Water heater size calculator

How to size a tank water heater

Tank size on the box is not the number that matters. What matters is the first-hour rating (FHR): how many gallons of hot water the heater can deliver in one hour, starting full. It's on the yellow EnergyGuide label as "Capacity (first hour rating)". The Department of Energy's advice is to pick a model whose FHR at least matches your peak-hour demand. Here is the math with the calculator's starting values.

1. 1

   Pick your busiest hour

   For most homes it's the morning: three people shower, someone shaves, someone rinses the breakfast dishes.

2. 2

   Add the gallons

   3 showers × 20 gallons + 1 shave × 2 + 1 hand dishwashing × 3 = 65 gallons of hot water in that hour.

3. 3

   Match the first-hour rating

   You need an FHR of 65 or more. A typical 40-gallon gas tank rates about 80, so it covers this home. A 40-gallon electric tank at about 55 would run short. A 50-gallon heat pump model, at a typical 67, just covers it.

How to size a tankless water heater

A tankless unit heats water as it flows, so it is rated by how many gallons per minute (gpm) it can heat by a given number of degrees. Two numbers decide the size:

1. 1

   Flow: add up what runs at once

   The DOE's example is one 2.5 gpm shower and one 0.75 gpm faucet: 3.25 gpm.

2. 2

   Temperature rise: hot setting minus cold inlet

   DOE suggests assuming 50°F water coming in and 120°F going out, a 70°F rise. In winter the water is colder, so size for winter.

3. 3

   Convert to burner or element size

   Gas: gpm × 500 × rise ÷ 0.95 efficiency = 3.25 × 500 × 70 ÷ 0.95 ≈ 120,000 Btu/h. Electric: gpm × rise ÷ 6.83 = 3.25 × 70 ÷ 6.83 ≈ 33 kW, or about 139 amps at 240 volts.

That electric figure is why electric tankless units work best in warm climates or for one fixture at a time. DOE puts the typical capacity at about 5 gpm for gas units and 2 gpm for electric ones at a 70°F rise. A common 199,000 Btu/h gas unit is rated about 5.4 to 6.0 gpm at the 67°F test rise.

Incoming water temperature by state

The colder your water, the more work every heater does. We estimate it with the method in the National Renewable Energy Laboratory's Building America protocols: the yearly average water main temperature is the yearly average air temperature plus 6°F, and it swings colder in winter depending on how cold your winters are. Air temperatures are NOAA's 1991–2020 state averages. Selected states:

Statewide averages. Mountain towns, well water and long buried runs differ; a thermometer under a cold tap in January gives you the real number. Pick your state in the calculator for its values.

Gas, electric, heat pump or tankless?

Efficiency is measured as the Uniform Energy Factor (UEF): the share of the energy you buy that ends up as hot water, under the DOE test. Higher is better, and a heat pump beats 1 because it moves heat from the air instead of making it.

* Estimates. Heat pump life is assumed similar to a tank; electric tankless UEF varies by model, with a federal minimum of 0.91.

• Heat pump water heaters cost the least to run almost everywhere power is not extremely expensive. They need a fair amount of surrounding air (check the installation manual for the minimum room size), a condensate drain, and they cool the space around them. Most switch to backup elements when demand is high.
• Gas tanks recover fast, so a small tank has a high FHR. A standard atmospheric model is the cheapest to buy where gas is already piped and vented.
• Gas tankless never runs out, but it often needs a larger gas line and new venting, and it must be sized for winter.
• Electric tanks are simple and cheap to install, and the most expensive to run.

Tax credits and rules in 2026

The federal Energy Efficient Home Improvement Credit (section 25C) used to cover 30% of a heat pump water heater, up to $2,000, and up to $600 for an efficient gas model. The 2025 budget law ended it early. The IRS says the credit is "not allowed for any property placed in service after Dec. 31, 2025," so a water heater installed in 2026 gets no federal tax credit. Separately, the Inflation Reduction Act's state-run rebates still exist: households under 150% of their area's median income can get up to $1,750 toward a heat pump water heater, in the states that have launched the program. Ask your state energy office and your utility, which often runs its own rebates.

A DOE efficiency standard adopted in 2024 takes effect for units made on or after May 6, 2029. It sets efficiency levels for electric tanks larger than 35 gallons, up to 120 gallons, that only heat pump models meet today. Tanks already installed are not affected.

Tips from the trade

• Read the label, not the box. Two "50-gallon" tanks can differ by 30 gallons of FHR.
• Bigger is not safer. An oversized tank keeps more water hot around the clock and loses more heat.
• Set 120°F. It's DOE's recommended setting for most homes, saves energy and lowers scald risk. Raise it only if a dishwasher without its own heater needs it.
• Low-flow heads change the math. A 2.0 gpm WaterSense showerhead uses 20% less than a 2.5 gpm one, which shrinks a tankless unit and stretches a tank.
• Hire licensed pros for gas and big electrical work. Gas lines and venting, the temperature and pressure relief valve, seismic straps in some states, and large electric circuits are code items that need a permit in most places. Carbon monoxide from bad venting is a real risk with gas units.

Sources and assumptions

• Sizing method, gallons per use, tankless flow and the 50°F/120°F defaults: U.S. Department of Energy, Energy Saver, Sizing a New Water Heater and Tankless or Demand-Type Water Heaters (archived copies from June and July 2026; the live pages were offline on October 29, 2026). Bath, 20 gallons: AHRI peak-hour worksheet.
• Fixture flow: federal limits of 2.5 gpm for showerheads and 2.2 gpm for faucets, 10 CFR 430.32; WaterSense showerheads at 2.0 gpm or less.
• Running cost formula: DOE test procedure, 10 CFR 430, Appendix E: energy per year = gallons a day × 365 × 8.24 lb/gal × temperature rise ÷ UEF, in Btu; 3,412 Btu per kWh and 100,000 Btu per therm. Daily use bins of 38, 55 and 84 gallons are the test's low, medium and high draw patterns. We use your temperature rise in place of the test's fixed 67°F, which is an approximation.
• UEF values: ENERGY STAR criteria and certified-product list (median 3.68 for 50-gallon-class heat pump models, 0.95 for gas tankless, October 2026); typical 50-gallon gas (0.62) and electric (0.92) tanks from manufacturer listings and the federal minimums. First-hour ratings: manufacturer specification pages (for example a 40-gallon atmospheric gas tank at 80 and a 40-gallon electric tank at 55) and ENERGY STAR heat pump medians (67, 79 and 91 for 50, 65 and 80 gallons).
• Water temperature: NREL, Building America House Simulation Protocols (2010), mains temperature equation, with NOAA Climate at a Glance 1991–2020 state averages (Anchorage for Alaska, Honolulu for Hawaii, Reagan National for DC). Hawaii's formula result (84°F) is above realistic tap temperatures, so we cap it at 77°F yearly and 75°F in winter. The U.S. default is the DOE test's 58°F, with DOE's 50°F for winter.
• Electricity: EIA Electric Power Monthly, Table 5.6.B, residential average, January–August 2026. Natural gas: EIA residential price by state, 2025 annual average (released September 30, 2026), converted to therms with each state's heat content. For CT, ME, MD, NC, RI and SC, where EIA shows no 2025 annual figure, we used the volume-weighted average of the 2025 months it does publish.
• Tax credit: IRS, One, Big, Beautiful Bill provisions (updated October 22, 2026) and Energy Efficient Home Improvement Credit. Rebates: DOE Home Energy Rebates. 2029 standard: DOE Consumer Water Heaters.
• Installed prices: HomeGuide water heater and tankless cost guides (January 2026), national ranges. Lifespans: DOE. Prices and rates checked October 29, 2026.

Common questions