How House Works

Water heater size calculator

Two calculations from the U.S. Department of Energy's sizing method: how many gallons per minute a tankless heater must heat and by how many degrees, and the first-hour rating to look for on a tank.

By How House Works EditorialBased on DOE sizing method, federal fixture limitsChecked against sources

Tankless: flow and temperature rise

Hot water devices used at the same time
UseDeviceHow manygpm each
ShowerFederal maximum for a showerhead (at 80 psi), also the figure in DOE's example. WaterSense showerheads use 2.0 gpm or less.
Faucet (DOE's example)The faucet in DOE's example.
Kitchen faucet at the federal maximumFederal maximum for a kitchen faucet (at 60 psi).
Bathroom faucet at the federal maximumFederal maximum for a lavatory faucet (at 60 psi). WaterSense bathroom faucets: 1.5 gpm or less.
DishwasherNREL's average for a dishwasher event, hot water only.
Clothes washerNREL's average for a clothes washer event, hot water only.
Bathtub fillingNREL's average for a bath event, hot and cold water combined, so the hot share is lower.

DOE's defaults: assume 50 °F incoming water unless you know otherwise (a thermometer under a cold faucet tells you), and 120 °F for most uses. In winter the incoming water can be colder, so use the coldest temperature you expect.

What real heaters deliver

Manufacturers list how many gallons per minute a model can heat at a given rise. All of these list a 67 °F rise; at a bigger rise the flow is lower.

Modelgpm at 67 °F riseSource
Navien NPE-150S23.4Navien NPE-2 install manual, p. 7
Navien NPE-180A24.3Navien NPE-2 install manual, p. 7
Navien NPE-210A25.0Navien NPE-2 install manual, p. 7
Navien NPE-240A25.6Navien NPE-2 install manual, p. 7
Rheem RTGH-68DVLN-33.4Rheem RTGH spec sheet, p. 1
Rheem RTGH-84DVLN-34.6Rheem RTGH spec sheet, p. 1
Rheem RTGH-90DVLN-35.2Rheem RTGH spec sheet, p. 1
Rheem RTGH-95DVLN-35.7Rheem RTGH spec sheet, p. 1

For scale, DOE says a 70 °F rise is typically possible at about 5 gpm through gas-fired tankless heaters and 2 gpm through electric ones, and that taking a shower and running the dishwasher at the same time can stretch a tankless water heater to its limit.

Tank: first-hour rating

Hot water use during the busiest hour
Use in your busiest hourGallons eachTimes
Shower
Shaving
Hand dishwashing or food prep
Automatic dishwasher
Clothes washer, top-loader
Clothes washer, front-loader (H-axis)

Gallons per use are DOE's worksheet averages. The starting numbers are DOE's example household: 3 showers, 1 shave and 1 hand dishwashing in the same hour.

The first-hour rating is the number of gallons of hot water a tank heater can supply in an hour, starting with a full tank (DOE). Under the FTC's labeling rule, the EnergyGuide label of a storage water heater shows both the tank size and the first-hour rating (16 CFR 305.17(a)(4)). DOE adds that manufacturers' product literature may list it too.

Gas piping, venting and electrical work for a water heater belong to a licensed professional following the heater's installation manual; Navien, for example, reserves gas connections for a licensed professional (NPE-2 Installation Manual, p. 20).

Methodology

Both calculations follow DOE's Energy Saver page “Sizing a New Water Heater”. That page is no longer on energy.gov (it returned a 404 error on October 12, 2026); the sources list links to an archived copy. The section with the worksheet is dated August 21, 2012, and DOE notes that the gallon figures are estimates based on averages from a variety of information published on websites.

  1. Tankless flow = the sum of the flow rates of the devices you expect to run at the same time.
  2. Temperature rise = hot water temperature − incoming water temperature (DOE's default: 120 °F − 50 °F = 70 °F).
  3. Heat delivered = flow (gpm) × rise (°F) × 8.33 lb per gallon × 60 minutes × 1 Btu per pound per °F, about 499.8 Btu/h for each gpm and °F. For electric units, kW = Btu/h ÷ 3,412, and amps at 240 V = watts ÷ 240.
  4. Tank peak hour demand = gallons per use × times used in your busiest hour, summed. DOE's advice is a first-hour rating at least that large.
  5. The usage groups (very small under 18 gallons, low 18 to under 51, medium 51 to under 75, high 75 or more) come from the federal test procedure, 10 CFR 430 Appendix E.

One correction to DOE's own example: its worksheet rows (3 showers × 20, 1 shave × 2, 1 hand dishwash × 3) add up to 65 gallons, while the page prints a total of 66. The calculator shows 65.

What the calculator does not account for

  • Seasonal and regional differences in incoming water temperature. Enter your own number if you know it.
  • The hot and cold mix at a shower or tub. The calculator counts the full flow of each device, as DOE's method does, although part of that flow is cold water.
  • Burner efficiency and gas input, altitude, and the length of the gas line and venting.
  • Recirculation systems.

Default values

DevicegpmSource
Shower2.510 CFR 430.32
Faucet (DOE's example)0.75DOE, Sizing a New Water Heater (archived)
Kitchen faucet at the federal maximum2.210 CFR 430.32
Bathroom faucet at the federal maximum2.210 CFR 430.32
Dishwasher1.39NREL, House Simulation Protocols, Table 12
Clothes washer2.2NREL, House Simulation Protocols, Table 12
Bathtub filling4.4NREL, House Simulation Protocols, Table 12

Sources

  1. U.S. Department of Energy. Sizing a New Water Heater (Energy Saver; no longer on energy.gov, archived copy of July 2, 2026). sections “Sizing Tankless or Demand-Type Water Heaters” and “Sizing Storage and Heat Pump (with Tank) Water Heaters”. Accessed Oct 12, 2026.
  2. U.S. Department of Energy. Tankless or Demand-Type Water Heaters (Energy Saver; no longer on energy.gov, archived copy of February 1, 2026). section “How Tankless Water Heaters Work”. Accessed Oct 12, 2026.
  3. Electronic Code of Federal Regulations. 10 CFR 430.32, Energy and water conservation standards (showerheads, faucets). paragraphs (o) and (p), current as of October 8, 2026. Accessed Oct 12, 2026.
  4. U.S. Environmental Protection Agency. WaterSense Specification for Showerheads, Version 1.1. July 26, 2018, section 3.1.1. Accessed Oct 12, 2026.
  5. U.S. Environmental Protection Agency. High-Efficiency Lavatory Faucet Specification, Version 1.0. October 1, 2007, section 2.2. Accessed Oct 12, 2026.
  6. National Renewable Energy Laboratory / U.S. Department of Energy. 2014 Building America House Simulation Protocols (NREL/TP-5500-60988). March 2014, Table 12, printed p. 15. Accessed Oct 12, 2026.
  7. Electronic Code of Federal Regulations. Appendix E to Subpart B of Part 430, Uniform Test Method for Measuring the Energy Consumption of Water Heaters. section 5.4.1, Tables I and II; current as of October 8, 2026. Accessed Oct 12, 2026.
  8. Navien, Inc.. NPE Condensing Water Heaters Installation Manual (NPE-150S2, NPE-180/210/240 A2 and S2). Version 2.0, January 2026, section 2.3 Specifications, p. 7. Accessed Oct 12, 2026.
  9. Rheem Manufacturing. RTGH Series Tankless Water Heaters spec sheet (form TK-RTGH SERIES). 10/24, p. 1, table “Hot Water Flow Rate Capacity”. Accessed Oct 12, 2026.
  10. U.S. Geological Survey. Facts About Water (Water Science School). “Water numbers”: 8.33 pounds per gallon. Accessed Oct 12, 2026.
  11. U.S. Energy Information Administration. British thermal units (Btu); Energy conversion calculators. 1 Btu raises 1 pound of water by 1 °F; 1 kWh = 3,412 Btu. Accessed Oct 12, 2026.
  12. Electronic Code of Federal Regulations (Federal Trade Commission rule). 16 CFR Part 305, Energy and Water Use Labeling for Consumer Products (sections 305.10(d) and 305.17(a)(4)). current as of October 8, 2026. Accessed Oct 12, 2026.