Water Heater Sizing Calculator

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

Size a standard storage water heater from the people in the house, the bathrooms that can run a shower, the busiest hour you actually have, and whether the replacement is gas or electric. Peak-hour demand follows the U.S. Department of Energy Energy Saver worksheet. That number is the first-hour rating (FHR) to match on the yellow EnergyGuide label. Tank gallons are then the smallest common storage size that can hit that FHR given typical gas vs electric recovery. This is a sizing starting point, not a substitute for a plumber’s assessment of actual fixture demand, especially for unusual household patterns.

Tank size vs FHR, and how to read the existing label: sizing guide. Running out of hot water: troubleshooting. Tank vs tankless is a different sizing problem: tank vs tankless. Plumbing index: plumbing calculators hub.

Basement groundwater is a different worksheet: Sump Pump Sizing Calculator (basin, inflow GPM, total head — starting point, not a named-model pump curve).

Horizontal drain slope on the same plumbing job: Drain Pipe Slope Calculator (IPC 2021 Table 704.1 / UPC 708.1 — prescriptive starting point, not a substitute for local code). Water-supply pipe diameter is a different worksheet (WSFU, not first-hour rating): water supply pipe sizing calculator. Not sure the fixture-unit totals for that house? fixture unit calculator.

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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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.

What This Number Is (and Is Not)

DOE does not size storage water heaters by “people × 10 gallons.” It sizes them by first-hour rating: how many gallons of hot water the heater can deliver in one hour starting from a full tank. That FHR has to meet your peak-hour demand. Tank gallons are a retail shorthand. A 50-gallon gas heater and a 50-gallon electric heater are not the same first-hour machine, because the gas burner recovers faster.

Assumptions baked into every result

DOE Energy Saver worksheet gallons as listed in the results. Peak-hour fixture mix from the scenario you picked, with shower count capped by household size and bathrooms. Usable storage treated as 70% of nominal tank volume (a field approximation of mixed water that stays usefully hot). Recovery: ~37 gal/hr gas, ~21 gal/hr electric at a 90°F rise. Standard tank sizes only: 30, 40, 50, 55, 60, 75, 80 gallons. Tankless GPM is a simultaneous-flow starting point using ~2.0 GPM showers, not a manufacturer curve at your groundwater temperature.

Inlet temperature will lie to you in January. A tankless spec that holds 4 GPM at a 70°F rise may drop below 3 GPM when the well is 40°F and you want 120°F at the tap. The FHR on a tank is a laboratory first hour, not your groundwater. Confirm both with a plumber.

When this tool is the wrong tool

Soaking tubs, multiple body-spray showers, a salon or in-law suite that behaves like a second dwelling, recirculation loops that constantly steal recovery, commercial kitchens, and anything a plumber would count in fixture units rather than a residential peak-hour worksheet. Those jobs need measured GPM, temperature rise, and often a different heater family. Treat every number here as a conversation starter, not a purchase order.

Already running out of hot water on a “big enough” tank?Sediment, thermostat, mixing valve, and a failed dip tube fake an undersized heater.

Troubleshooting →

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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.

Water heater anode rod

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.

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Thermostatic mixing valve

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.

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Pipe wrench

Pipe Wrench

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

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Temperature and pressure relief valve

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

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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Water heater sizing calculator

Run the Peak-Hour Numbers

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

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