What size water heater do I need?
For a family of four on standard shower heads: a 40 or 50 gallon gas tank, or a 50 to 80 gallon electric. What decides the size is how many gallons of hot water your busiest hour pulls. Tally that hour below; the math updates as you type.
How the sizing works
Tank water heaters are sized by matching one label number, the first hour rating, to your peak hour demand. FHR is a tested figure: start with a full hot tank, draw at 3 gallons a minute, and count what comes out in 60 minutes while the burner or element fights back. Peak hour demand is the worksheet above, using the per-use gallons DOE and AHRI publish: 20 for a shower, 20 for a bath, 14 for a dishwasher load, 32 for a hot-wash laundry load, small change for the sink. Add up your busiest hour, then buy a tank whose FHR meets it.
Worked example, same one AHRI prints: three showers, one shave, one shampoo, one round of hand dishes is 60 + 2 + 4 + 4 = 70 gallons of peak demand. A 50-gallon gas tank (FHR around 78) covers it; a 50-gallon electric (FHR around 55) does not, which is how families end up "running out of hot water" with a tank that sounded big enough. The gallons on the sticker were never the operative number.
One honest caveat, once: our FHR figures are an approximation (70% of the tank plus one hour of recovery) so the tables can exist, and they run within a few gallons of published spec sheets. When you shop, read the actual FHR off the yellow EnergyGuide label; the FHR guide shows where it hides.
Quick reference: showers in the hour, tank that covers it
Each row adds one shave and one round of hand dishes to the showers. Computed by the same engine as the calculator above.
| Showers in busiest hour | Peak demand | Gas tank | Electric tank | Demand, 2.0 GPM heads | Gas | Electric |
|---|---|---|---|---|---|---|
| 1 | 26 gal | 30 gal | 30 gal | 19 gal | 30 gal | 30 gal |
| 2 | 46 gal | 30 gal | 40 gal | 32 gal | 30 gal | 30 gal |
| 3 | 66 gal | 40 gal | 80 gal | 45 gal | 30 gal | 40 gal |
| 4 | 86 gal | 75 gal | none fits | 58 gal | 40 gal | 65 gal |
| 5 | 106 gal | 75 gal | none fits | 71 gal | 50 gal | 80 gal |
The two-tank rows are not a typo. Four standard showers in one hour is 86 gallons of hot water, and no residential electric tank delivers that; this is exactly the household where gas, tankless, or staggering the showers earns its keep.
Questions people ask
What size water heater does a family of 4 need?
A 40 or 50 gallon gas tank, or a 50 to 80 gallon electric, and the spread is not vagueness: it depends on how many of the four shower inside the same hour and what heads are on the walls. Three standard 2.5 GPM showers back to back is 66 gallons of demand, which a 40-gallon gas tank covers (its first hour rating runs about 71) but needs an 80-gallon electric. Swap in 2.0 GPM heads and the electric answer falls to 50 gallons. The family-of-4 page walks the whole table.
Which matters more, tank gallons or first hour rating?
First hour rating. It is the tested number of hot gallons the unit delivers in 60 minutes starting full, and it is printed on every EnergyGuide label. A 40-gallon gas tank outdelivers a 65-gallon electric in that hour (roughly 71 gallons against 66) because the burner replaces water as fast as the showers pull it. Match FHR to your peak hour and the gallons take care of themselves.
Why does an electric tank need to be so much bigger than gas?
Recovery speed. A standard 40,000 BTU gas burner reheats about 43 gallons an hour through a 90°F rise; a 4,500-watt element manages about 20. The gas tank refills the hour as it goes, so it can be smaller. That single number is most of the gas-vs-electric sizing story, and the recovery guide shows the arithmetic.
Can a water heater be too big?
Yes, but the failure is money rather than safety. An oversized tank holds water hot around the clock, so you pay standby losses on gallons you never use, it costs more up front, and an 80-gallon electric is a genuinely awkward object to fit through a basement door. Size to the busiest real hour, not to the worst hour you can imagine.
Should I skip the tank and go tankless?
Different math entirely. A tank is sized to an hour of demand in gallons; a tankless is sized to a moment of demand in gallons per minute, against your winter inlet temperature. In Minnesota, two simultaneous showers can outrun the biggest residential gas unit; in Florida the same unit handles them easily. Run the tankless calculator before deciding.