What Size Water Heater Do I Need? Calculator

Updated By the KnowTheFix editorial team · How we research

For a tank, size by your busiest hour, not by the number of people. Add up the hot water you use in your peak hour, then pick a tank whose first-hour rating (on the yellow EnergyGuide label) is at least that much. For tankless, add up the flow of everything that runs at once and check it against the unit’s capacity at your temperature rise.

Tank: peak-hour demand calculator

How many of each happen in your home’s busiest hour, typically weekday mornings?

Peak-hour demand: 0 gallons

Choose a tank with a first-hour rating of at least 0 gallons.

Tank typeTypical first-hour ratingFits?

Ranges are typical for each type. Check the exact model’s EnergyGuide label or spec sheet before you buy.

Tankless: flow and temperature-rise calculator

What runs at the same time at your busiest moment?

You need about 0 gpm at a 0°F rise

    Tankless unit showing a code? See Navien, Rinnai or Rheem codes.

    Gas estimate uses BTU × 95% efficiency ÷ (500 × rise), derated about 3% per 1,000 ft above 2,000 ft. Manufacturer charts vary, so check the specific model’s flow chart.

    Quick guide by household size

    PeopleTank size (Rheem guidance)
    1–230–40 gallons
    2–440–50 gallons
    4–550–55 gallons
    5+75 gallons, or tankless

    Fuel matters as much as gallons: a 50-gallon gas tank delivers far more in an hour than a 50-gallon electric. Use the calculator above for a real answer, then see what each size costs to replace.

    Where the numbers come from

    • Gallons per use and the first-hour-rating method: U.S. DOE Energy Saver, “Sizing a New Water Heater” (2024 version). DOE’s older worksheet used lower numbers (10 gallons per shower). We use the current values.
    • First-hour ratings: ENERGY STAR certified product data and manufacturer spec sheets (Rheem, A.O. Smith, Bradford White).
    • Fixture flows: EPA WaterSense (showers ≤2.0 gpm, bathroom faucets ≤1.5 gpm), DOE examples and Rheem’s fixture flow guide.
    • Groundwater temperatures: Rinnai’s US groundwater temperature map. Tankless capacity and altitude derate: Navien and Rinnai spec sheets.

    Typical ranges: 50-gal electric 58–63 gal · 40-gal heat pump 45–66 gal · 50-gal heat pump 50–77 gal · 40-gal standard gas 67–81 gal · 50-gal standard gas 75–85 gal · 40-gal high-efficiency gas 86–88 gal · 50-gal high-efficiency gas 95 gal · 80-gal heat pump 88–95 gal.

    Frequently asked questions

    What size water heater do I need for a family of 4?

    Rheem’s rule of thumb is 40–50 gallons for 2–4 people and 50–55 gallons for 4–5. Sizing by your busiest hour is more accurate: four back-to-back showers alone use about 80 gallons by DOE’s current worksheet, which calls for a tank with a first-hour rating of at least 80 gallons, such as some 50-gallon gas tanks or a high-efficiency gas model.

    What is a first-hour rating?

    The first-hour rating (FHR) is how many gallons of hot water a tank can deliver in an hour starting with a full tank. It’s printed on the yellow EnergyGuide label as “Capacity (first hour rating).” DOE recommends choosing a tank whose FHR is at least your peak-hour demand.

    Why does a 50-gallon gas tank deliver more than a 50-gallon electric?

    Gas burners reheat water much faster than electric elements, so a 50-gallon gas tank typically has a first-hour rating around 75–85 gallons, while a 50-gallon electric tank is around 58–63 gallons.

    What size tankless water heater do I need?

    Add up the flow rates (gallons per minute) of the fixtures you run at the same time, then work out your temperature rise: 120°F minus your incoming water temperature. A typical 199,000 BTU gas tankless unit delivers about 5 gpm at a 70°F rise. Colder groundwater or high altitude lowers that.

    Does altitude affect tankless water heaters?

    Yes. Navien’s guidance, for example, reduces gas output about 3% per 1,000 ft above 2,000 ft on natural gas, about 10% in Denver. Colorado homes should size up accordingly.