What Size Water Heater Do I Need?
The rule of thumb — “a family of four needs a 50-gallon tank” — is a decent starting guess and a poor final answer. The right size depends on your busiest hour, not your household headcount, because two showers running while the dishwasher fills is what actually empties a tank.
Size by the busiest hour, not the tank label
A storage water heater has two capacities that matter. The tank volume is how much hot water it holds; the first-hour rating (FHR) is how much it can deliver in one busy hour — the stored water plus what it reheats while you draw. FHR, not raw gallons, is what you match to your demand, because a well-fired 40-gallon tank can out-deliver a weak 50-gallon one in a peak hour.
Estimate your peak-hour demand
Add up the hot water your household draws in its single busiest hour — typically the morning rush. The water heater size calculator uses the standard approach:
peak-hour demand = showers × gallons per shower + shaving + kitchen use
Worked example. Two showers at 20 gallons each is 40 gallons, plus 2 gallons for shaving and 4 for the kitchen — a peak-hour demand of about 46 gallons. You then choose a unit whose first-hour rating is at least 46, which typically points to a 50-gallon tank. Notice the driver was two simultaneous showers, not the number of people in the house; a household of five that never overlaps its draws can need a smaller unit than a household of three that all leaves at 7 a.m.
Recovery rate: how fast it refills
Once the peak passes, the burner reheats the tank. Recovery rate is how many gallons per hour it can raise to temperature, and it depends on burner input, efficiency and the temperature rise. Gas units recover far faster than electric ones of the same size, which is why an electric tank often needs to be physically larger to serve the same household. Check yours with the recovery rate calculator:
gas GPH = BTU/hr × efficiency ÷ (8.34 × temperature rise)
Worked example. A 40,000 BTU/hr gas burner at 80% efficiency, raising water 70°F, recovers 40,000 × 0.80 ÷ (8.34 × 70) ≈ 54.8 gallons per hour. Add that recovery to the stored volume and you have the first-hour rating — the number that really has to clear your 46-gallon peak.
Rough starting points by household
- 1–2 people: often a 30–40 gallon tank (or a well-sized tankless).
- 3–4 people: commonly 40–50 gallons, depending on overlap and whether it is gas or electric.
- 5+ people or multiple simultaneous showers: 50–80 gallons, or a properly sized tankless.
Treat these as a sanity check on the calculated demand, not a substitute for it.
Don’t oversize by default
Bigger is not automatically better. An oversized tank costs more to buy, takes more space, and wastes energy keeping water you never use hot — standby loss runs 24 hours a day. Size to your real peak plus a sensible margin, not to the largest tank that fits. If your peak is driven by rare events (a full house at the holidays), it may be cheaper to stagger showers than to buy a tank sized for the once-a-year crush.
Tankless is sized differently
If you are leaning tankless, none of the gallon math applies — a tankless unit has no storage, so you size it by the flow rate it must serve and the temperature rise it must achieve at once. That is a separate calculation covered in how to size a tankless water heater and the tankless sizing tool.
Fuel type changes the sizing
Gas and electric tanks of the same nominal volume do not serve the same household, and the reason is recovery. A gas burner reheats water several times faster than a pair of electric elements, so a gas 40-gallon tank has a much higher first-hour rating than an electric 40-gallon tank. That is why the same family that is comfortable on a 40-gallon gas unit often needs a 50-gallon (or larger) electric one — the electric tank leans harder on stored volume because it cannot refill quickly during the peak. When you compare quotes across fuel types, compare first-hour ratings, not tank gallons, and use the recovery rate calculator to see the gap. Heat-pump (hybrid) electric tanks change the picture again: they recover slowly but cost far less to run, so they trade first-hour rating for efficiency — check the payoff with the heat-pump savings calculator.
Signs your current heater is undersized
If you are replacing a unit, the old one has already told you whether it was big enough. The classic symptom of an undersized heater is running out of hot water during the morning rush — the shower goes lukewarm after the second person, or a shower and the dishwasher together drain it. That is a first-hour-rating shortfall, not a broken heater, and simply buying the same size again repeats the problem. Before you re-order, total your real peak-hour demand in the tank size calculator and compare it against the outgoing unit’s rating: if your household grew, added a bathroom, or installed a big soaking tub since the last heater, size up. Conversely, if you never run short and the house has fewer occupants than before, you may be paying to keep more water hot than you use — a chance to size down and cut standby loss.
Confirm before you buy
These are planning estimates. Fixture use varies, incoming water temperature swings with the season, and a water-heater swap usually needs a permit and a licensed installer — gas, venting, the temperature-and-pressure valve and seismic strapping are safety-critical. Size it here, then confirm the install with a professional.