Worked examples
Water heater size for a family of 2, 3, 4, 5 or 6
Headcount is only a proxy. What sizes a tank is how many showers and other uses land in your busiest hour; what sizes a tankless unit is what runs at the same moment. Below are worked busiest hours for 2 to 6 people, each calculated at four incoming water temperatures.
Updated
Quick answer, 50°F incoming water
Tank water heaters: minimum First Hour Rating
Each row is one plausible busiest hour for that household, with standard 2.5 gpm showers of 8 minutes and a 120°F tank. Read across to your incoming water temperature.
| Household | Busiest hour | 40°F in | 50°F in | 60°F in | 70°F in |
|---|---|---|---|---|---|
| 2 people | 2 standard showers, 2 bathroom faucets, 1 kitchen faucet | 39 | 38 | 36 | 34 |
| 3 people | 3 standard showers, 2 bathroom faucets, 1 kitchen faucet, a dishwasher load | 61 | 59 | 56 | 53 |
| 4 people | 4 standard showers, 3 bathroom faucets, 1 kitchen faucet, a dishwasher load | 78 | 76 | 73 | 68 |
| 5 people | 4 standard showers, 3 bathroom faucets, 1 kitchen faucet, a bath, a dishwasher load | 106 | 103 | 98 | 92 |
| 6 people | 5 standard showers, 4 bathroom faucets, 1 kitchen faucet, a bath, a dishwasher load, a hot/warm wash | 133 | 130 | 124 | 117 |
Cold incoming water raises the numbers because blended fixtures draw a larger share from the heater. It also lowers what a tank actually delivers, because FHR is rated with 58°F water.
Family of 2
Two back-to-back morning showers, a sink or two, and the kitchen tap. That hour needs a tank with an FHR of at least 38 gallons at 50°F incoming water (39 at 40°F, 34 at 70°F). With 2.0 gpm water-saving showerheads, the 50°F figure drops to 32. One shower while someone uses the kitchen tap. For tankless, that peak needs 3.9 gpm at a 70°F rise. Open the tank example · open the tankless example
Family of 3
Three showers in the morning rush, sinks, the kitchen tap and a dishwasher load. That hour needs a tank with an FHR of at least 59 gallons at 50°F incoming water (61 at 40°F, 53 at 70°F). With 2.0 gpm water-saving showerheads, the 50°F figure drops to 49. Two showers at the same time. For tankless, that peak needs 3.9 gpm at a 70°F rise. Open the tank example · open the tankless example
Family of 4
Four showers packed into one hour, bathroom sinks, the kitchen tap and a dishwasher load. That hour needs a tank with an FHR of at least 76 gallons at 50°F incoming water (78 at 40°F, 68 at 70°F). With 2.0 gpm water-saving showerheads, the 50°F figure drops to 63. Two showers plus the kitchen tap. For tankless, that peak needs 5.8 gpm at a 70°F rise. Open the tank example · open the tankless example
Family of 5
Four showers and a child's bath in the same hour, sinks, kitchen tap and a dishwasher load. That hour needs a tank with an FHR of at least 103 gallons at 50°F incoming water (106 at 40°F, 92 at 70°F). With 2.0 gpm water-saving showerheads, the 50°F figure drops to 90. Two showers, a bathroom sink and the kitchen tap. For tankless, that peak needs 7.0 gpm at a 70°F rise. Open the tank example · open the tankless example
Family of 6
Five showers and a bath in one hour, sinks, kitchen tap, a dishwasher load and a warm wash. That hour needs a tank with an FHR of at least 130 gallons at 50°F incoming water (133 at 40°F, 117 at 70°F). With 2.0 gpm water-saving showerheads, the 50°F figure drops to 114. Three showers plus the kitchen tap. For tankless, that peak needs 7.8 gpm at a 70°F rise. Open the tank example · open the tankless example
Tankless: flow for the same households
Tankless units are sized for the worst moment, not the worst hour. These peaks assume a 120°F outlet.
| Household | Running at once | 40°F in | 50°F in | 60°F in | 70°F in |
|---|---|---|---|---|---|
| 2 people | 1 standard shower, 1 kitchen faucet | 4.0 | 3.9 | 3.7 | 3.5 |
| 3 people | 2 standard showers | 4.1 | 3.9 | 3.8 | 3.5 |
| 4 people | 2 standard showers, 1 kitchen faucet | 6.0 | 5.8 | 5.6 | 5.3 |
| 5 people | 2 standard showers, 1 bathroom faucet, 1 kitchen faucet | 7.2 | 7.0 | 6.7 | 6.3 |
| 6 people | 3 standard showers, 1 kitchen faucet | 8.0 | 7.8 | 7.5 | 7.0 |
Remember that the rise changes across those columns too: 80°F at 40°F incoming water, 50°F at 70°F. A unit that covers 5.3 gpm in a warm climate must deliver 6.0 gpm at a much bigger rise in a cold one. Whole-house tankless sizing converts these to BTU/hr and kW.
What changes the answer most
- Back-to-back showers. Each extra standard shower adds about 16 hot gallons to the busiest hour at 50°F incoming water.
- A bathtub. A 35-gallon tub fill adds more to the peak hour than a shower, and at 5 gpm it is the biggest single flow for tankless.
- Showerheads. Switching from 2.5 to 2.0 gpm heads cuts shower demand by a fifth.
- Winter water. The same household needs noticeably more in January than in July.
None of these rows is a rule. They are starting points. Put your own busiest hour into the calculator.
Household sizing questions
What size water heater does a family of 4 need?
In our worked example (four showers, three sink uses, the kitchen tap and a dishwasher load in one hour), a family of four needs a tank with a First Hour Rating of about 76 gallons with 50°F incoming water, or 78 gallons with 40°F water. For tankless, two showers plus the kitchen tap need 5.8 gpm at a 70°F rise. Your own busiest hour may be lighter or heavier.
Is a 40 gallon water heater enough for a family of 4?
Sometimes. Compare the heater's First Hour Rating, not its tank size, with your busiest hour. The 40-gallon gas models we checked rate 64–84 gallons and the 40-gallon electric models 53–55, against the 76 gallons our four-person example needs at 50°F incoming water. See 40 vs 50 gallon for the models.
How many gallons of hot water per person per day?
Daily use is not what sizes a water heater. A tank runs out in the busiest hour, so size by peak-hour demand. Daily use matters for energy cost, not for whether you run out of hot water.
What size tankless water heater for a family of 4?
Size tankless by what runs at once, not by headcount. Two showers plus the kitchen tap need 5.8 gpm at a 70°F rise, about 203,500 BTU/hr of heat into the water, and 6.0 gpm with 40°F winter water.
Sources
Water Heater Sizing Calculator for iPhone
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