What size water heater do I need?
Most people size a water heater by household headcount and tank gallons. That works as a rough guide, but the better number for a tank is its first-hour rating, and for a tankless unit it is the flow rate at your temperature rise. Both come down to one question: how much hot water does your household use in its busiest hour?
Quick rule of thumb for tank heaters
| Household | Typical tank size (gas) | Typical tank size (electric) |
|---|---|---|
| 1 to 2 people | 30 to 40 gallons | 30 to 40 gallons |
| 2 to 3 people | 40 gallons | 40 to 50 gallons |
| 3 to 4 people | 40 to 50 gallons | 50 to 66 gallons |
| 5 or more | 50 to 75 gallons | 66 to 80 gallons |
Electric tanks usually need to be a bit larger because their elements reheat water more slowly than a gas burner.
The better way: first-hour rating
The first-hour rating is how many gallons of hot water the heater can deliver in one hour starting with a full, hot tank. It combines tank size and recovery speed, which is why two 50-gallon tanks can perform very differently. You'll find it on the yellow EnergyGuide label or in the spec sheet.
To find what you need, estimate your peak hour. Typical hot water use per activity:
| Activity | Approx. hot water per use |
|---|---|
| Shower | 10 gallons |
| Washing hands and face, shaving | 2 to 4 gallons |
| Dishwasher | 6 gallons |
| Washing dishes by hand | 4 gallons |
| Clothes washer (warm cycle) | 7 gallons |
| Food prep | 5 gallons |
Example: on a weekday morning, three people shower (30 gallons), someone shaves (2), and the dishwasher runs (6). That's about 38 gallons in the peak hour. A tank with an FHR in the high 30s or 40s would cover it. If showers are long or the showerheads are older and higher-flow, add a margin.
Sizing a tankless water heater
Tankless units heat water on demand, so there's no tank to run out of, but there is a limit on how much hot water they can make at the same time. Two numbers matter:
- Peak flow (gallons per minute). Add the fixtures you realistically run together. A modern showerhead uses about 1.5 to 2.0 gpm, a kitchen faucet around 1.5, a dishwasher about 1 to 1.5 while filling.
- Temperature rise. Subtract your incoming groundwater temperature from your target (usually 120°F). Groundwater in northern states can be around 40 to 50°F in winter, which means a rise of 70 to 80°F. In warm southern states it may be 60 to 70°F, so the rise is smaller.
Manufacturers publish a chart of gpm at different temperature rises. Example: two showers at 2.0 gpm each in a cold climate need about 4 gpm at a 75°F rise. A unit rated for that will keep up; a smaller one will deliver lukewarm water when both showers run.
Gas, electric or heat pump?
- Gas tanks recover quickly and suit larger households, but need venting.
- Electric tanks are simple to install and maintain but recover more slowly.
- Heat pump water heaters are electric units that move heat from the surrounding air. They use far less electricity than standard electric tanks, but they need space around them and cool the room they're in. Their FHR can drop in "heat pump only" mode, so compare ratings in the mode you'll actually use.
Common sizing mistakes
- Buying the same size as the old unit without checking whether the household has changed.
- Ignoring a large soaking tub. Filling one can take most of a 40-gallon tank.
- Oversizing "just in case." A bigger tank keeps more water hot around the clock, which costs energy every day.
- Choosing tankless without checking the gas line size or electrical capacity. Whole-house electric tankless units often need major electrical upgrades.
This guide is general information for typical U.S. homes. Your equipment's manual and local codes take priority. If anything involves gas, electrical panels or refrigerant and you're unsure, stop and call a licensed pro.