Reference data
Water heater sizing
Tank size and first hour rating by household, and the flow a tankless unit delivers at each climate's temperature rise.
Written and edited by Pablo Ruiz Quintero
Last reviewed 2026-09-16
What size water heater do I need for my household?
For a tank, whatever covers your busiest hour — which is not the same as the capacity on the label. For a tankless, whatever delivers your peak flow at your temperature rise, which is not the flow on the box.
Tank size by household
| People | Hot water a day | Gas tank | Its FHR | Electric tank | Its FHR |
|---|---|---|---|---|---|
| 1 | 20 gal | 30 gal | 45 gal | 30 gal | 38 gal |
| 2 | 36 gal | 30 gal | 45 gal | 30 gal | 38 gal |
| 3 | 50 gal | 30 gal | 45 gal | 30 gal | 38 gal |
| 4 | 64 gal | 30 gal | 45 gal | 30 gal | 38 gal |
| 5 | 76 gal | 30 gal | 45 gal | 30 gal | 38 gal |
| 6 | 88 gal | 30 gal | 45 gal | 30 gal | 38 gal |
Fixture flow rates
| Fixture | Gallons per minute | Note |
|---|---|---|
| Shower | 2.1 | A current low-flow head. An older one can be 2.5 or more. |
| Bath tub filling | 4 | The heaviest single draw in a house, and the one that empties a tank. |
| Kitchen sink | 1.5 | Higher than a bathroom tap, and often running while someone showers. |
| Bathroom sink | 1 | Small, but they add up in a house full of people getting ready. |
| Dishwasher | 1.5 | Most modern machines heat their own water, which takes them out of the peak. |
| Washing machine | 2 | Only if it is run on a hot cycle, which is increasingly rare. |
What a tankless unit really delivers
| Zone | Water in | Rise | 120k BTU/h | 150k BTU/h | 199k BTU/h |
|---|---|---|---|---|---|
| Zone 1 | 75°F | 45°F | 4.5 | 5.7 | 7.5 |
| Zone 2 | 70°F | 50°F | 4.1 | 5.1 | 6.8 |
| Zone 3 | 62°F | 58°F | 3.5 | 4.4 | 5.8 |
| Zone 4 | 55°F | 65°F | 3.1 | 3.9 | 5.2 |
| Zone 5 | 50°F | 70°F | 2.9 | 3.6 | 4.8 |
| Zone 6 | 45°F | 75°F | 2.7 | 3.4 | 4.5 |
| Zone 7 | 42°F | 78°F | 2.6 | 3.3 | 4.3 |
| Zone 8 | 38°F | 82°F | 2.5 | 3.1 | 4.1 |
Reading the third table
Every column is the same machine. A 199,000 BTU/h unit — the largest residential size — delivers 7.5 gallons a minute where the water arrives at 75°F and 4.1 where it arrives at 38. That is not a fault; it is arithmetic. The burner can add a fixed amount of heat per hour, and colder water needs more of it per gallon.
One shower and a kitchen sink together is about 3.6 gallons a minute. Read across the bottom rows and you can see how quickly the comfortable margin disappears in a cold climate — which is why a tankless unit that was fine in September goes cold in January.
Why first hour rating and not gallons
A storage tank is refilling while you draw from it, so what you get in a busy hour is the capacity plus the recovery. A gas burner adds heat roughly twice as fast as a pair of electric elements, which is why the same 50 gallons is worth about 75 in gas and 62 in electric.
That difference is the reason swapping a gas heater for an electric one of the same capacity leaves a household with noticeably less hot water than it had.
Using it
The water heater calculator applies all three tables at once: your household, your fixtures, your climate — and then prices every type over its whole life, including the installation extras that make a tankless quote differ so much from a tank one.
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About the author
Pablo Ruiz Quintero — Editor
Editor of MaterialsEstimator. Decides which documents count as a source, what every default value is, and where the line sits between what the reader parses and what the engines compute.
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