Tank vs Tankless Sizing
Updated July 2026
Tanks are sized to the busiest hour in gallons; tankless to the worst instant in GPM. Households that fail one test often pass the other. How to run both.
The two architectures fail in opposite ways, so they are sized on opposite questions. A tank fails by running out: it holds a finite number of hot gallons, so the test is the busiest hour, in gallons. A tankless fails by being outrun: it makes hot water forever but only so fast, so the test is the worst instant, in gallons per minute. Neither number predicts the other, and most bad installs come from answering the wrong question.
The same family, both tests
Take four people, standard heads. Tank test: three showers plus sink use in the peak hour is 66 gallons, which a 40-gallon gas tank covers (FHR about 71) and an 80-gallon electric covers with less grace. Tankless test: two showers plus the kitchen sink running at once is 6.5 GPM, which at a northern 70°F rise wants 227,500 BTU per hour of delivered heat, more than one 199,000 BTU unit provides. Same family: passes the tank test easily, fails the single-tankless test in a cold state. A Texas copy of this family passes both.
The reverse case exists too. Seven people who shower in relays never stress a moment (one shower, 2.5 GPM, trivial for a mid-size tankless) but pull 130-plus gallons across the hour, which no single electric tank survives. Relay households are what tankless was invented for.
How to decide, in order
Run the tank calculator on your real busiest hour, then the tankless calculator on your real worst moment. If both come back green, the decision is about fuel, venting, and the $1,000-plus install-cost gap, none of which is a sizing question. If only one passes, the sizing has decided for you. If neither passes, the answer is two of something, and that quote needs a person in your basement.
One asymmetry worth knowing: a tank forgives a bad guess with a lukewarm hour once a month; an undersized tankless reminds you at every collision, because it throttles flow to hold temperature. Undersizing hurts more on the tankless side. Size the moment honestly.