Tank sizing tables are everywhere and they all say roughly the same thing. They are a reasonable starting point and they leave out the three things that decide the answer.
The usual table
| Household | Gas | Electric |
|---|---|---|
| 1–2 people | 30–40 gal | 40–50 gal |
| 2–3 people | 40 gal | 50 gal |
| 3–4 people | 40–50 gal | 50–60 gal |
| 5+ people | 50–75 gal | 66–80 gal |
Electric is a size larger throughout, because it recovers at roughly half the rate of gas.
What the table leaves out
Whether the demand coincides. Two people showering simultaneously is a bigger peak than four showering across two hours. This is the single largest factor and no table can know it.
Incoming water temperature. A house in the northern tier has a much larger temperature rise in winter, which cuts the recovery rate and therefore the hourly capacity. The same table gives an answer that is adequate in July and short in January.
Fixture flow rates. A high-flow rain shower head uses twice what a low-flow one does. Two of those change the calculation entirely.
A soaking tub, which can empty a 50-gallon tank in one fill.
A whirlpool or a large tub, which frequently requires a heater sized specifically for it.
The better approach
Estimate the peak hour in gallons of hot water, from the household's actual routine:
Shower 10–20 gallons. Bath 20. Dishwasher 6. Clothes washer 7–15 on warm. Kitchen sink 4. Basin uses 1–2 each.
Add up what genuinely happens in the busiest hour, then choose a heater whose first hour rating meets it. The FHR is on the EnergyGuide label and it already combines the stored volume with an hour of recovery, which is exactly the question being asked.
When the answer is too big
Households sometimes find the calculation points at a tank larger than will fit, or larger than is sold.
Three responses, in order of cost:
Stagger the demand. Move the dishwasher and clothes washer out of the peak hour.
Reduce the flow. Low-flow shower heads, which cut the largest component directly.
Store hotter and mix down. A tank at 140°F with a thermostatic mixing valve at the outlet delivers more usable hot water from the same volume — often the difference between adequate and not, for the price of a fitting.
Two heaters, in series or serving different parts of the house, which is normal in large houses.
Do not oversize by default
A larger tank costs more, occupies more space and loses more heat standing idle.
The standby loss is modest on a modern tank and it is continuous, and for a heat pump unit a larger tank also means a longer reheat in heat pump mode.
Size to the peak with a modest margin rather than buying the biggest that fits.
Revisit it when the household changes
The tank installed for a family of five is oversized for the two people who remain, and the tank sized for a couple is short when children arrive.
A replacement is the moment to redo the arithmetic rather than matching what is there — which, as with most appliances, is how the previous occupant's requirement gets carried forward for another decade.
Physical fit
The tank has to get into the space, which for a basement with a tight stairway or a closet with a narrow door is a real constraint.
Manufacturers publish height and diameter, and short or lowboy tanks exist for crawl spaces and under-counter installations at the same capacity in a wider, shorter form.
Clearances also matter: a gas heater needs combustion air and service access, and a heat pump unit needs a specified air volume around it.
Replacing like for like
The tank standing there is evidence of what a previous owner or builder chose, not of what the household needs.
Where hot water has never run short, matching the size is reasonable. Where it has, matching it guarantees the complaint continues — and the first hour rating of a same-size replacement may differ from the original anyway, since efficiency standards changed insulation thicknesses and usable volumes.