size does. The Electricity Cost Calculator and the Natural Gas Cost Calculator turn kWh and therms into a bill, and the BTU Calculator covers space heating, which a combination system would share with your hot water.

How DOE estimates peak-hour demand

DOE's sizing method starts with your busiest hour of the day, usually the morning shower rush or the evening cleanup. Count how many times each use happens in that hour, multiply by the average gallons of hot water per use, and add. The total is your peak-hour demand, and the tank's first hour rating should at least match it. These are the per-use figures from DOE's Energy Saver worksheet:

The worksheet estimates one hour, not a whole day. DOE also notes that its figures are averages; a long shower under an old showerhead can use more than 20 gallons, and a quick one under a WaterSense head far less. If you know your habits, adjust the counts rather than the gallons.

Worked example: a family's morning rush

A family of four runs three showers, one round of food prep and one dishwasher load in its busiest morning hour:

Now compare with real equipment. The median DOE-certified 40-gallon gas tank has a first hour rating of exactly 70 gallons, and the median 50-gallon gas tank 82, so a typical gas model of either size works. On the electric side, the median 50-gallon resistance tank reaches only 63 gallons, and the median 50-gallon heat pump model 69, just short; the median 65-gallon heat pump model, at 80 gallons, clears it. A first hour rating of 70 falls in DOE's medium draw pattern (51 up to 75 gallons), which is rated at 55 gallons of hot water a day. These are the calculator's default inputs.

First hour ratings by tank size and fuel

Tank size and first hour rating are not the same thing: a powerful gas burner refills a 40-gallon tank faster than an electric element refills a 50-gallon one. We pulled all 5,102 consumer water heater listings in DOE's Compliance Certification Database (updated September 25, 2026), removed duplicate brand listings, and grouped the 522 unique storage models in common sizes by nominal size, counting electric tanks with a UEF of 1.5 or more as heat pumps:

Three patterns stand out. Gas tanks deliver about 15 to 20 gallons more in the first hour than electric resistance tanks of the same size. Heat pump models need a bigger tank to match a gas unit, because the compressor heats slowly. And no electric resistance tanks above 55 gallons appear in the electric storage class at all: the federal standard for those sizes already requires heat pump efficiency, and the only exception is a separate grid-enabled class built for utility load-control programs.

How to size a tankless water heater

A tankless or demand heater warms water only as it flows, so DOE sizes it by flow rate and temperature rise. List the fixtures you expect to use at the same time, add their gallons per minute, and subtract the incoming water temperature from the output you want, normally 120°F. The heat output then follows from the weight and specific heat of water:

The 8.24 lb per gallon is the density of water at 125°F used in DOE's test procedure; the familiar shortcut of 500 per gpm per degree rounds it up by about 1%. The flows you add come from the fixtures themselves. Federal rules cap them, and WaterSense models go lower:

Worked example: DOE's shower-and-faucet case

DOE's own example runs a 0.75 gpm faucet and a 2.5 gpm shower together, so the heater must deliver 3.25 gpm. Incoming water at the default 50°F heated to 120°F is a 70°F rise. The output is 3.25 × 494.4 × 70 = 112,476 Btu/h. A condensing gas unit at 95% efficiency needs 112,476 ÷ 0.95 = 118,396 Btu/h of input, comfortably inside a 140,000 Btu/h model, which delivers 3.84 gpm at that rise. An electric unit needs 112,476 ÷ 3,412 = 33.0 kW, or 137.4 amps at 240 volts, which only the largest whole-house electric models reach.

Climate changes the answer. Move the same family to Minneapolis, give them two 2.0 gpm WaterSense showers at once, and the inlet water averages about 49°F: 4.0 gpm through a 71°F rise is 140,410 Btu/h, so a 140,000 Btu/h gas unit falls just short (3.79 gpm) and a 160,000 Btu/h unit is the smallest that fits. In Houston, with 73°F inlet water, the same 4.0 gpm needs only 92,947 Btu/h.

Inlet water temperature by region

DOE's default of 50°F is a reasonable guess for much of the country, but the real figure ranges from the low 40s to about 80°F. USGS found that groundwater 30 to 60 feet down runs 2 to 3°F above the mean annual air temperature, so the calculator adds 2.5°F to each reference city's NOAA 1991-2020 normal and rounds to a whole degree:

A Duluth home needs almost twice the burner of a Miami home for the same 5 gpm. If your utility draws from a river or lake rather than wells, winter water can be colder than this table, close to the monthly air temperature, so a thermometer under a cold tap in January is the best input.

What common tankless sizes deliver

Turning the formula around shows the flow a given heater can sustain: maximum gpm = input × efficiency ÷ (494.4 × rise). The middle column uses DOE's 70°F default rise and the right column Houston's 47°F:

The electric rows explain DOE's remark that electric demand heaters typically manage about 2 gpm at a 70°F rise, against roughly 5 gpm for gas. They also show the electrical cost of going tankless with electricity: a 27 kW unit draws 112.5 amps, more than many older homes have spare.

Yearly operating cost: electric, gas or heat pump

DOE rates every water heater with a uniform energy factor (UEF) measured under one of four draw patterns, and its test procedure turns that into yearly energy: 365 days × gallons per day × 8.24 lb per gallon × 67°F (58°F in, 125°F out) ÷ UEF. The table applies that formula with EIA's 2025 residential averages of 17.30 cents per kWh and $1.48 per therm:

The UEFs are the medians of DOE-certified 50-gallon models for electric (0.92) and heat pump (3.75) tanks, the 40-gallon gas median (0.64), and the ENERGY STAR floor for gas tankless (0.95). At medium use, a heat pump water heater saves about $461 a year over an electric tank. Gas units also use a little electricity for fans and controls, which the comparison leaves out.

How to use this calculator

1. Choose Tank or Tankless. Tank mode sizes by the first hour rating, tankless mode by flow and temperature rise.
2. Tank: enter how many showers, shaves, dishwasher loads and washer loads happen in your busiest hour. The result is the first hour rating to look for, with a verdict for every common tank size and fuel.
3. Tankless: set each fixture's flow and how many run at once, then pick your region's inlet temperature or enter a measured one. The result gives gas input, electric kW and amps, and which common sizes keep up.
4. Compare running costs in the bottom card. The draw pattern follows your result; tap another pattern or change the prices to match your bills.

It is meant for homeowners replacing a failed heater who want to avoid buying too small, buyers weighing a heat pump or tankless upgrade, and plumbers who need a quick, sourced number to show a client.

Key terms

• First hour rating (FHR): gallons of hot water a tank delivers in one hour starting fully heated, printed on the EnergyGuide label.
• Peak-hour demand: the hot water your home uses in its busiest hour; the FHR should match or beat it.
• Gallons per minute (gpm): flow rate; a tankless heater's rated maximum GPM is measured at a 67°F rise.
• Temperature rise: output temperature minus incoming water temperature.
• Uniform energy factor (UEF): DOE's efficiency rating, hot water energy delivered divided by energy used.
• Draw pattern: one of DOE's four test profiles (very small, low, medium, high), chosen from the FHR or maximum GPM.

What changes the result

• Back-to-back showers. Each adds 20 gallons to the peak hour; a fourth shower moves 70 gallons to 90.
• Washer type. A top-loader adds 25 gallons, a front-loader 15.
• Inlet temperature. Heating 5 gpm needs 200,362 Btu/h of gas input in Duluth and 104,084 Btu/h in Miami.
• Output temperature. Raising it from 120°F to 140°F for a dishwasher without its own heater turns a 70°F rise into 90°F, about 29% more burner.

Limitations

The per-use gallons are DOE averages, and DOE's own worksheet example (three showers, a shave and hand dishwashing) is printed with a total of 66 gallons although the listed items add to 65; this page uses the itemized figures. Median first hour ratings describe typical models, not yours, so read the label before you buy. Tankless sizing assumes full fixture flow at 120°F, which is conservative for showers that mix in cold water. Yearly costs use DOE's standard test conditions and national average prices, so a cold-climate home with expensive power will pay more.

Related calculators

Size the rest of the home's heating with the BTU Calculator, cut the heat load with the Insulation Calculator, and check whether your panel can carry an electric unit with the Watts to Amps Calculator. The Electricity Cost Calculator and Natural Gas Cost Calculator price the yearly energy, and the Celsius to Fahrenheit Converter helps with metric spec sheets.

Sources

• U.S. Department of Energy, Energy Saver, Sizing a New Water Heater (archived copy of July 2026; the page has since been retired from energy.gov): peak-hour worksheet, tankless example, 50°F and 120°F defaults.
• Electronic Code of Federal Regulations, 10 CFR 430, Subpart B, Appendix E (water heater test procedure, draw patterns, annual energy formula) and 10 CFR 430.32 (UEF standards, 2029 levels, faucet and showerhead flow limits).
• U.S. Department of Energy, Compliance Certification Database, Water Heaters (data updated September 25, 2026).
• ENERGY STAR (EPA), Water Heater Key Product Criteria.
• EPA WaterSense, Showerheads and Bathroom Faucets.
• U.S. Geological Survey, Collins (1925), Temperature of Water Available for Industrial Use in the United States (Water-Supply Paper 520-F).
• NOAA National Centers for Environmental Information, U.S. Climate Normals 1991-2020 (annual mean temperature).
• U.S. Energy Information Administration, average residential electricity price, 2025, residential natural gas price, 2025 and Btu conversion factors.