Water heater sizing calculator for tank and tankless

How it works

Tank mode adds up the hot water your household uses in its busiest hour, using DOE's worksheet. It gives the first-hour rating to look for on the EnergyGuide label. Tankless mode adds up the flow of the fixtures running at the same time and works out the heat needed to raise your coldest inlet water to the setpoint, with every step in the Show your work panel.

How we calculate it

These are the same steps, in the same order, as the Show your work panel under each result. The tool calculates with unrounded values, so the last digit can differ from multiplying the rounded numbers shown.

Tank or heat pump with a tank

1. Peak-hour demand. For each use, multiply the gallons of hot water per use by the times in your busiest hour. Add the rows. DOE's example (3 showers, 1 shave, 1 sink of dishes) is 20 × 3 + 2 × 1 + 3 × 1 = 65 gallons. DOE's page prints 66 for this example, but its rows add to 65.
2. Which gallons per use. DOE's current worksheet uses 20 gallons per shower. Its pre-2022 edition, still quoted on many sites, used 10. AHRI's 2011 worksheet uses 20 and adds bath, shampoo and hand-washing rows. Pick the profile in the form. Each one is listed in full below.
3. Shower gallons from your showerhead (optional). gpm × minutes × (tap temperature − inlet) ÷ (setpoint − inlet). For example, 2.0 gpm × 8 min × (105 − 50) ÷ (120 − 50) = 12.6 gallons of hot water.
4. First-hour rating to look for. DOE: at least your peak-hour demand. The first-hour rating is the gallons of hot water a tank delivers in an hour, starting full. It is on the EnergyGuide label as "Capacity (first hour rating)".
5. Your current heater (optional). Its first-hour rating minus your peak-hour demand. A negative number is how many gallons short it is.

Tankless

1. Inlet water temperature. If you are not sure, DOE says assume 50°F. For a station, take NOAA's 1991-2020 monthly normals. Annual mean T = the 12 months ÷ 12. Range R = warmest month − coldest month. Then ratio = 0.4 + 0.01 × (T − 44), and coldest inlet = (T + 6) − ratio × R ÷ 2, rounded to a whole degree. This is the NREL correlation used in EnergyPlus. For Minneapolis: T = 46.86, R = 58.1, ratio = 0.4286, coldest inlet = 40.41°F around February 16, so 40°F is used.
2. Temperature rise. Setpoint − inlet. 120 − 40 = 80°F.
3. Flow. Add up gpm × count for every fixture running at the same time (DOE's method). Two 2.5 gpm showers = 5.00 gpm.
4. Heat output. 500 × gpm × rise, in Btu/h. 500 = 8.33 lb per gallon × 60 minutes per hour × 1 Btu per lb per °F. 500 × 5.00 × 80 = 200,000 Btu/h.
5. Gas and electric input. Gas input = output ÷ 0.82 (non-condensing) and ÷ 0.92 (condensing). Electric kW = output ÷ 3,412.14 ÷ 0.98, and amps = watts ÷ volts. A gas cell at 200,000 Btu/h or more is flagged, because one residential gas tankless unit is rated under that (10 CFR 430.2). An electric cell over 200 A is flagged too.
6. Blended load. Showers and taps are mixed with cold, so each mixed fixture counts gpm × count × (tap temperature − inlet). Appliances such as a dishwasher count at the setpoint. Multiply the total by 500 for Btu/h, then divide by 500 × rise for gpm of setpoint water. Two mixed showers at 105°F: 5.00 × 65 × 500 = 162,500 Btu/h, or 4.06 gpm of 120°F water.
7. Your unit (optional). Gas: input × efficiency. Electric: kW × 3,412.14 × efficiency. Divide the output by 500 × rise to get gpm at your rise. The EnergyGuide Max GPM is tested at about a 67°F rise (58°F in, 125°F out), so the tool converts it: Max GPM × 67 ÷ your rise.

Plumbers can carry the result into a quote with the estimate and invoice generator. Replacing an AC, heat pump or furnace as well? The HVAC repair or replace calculator compares the yearly cost of each choice.

DOE Energy Saver worksheet, current edition (gallons of hot water per use)

DOE: "Estimates are based on averages from a variety of information published on websites."

DOE Energy Saver worksheet, pre-2022 edition (gallons of hot water per use)

Standard usage with no water conservation measures. Adapted by DOE from the FEMP Energy Cost Calculator. DOE has replaced these figures with the current edition above; many sites still quote them.

AHRI 2011 worksheet (gallons of hot water per use)

Assumes no water conservation measures. The wringer clothes washer row (26 gal) is left out as obsolete equipment.

Tankless fixture flows used as defaults

Estimated coldest-month inlet temperature by state

We checked four of these estimates against figures water utilities publish (S11-S14). Minneapolis Water Works' river water averaged 37-38°F in January and February 2026, close to our 40°F. Seattle Public Utilities measured about 45°F in late February, against our 53°F. New York City's and Phoenix's distribution samples reached 35-38°F and 47-57°F at their coldest, well below our 50°F and 67°F. If your utility publishes a winter figure, use it.

Sources

1. S1: DOE Energy Saver, "Sizing a New Water Heater". The live page returned "Page not found" on 2026-10-14. Current worksheet read from Wayback Machine captures dated 2025-08-01, 2026-01-01 and 2026-07-02; pre-2022 edition read from a PDF capture.
2. S2: AHRI, "Worksheet for Estimating Peak Hour Demand/First Hour Rating", © 2011.
3. S3: DOE Building America, "Measure Guideline: Transitioning to a Tankless Water Heater", 2012.
4. S4: 10 CFR 430.32(o) and (p), eCFR.
5. S5: EPA WaterSense showerhead and faucet specifications.
6. S6: 10 CFR 430 Subpart B, Appendix E.
7. S7: 10 CFR 430.2.
8. S8: Burch & Christensen (NREL), mains water temperature, 2007; EnergyPlus Engineering Reference.
9. S9: NOAA NCEI U.S. Climate Normals 1991-2020.
10. S10: NIST SP 811, 2008.
11. S11: Seattle Public Utilities, Quarterly Surface Water Analysis, February 2024, 2025 and 2026.
12. S12: NYC DEP, Drinking Water Supply and Quality Report, 2023-2025.
13. S13: City of Phoenix, Water Quality Report, 2023-2025.
14. S14: Minneapolis Water Works, Monthly Plant Effluent Water Analysis, January and February 2026.