Enter the number of people, hot water usage pattern, and fuel type — get the correct gallon capacity, required first hour rating, and a realistic installed cost to build your quote.
Pre-built quote forms for water heater installs (tank + tankless), drain jobs, and service calls — professional, ready to print.
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The most common sizing mistake on a water heater replacement is ordering the same gallon capacity as the unit that's coming out. That tank may have been undersized when installed 12 years ago — or a family of five now lives in a house that was sized for two. First hour rating (FHR), not gallon capacity, is the number that actually determines whether a tank will keep up with demand.
FHR is the number of gallons of hot water a heater can deliver in the first hour of operation, starting with a full tank. It combines the stored hot water with the recovery rate. A 50-gallon gas tank with a 40,000 BTU burner might have an FHR of 80–90 gallons. A 50-gallon electric tank with a 4,500W element might have an FHR of 58–65 gallons. Same physical size, very different performance. The FHR is listed on the EnergyGuide label of every tank — match it to your household's peak hour demand, not the gallon number.
Gas tanks recover approximately 1°F per minute (roughly 40 gallons per hour at a 90°F rise). Electric tanks recover significantly slower — a 4,500W element heats about 20 gallons per hour. This means for larger households with sequential showers, you can often downsize a gallon tank when switching from electric to gas and still have more hot water available. A 50-gallon gas tank frequently outperforms a 65-gallon electric in real-world peak demand situations.
HPWHs (Rheem ProTerra, AO Smith Voltex, A.O. Smith HPTU) need adequate ambient air volume — at least 700–1,000 cubic feet of unconditioned space to operate in heat pump mode efficiently. They also exhaust cold, dry air, which can cause comfort issues in small utility rooms. They're not a straight swap in every location. Verify the space requirements before recommending one.
In closed plumbing systems (any home with a pressure-reducing valve, check valve, or backflow preventer on the main), thermal expansion has nowhere to go when the water heater heats water. Code in most jurisdictions now requires a thermal expansion tank on all new water heater installations in closed systems. Include it in every quote — it's a $40–60 part and 15 minutes of labor, and it's required. Don't let the customer argue it out of the quote.
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Tank water heater sizing should be based on first hour rating (FHR) — not gallon capacity alone. This calculator determines the correct FHR and tank size for a household based on occupancy and usage pattern, then builds an installed cost estimate from unit cost, labor time by access difficulty, permit fees, and your markup. Use it to size replacements correctly and stop defaulting to the same size as the old unit.