Water Heater Sizing Calculator

Recommended tank gallons and first-hour rating from household size, bathrooms, peak-hour use, and gas vs electric recovery

Starting point only — not a substitute for a plumber counting actual fixture demand. Tank size and first-hour rating here come from the U.S. Department of Energy Energy Saver peak-hour demand worksheet (fixture gallons: shower 20, shaving 2, hand dishwashing/food prep 3, dishwasher 7, H-axis clothes washer 15) plus a transparent gas-vs-electric recovery assumption used to translate that first-hour number into a common storage tank size. Unusual household patterns (soaking tubs, body sprays, a hot-tub fill, a salon sink, stacked laundry during showers) are outside this worksheet. Confirm the yellow EnergyGuide first-hour rating on the unit you buy, inlet water temperature, and simultaneous fixture GPM with a plumber.

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Water Heater Service Gear

Sizing gets you a tank or a tankless spec. Sediment, anodes, mixing valves, and T&P valves are why a “big enough” heater still runs out of hot water.

Sacrificial Anode Rod

A spent anode does not shrink the tank, but it lets the steel rust from the inside. Check it before you assume you need more gallons.

View on Amazon →

Thermostatic Mixing Valve

Store hotter, deliver 120°F at the tap. A mixing valve changes how much usable hot water you actually get from the same tank.

View on Amazon →

Pipe Wrench

Anode, drain valve, and dielectric unions still come apart with a wrench. Sizing math does not replace a drain-and-flush.

View on Amazon →

T&P Relief Valve

Safety first. A weeping T&P is not a sizing problem. Replace the valve; do not cap it.

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Thermal Expansion Tank

Closed municipal systems need somewhere for heated water to go. Expansion tanks are not first-hour rating, but they save the T&P from dripping all winter.

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Run the Peak-Hour Numbers

Household size, bathrooms, peak scenario, gas or electric. Tank gallons and FHR starting point.

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How to Use This Water Heater Sizing Calculator

Count the people who live here, count the bathrooms that can actually run a shower, pick the busiest hour you really have (not the one you wish you had), and say whether the replacement is gas or electric. The calculator builds a DOE peak-hour demand from those choices, reports that number as the first-hour rating to match, then picks the smallest common storage tank whose estimated FHR clears it given gas or electric recovery.

Peak-hour demand (FHR target) = Σ (DOE fixture gallons × uses in 1 hour) · Estimated FHR ≈ 0.70 × tank gallons + recovery gal/hr

Tank size vs FHR, and how to read the yellow label on the heater already in the basement: sizing guide. Running out of hot water before you blame the gallon number: troubleshooting. If you are deciding tank vs tankless, sizing is not even the same unit: tank vs tankless.

Water Heater Sizing Calculator