How to Choose the Right Water Heater Size
For tank water heaters, size by First Hour Rating (FHR) — the gallons of hot water it delivers in the first hour of use. A household of 1–2 people needs a 30–40 gallon tank (FHR 50–60), 3–4 people need a 40–50 gallon tank (FHR 60–80), and 5+ people need a 50–80 gallon tank (FHR 80–100). For tankless units, size by flow rate in GPM: add up the GPM of every fixture you'd run simultaneously, then choose a unit rated to deliver that flow at your required temperature rise. Undersizing causes cold showers; oversizing wastes energy and money.
Cost Breakdown
| Service | Low | High | Notes |
|---|---|---|---|
| 40-gallon gas tank water heater (installed) | $900 | $1,500 | Most common residential size |
| 50-gallon gas tank water heater (installed) | $1,100 | $2,000 | For larger households (4–5 people) |
| Whole-home gas tankless (installed) | $2,500 | $4,500 | Includes gas line upgrade if needed |
| Whole-home electric tankless (installed) | $1,500 | $3,000 | May need electrical panel upgrade |
Tank Water Heaters: Size by First Hour Rating
The First Hour Rating (FHR) printed on every tank water heater's EnergyGuide label tells you the actual gallons of hot water it produces in one hour of continuous demand. This matters more than tank size because a well-designed 40-gallon tank with a powerful burner can outperform a poorly designed 50-gallon tank with a weak one.
To find your FHR need: pick your busiest hot-water hour (usually morning showers and getting ready) and add up the gallons used. A shower uses 10–15 gallons, a dishwasher cycle uses 6 gallons, and each sink faucet running hot uses 2 gallons per minute.
Tank Size Guide by Household
- 1–2 people: 30–40 gallon tank, FHR 50–60. Handles one shower plus one sink running simultaneously.
- 3–4 people: 40–50 gallon tank, FHR 60–80. Handles two back-to-back showers plus a dishwasher load. This is the most common residential size.
- 5+ people: 50–80 gallon tank, FHR 80–100. Handles multiple simultaneous showers or a large household morning routine. Consider two smaller tanks or a tankless unit at this level.
Tankless Water Heaters: Size by Flow Rate
Tankless units are sized by gallons per minute (GPM) at a specific temperature rise. Here's how to calculate:
- List every fixture you'd run at the same time. Shower: 2.0–2.5 GPM. Kitchen faucet: 1.5 GPM. Dishwasher: 1.0–1.5 GPM. Washing machine: 1.5–2.0 GPM.
- Add the GPM values. Example: two showers + kitchen faucet = 5.5–6.5 GPM.
- Determine your temperature rise. Subtract your incoming groundwater temperature from your desired output (typically 120°F). In northern states, groundwater is 40–50°F, requiring a 70–80°F rise. In southern states, groundwater is 60–75°F, requiring only a 45–60°F rise.
- Match to a unit. A unit rated for 5.5 GPM at a 70°F rise handles the example above in cold climates. The same demand in warm climates could be handled by a smaller unit rated for 5.5 GPM at a 45°F rise.
Gas vs. Electric Sizing Differences
Gas tank heaters recover faster (30–40 gallons/hour) than electric models (20–25 gallons/hour), so you can often go one size smaller with gas. For tankless, gas units produce 5–10 GPM while most whole-home electric tankless units max out at 3–5 GPM. In cold climates, electric tankless often can't supply more than two fixtures simultaneously — gas is the better choice for whole-home tankless in northern states.
Related Questions
Is a 40-gallon water heater enough for a family of 4?
A 40-gallon gas water heater with a First Hour Rating of 65+ works for most families of four. If you frequently run multiple showers at the same time or have a soaking tub, step up to a 50-gallon unit. Electric 40-gallon tanks recover more slowly and may run short for a family of four during peak usage.
What size tankless water heater do I need for a 3-bathroom house?
For a 3-bathroom house, assume a worst case of 2 showers and 1 faucet running simultaneously (roughly 6–7 GPM). In cold climates (70°F rise), you need a gas tankless unit rated for at least 7 GPM. In warm climates (45°F rise), a 5–6 GPM unit handles the same demand.
Does a bigger water heater cost more to run?
Yes, but the difference is modest. A 50-gallon tank costs roughly $30–$50 more per year in standby heat loss compared to a 40-gallon tank. The bigger concern is oversizing by two categories — an 80-gallon tank heating water for two people wastes $100–$150/year.