What Size Water Heater Do I Need?
Asking "what size water heater do I need?" is the right first step — buy too small and the morning showers run cold; buy too big and you pay to heat water you never use. Enter your household details below to get an estimated first-hour rating, the right tank size, and a tankless alternative flow rate.
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How to use this calculator
Enter how many people live in your home and how much hot water you use overall. Low means quick showers and conservative habits; average is typical daily use; high means long showers, frequent laundry, and high bath use. Then enter how many showers usually run during the busiest hour of the day — that peak demand is what the sizing is really based on.
Pick your fuel type (gas or electric) so the guidance reflects how each heats water, and fill in the tankless section if you're considering going tankless: how many showers run at the same time and whether the dishwasher usually joins in. Your results update instantly as you type.
How it works
Water heaters are sized by first-hour rating (FHR) — the gallons of hot water a unit can deliver during your busiest hour. The rule of thumb used here: FHR needed ≈ people × 12 gallons for average use (10 for low, 14 for high). If your busiest hour has more showers than the per-person estimate covers, the calculator uses the larger of the two figures, since peak demand is what actually empties a tank.
FHR is not the same as tank size. A heater's FHR equals roughly 70% of its tank capacity for gas models and 60–65% for electric ones, because gas burners reheat water faster. So the recommended tank size is the smallest standard size (30, 40, 50, 65, 75, or 80 gallons) that covers your needed FHR — and a gas unit at that tank size will generally deliver more hot water per hour than an equivalent electric one.
For the tankless alternative, flow rate is what matters, not gallons: about 2.5 GPM per simultaneous shower plus 1.5 GPM for a running dishwasher. Choose a tankless unit rated above that total.
These are planning estimates based on industry rules of thumb, not a guarantee. Actual performance depends on incoming water temperature, your specific fixtures, and the model you buy — always check the FHR or GPM on the EnergyGuide label, and consult a plumber for final sizing.
Frequently asked questions
What size water heater do I need for a family of 4?
A family of 4 with average hot-water use typically needs a 50-gallon tank. That covers about 48 gallons of first-hour demand, which the 50-gallon size handles. If your family takes back-to-back long showers, size up to 65 gallons.
What is first-hour rating (FHR), and how is it different from tank size?
Tank size is how many gallons the tank holds. First-hour rating (FHR) is how many gallons of hot water the heater can deliver in the busiest hour, combining stored water plus what the burner or element reheats. Two heaters with the same tank size can have different FHR values, which is why FHR matters more than tank size alone.
Is a gas or electric water heater better for the same tank size?
Gas burners typically deliver higher first-hour ratings for the same tank size because they heat water faster. Electric models recover more slowly, so an electric heater may need a slightly larger tank to match the FHR of a gas one. Check the FHR number on the EnergyGuide label rather than tank size alone when comparing.
Should I switch from a tank to a tankless water heater?
Tankless units never run out of hot water, but they must handle your peak flow rate. Add up everything running at once — roughly 2.5 GPM per shower plus 1.5 GPM for a dishwasher — and pick a unit rated above that total. Tankless heaters also cost more up front and may need upgraded gas lines or electrical service.
Do low-flow showerheads mean I can buy a smaller water heater?
Low-flow showerheads (around 1.75 GPM vs. 2.5 GPM for standard heads) do reduce peak-hour demand, which can let you drop one tank size or run a smaller tankless unit. However, oversizing slightly is safer than running short on hot water during the morning rush.