Tank vs tankless: which one actually fits your house?
A tank fails one way, by running out of gallons in the busiest hour. A tankless fails the other way, by being outrun in the worst instant. So the honest comparison runs both numbers for the same house: the simultaneous GPM a tankless must make against your winter rise, and the first hour rating a tank needs for the same morning. Set your numbers below and read both answers at once.
Reading both numbers
The two architectures answer two different questions, and most bad installs come from answering the wrong one. A tank is sized in gallons per hour: start full, and the first hour rating is the tested count of hot gallons it hands out in 60 minutes while the burner or element fights back. A tankless is sized in gallons per minute: it never runs dry, but it can only heat so much water as it flows past, so it caps how many fixtures run at once. A house can pass one test and fail the other, which is exactly why a single pros-and-cons list never answers "which fits me."
The instant is where the wrong numbers live. You will see a family of six quoted at 15 to 25 GPM, which is not a flow rate at all; it is a peak-hour gallons figure wearing the wrong units. The real simultaneous instant for six people is three showers at once, 7.5 GPM. And here is the part the inflated numbers skip: a tankless unit is advertised at the GPM it makes at a 35°F rise, which no US winter provides. The biggest residential gas unit, 199,000 BTU, delivers about 4.7 GPM at a northern 70°F rise and about 6.8 GPM on the Gulf coast. Same box, and the winter rise cuts it nearly in half.
Family by family, at the DOE-default 50°F inlet
Northern winter water (50°F) lifted to 120°F, a 70°F rise. The instant is the simultaneous scenario each household is most likely to hit; the hour is that many practical showers plus one shave and a round of hand dishes. Move the house south and the tankless column improves fast, because the rise shrinks.
| Household | Worst instant | Single tankless (70°F rise) | Busiest hour | Gas tank | Electric tank | Simpler pick |
|---|---|---|---|---|---|---|
| Family of 2 | 3.5 GPM | 160,000 BTU unit | 46 gal | 30 gal | 40 gal | Either |
| Family of 3 | 5 GPM | two units | 66 gal | 40 gal | 80 gal | Tank |
| Family of 4 | 6.5 GPM | two units | 66 gal | 40 gal | 80 gal | Tank |
| Family of 5 | 7 GPM | two units | 86 gal | 75 gal | none fits | Tank |
| Family of 6 | 7.5 GPM | two units | 106 gal | 75 gal | none fits | Tank |
Two things fall out of the table. Up north a single tankless only covers the smallest household's instant; three simultaneous showers is past one unit even before you count the kitchen. And the electric tank column runs out at five people, because no single residential electric tank recovers fast enough to carry that hour. Both facts point big northern households toward gas storage or two tankless units, not one big tankless.
How to decide, in order
Run your real instant and your real hour above. If both come back green, sizing has not decided anything, and the choice is fuel, venting, and the install-cost gap between a drop-in tank and a tankless that may need a bigger gas line and new venting. If only one passes, the sizing has decided for you. If neither passes, the answer is two of something, and that quote needs a plumber standing in your mechanical room. A size is where a quote starts, not where it ends.
Questions people ask
Do I need 15 to 25 GPM for a family of 6?
No, and that number is the reason this tool exists. It reads like a peak-hour gallons figure smuggled in as a flow rate. The honest simultaneous instant for six people is three showers running at once, which is 7.5 GPM at 2.5 GPM a head. Even that 7.5 GPM is past a single residential tankless at a northern winter rise, so the right move is sizing the real instant, not chasing a made-up 20.
Which is cheaper to run, tank or tankless?
That is an operating-cost question this sizing tool does not answer, because the honest number depends on your gas or electric rate, your standby losses, and how you use hot water. Sizing decides which architecture physically fits your instant and your hour. Fuel economics is a separate call, and any installer quoting you a payback should show their rate assumptions.
Why does my house pass the tank test but fail the tankless test?
Because a tank hides simultaneity in stored gallons and a tankless cannot. A 40-gallon gas tank hands out 71 gallons in the busiest hour no matter how they overlap, while a tankless has to make every gallon on the spot. Two showers at once is 5 GPM, and the biggest residential gas unit delivers only about 4.7 GPM at a northern 70°F rise. Same house, opposite answers.
Can I make a tankless fit by staggering showers?
Yes, and that is often the real fix. Tankless sizing runs on the worst instant, so moving one shower or the dishwasher out of the collision drops the GPM and can bring the draw inside a single unit. A tank forgives a bad guess with one lukewarm hour a month; an undersized tankless throttles the flow at every overlap, so the trimming has to be a habit, not a hope.