Water Heaters · Calculator
Water Heater Sizing Calculator: First-Hour Rating from Peak Hour Use
Tank water heaters should be sized by first-hour rating (FHR), the gallons of hot water they can deliver in one busy hour starting full, not by tank volume alone. Add up the hot water your household uses in its busiest hour and choose a heater whose FHR meets or exceeds it. Three showers, a dishwasher load, a warm laundry load and some sink use come to about 56 gallons, which a typical 40-gallon gas or 50-gallon electric tank covers. As a quick check by household size: 1–2 people usually need 30–40 gallons, 3–4 need 40–50, and 5 or more need 50–80, with gas at the smaller end because it reheats faster.
Water heater size calculator
What first-hour rating means
A tank stores hot water and reheats it at a fixed rate. During a busy hour you draw the stored water and whatever the burner or elements can add in that hour. The first-hour rating captures both, measured under the US Department of Energy test procedure. It is printed on the yellow EnergyGuide label and on the manufacturer's spec sheet.
Two 50-gallon tanks can have very different FHRs. A gas burner of 40,000 BTU/h recovers roughly 40–45 gallons an hour through a 90 °F rise; a standard electric tank, whose 4,500 W elements normally run one at a time, recovers closer to 20 gallons. That is why a 40-gallon gas heater often outperforms a 50-gallon electric one in the morning rush.
The calculation
Peak-hour demand (gal) = Showers × 12 + Baths × 22 + Dishwasher × 6 + Laundry × 8 + Sink uses × 3
Choose a heater with FHR ≥ Peak-hour demand
The per-use figures are estimates of hot water, after mixing with cold. A shower at 2.0 GPM for 8 minutes uses 16 gallons, but about three-quarters of that is hot water from a 120 °F tank when the cold supply is around 50 °F, so roughly 12 gallons. Adjust if your household is different: longer showers, 2.5 GPM heads or a large soaking tub all raise the total. Front-loading washers and newer dishwashers use less.
Quick estimate by household size
If you do not want to count showers, the calculator also gives a rule-of-thumb tank size from the number of people and the fuel:
| People | Gas | Electric | Heat pump |
|---|---|---|---|
| 1–2 | 30–40 gal | 40 gal | 40–50 gal |
| 3–4 | 40 gal | 50 gal | 50–65 gal |
| 5–6 | 50 gal | 65–66 gal (5), 80 gal (6) | 80 gal |
| 7+ | 75 gal or two tanks | 80 gal | 80 gal |
A gas burner reheats water roughly twice as fast as a standard electric tank, so a smaller gas tank reaches the same first-hour rating. Heat pump models recover slowest of all in their efficient mode, so they are listed a step larger. Treat this as a starting point and confirm with the peak-hour count, which reflects your actual habits.
Worked example
A family of four's weekday morning: three showers, the dishwasher from last night finishing, one warm laundry load and two sink uses.
- Showers: 3 × 12 = 36 gal
- Dishwasher: 1 × 6 = 6 gal
- Laundry: 1 × 8 = 8 gal
- Sinks: 2 × 3 = 6 gal
- Total = 56 gallons
Look for an FHR of at least 56. The quick estimate agrees: four people on gas gives a 40-gallon tank (50 gallons on electric). A typical 40-gallon gas tank (FHR often 65–75) covers it. On electric, a standard 40-gallon (FHR often around 50–55) would fall short, so a 50-gallon is the right size. If someone also fills a bathtub that hour, demand jumps to 78 gallons, which calls for a 50-gallon gas or an 80-gallon electric tank.
Typical first-hour ratings by tank
Ranges vary by model, burner size and element wattage. Always confirm the FHR on the label.
| Tank size | Gas (atmospheric or power vent) | Electric resistance | Heat pump (hybrid mode) |
|---|---|---|---|
| 30 gal | about 50–60 | about 40–45 | uncommon |
| 40 gal | about 65–75 | about 50–58 | about 50–60 |
| 50 gal | about 80–90 | about 60–65 | about 60–70 |
| 65–66 gal | uncommon | about 70–75 | about 70–80 |
| 75 gal | about 120–135 | uncommon | uncommon |
| 80 gal | uncommon | about 85–95 | about 80–90 |
Heat pump water heaters recover slowly in heat-pump-only mode. If you plan to run them in their most efficient mode, size one step larger than the table suggests.
Tank, tankless or heat pump?
Tanks are sized by the busiest hour. Tankless units are sized by the busiest minute: the flow of fixtures running together. If your household has long, staggered showers rather than everyone at once, tankless can work well; use the tankless sizing calculator to see the gas input or kW required. The tankless vs tank guide compares the two, and gas vs electric explains how fuel prices and heat pump efficiency affect running costs. Heat pump water heaters also cool and dehumidify the space they sit in, a bonus in a damp basement and a drawback in a cold one.
Signs your current heater is the wrong size
- Too small: the last shower of the morning runs cold, or you schedule laundry and dishes around showers. Before replacing, check that the thermostat is not set too low, that the dip tube is intact (a broken one mixes cold water into the outlet), and that sediment has not buried the lower element or insulated the burner.
- Too large: rarely noticed directly, but it shows as higher standby losses. A 75-gallon tank for two people is the common example.
- Plenty of hot water, but slow to arrive: the heater is not the problem; the pipe run is. See the recirculation options.
Running cost depends on fuel and efficiency as well as size; the heating cost calculator can compare fuels for your annual water-heating energy.
Safety and setpoint
Most heaters ship set near 120 °F, which limits scald risk while reducing bacterial growth. Some households set 130–140 °F to stretch capacity, with a thermostatic mixing valve at the outlet to bring delivered water back to a safe temperature. If you add a recirculation pump, its heat loss adds to the load the heater has to recover.
Gas water heater replacement involves gas piping, venting and a temperature-and-pressure relief valve that must discharge safely; it is regulated work in most areas and usually requires a permit. Electric units need the correct breaker and wire size.
Frequently asked questions
What size water heater do I need for a family of four?
Most families of four need a first-hour rating of about 55–70 gallons. That is typically a 40-gallon gas tank (50 gallons if showers really stack up), or a 50- to 65-gallon electric or heat pump tank. The calculator's quick estimate uses the same household-size rule, while the peak-hour count reflects your own routine.
Is a bigger tank always better?
No. A tank much larger than you need loses more heat through its walls around the clock. Size by first-hour rating and choose the smallest tank that covers your busiest hour.
Where do I find a water heater's first-hour rating?
On the yellow EnergyGuide label, the spec sheet and often the rating plate on the tank, listed as "First Hour Rating" or "FHR" in gallons.
Last reviewed 2026-10-28