ClutchCalcs

Construction

Tank Water Heater Sizing Calculator

Size a tank water heater the way plumbers do: by First Hour Rating (FHR). FHR is gallons of hot water available during the first hour of peak demand. Add up your peak-hour fixtures, account for overlap, and pick the right tank size + fuel type.

Peak hour fixtures

How many of each could run during your busiest hour (typically morning).

Recommended tank size

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Peak demand (1 hr)
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Required FHR
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Recovery rate
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Temp rise needed
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Peak hour breakdown

    Tank options

    Standard residential tank sizes with typical FHR.

    Tank FHR (gas) FHR (electric) Fits your needs?

    Consider tankless if:

    • Peak demand exceeds 80 gal/hr (large family, multi-bathroom)
    • You have space constraints (tankless mounts on wall, no floor space)
    • Your home has natural gas service (electric tankless needs 100-200A dedicated)
    • You want endless hot water (no recovery wait between long shower + bath)

    → Use the tankless sizing calculator

    First Hour Rating (FHR) — the right way to size

    FHR is the gallons of hot water a tank can deliver in the first hour of use, starting fully heated. It accounts for both stored water AND how fast the heater can reheat. This is the number printed on every Energy Star label.

    Sizing by tank gallons alone undersizes constantly — a 40 gal electric tank might have an FHR of only 50 gal, while a 40 gal gas tank can do 70+. Always compare FHR to your peak hour demand.

    FAQ

    Why does pipe size matter? +
    A 1/2" supply line only carries about 5 gpm at typical pressure. If your peak hour requires 8-10 gpm, you will get pressure drop and lukewarm water no matter how big the tank is. Most new homes use 3/4" main.
    Why electric recovery is slower? +
    Electric elements deliver ~15,000 BTU/hr. Natural gas burners run 30-40,000 BTU/hr. Same tank size, gas reheats in roughly half the time.
    Heat pump water heater notes? +
    Heat pump (hybrid) heaters are 2-3x more efficient than standard electric but recover slower — usually need bigger tank size for same FHR. They also cool the room where they sit.
    Does showerhead matter? +
    Massively. Going from 2.5 gpm to 1.75 gpm low-flow cuts shower demand by 30%. A family of 4 may go from needing a 50 gal to a 40 gal tank.
    120°F or 140°F? +
    120°F is OSHA-recommended for residential — safe for children, prevents scalding. 140°F prevents Legionella but requires a tempering valve at fixtures. Most plumbers set to 120-130°F.
    What is wrong with the "bedroom rule"? +
    The rule (40 gal for 1-2 BR, 50 for 3-4 BR, 80 for 5+) ignores fixture density and habits. A 2-bedroom with two long-shower teens beats a 4-bedroom with four quick showers.