What size tankless water heater do I need?
Two simultaneous showers is 5.0 GPM. Up north, where incoming water is 50°F and the heater must lift it 70°F, that draw takes the biggest residential gas unit made, 199,000 BTU. In Florida the same showers need barely half that. Tankless sizing is those two numbers, flow and rise; set yours below.
How tankless sizing works
A tankless heater has no reserve. Every second, it must add enough heat to the water passing through to lift it from your inlet temperature to 120°F. The heat required is flow times rise: BTU per hour = 500 × GPM × °F of rise (500 is a gallon's 8.33 pounds times 60 minutes). Two showers at 5 GPM with 50°F water is 500 × 5 × 70 = 175,000 BTU per hour of delivered heat, which at a real-world 82% efficiency means a unit fed about 213,000 BTU of gas. That is past the 199,000 BTU residential ceiling, and now you know why northern plumbers squint when someone says "two showers and the dishwasher."
The trap in every brochure: units are advertised at the GPM they deliver at a 35°F rise. Nobody's winter water needs a 35°F rise to reach 120°F. Divide the advertised flow roughly in half for a northern winter and you are close to the truth.
What the biggest residential unit really delivers
One 199,000 BTU input unit at 82% efficiency, by region. Each shower is 2.5 GPM.
| Region | Winter inlet | Rise to 120°F | Real delivery |
|---|---|---|---|
| Far north (MN, ND, MT, ME, VT) | 42°F | 78°F | 4.2 GPM |
| North (NY, MA, MI, PA, WA, CO) | 50°F | 70°F | 4.7 GPM |
| Middle band (VA, KY, MO, KS, NV) | 58°F | 62°F | 5.3 GPM |
| South (GA, AL, TX, AZ, most of CA) | 66°F | 54°F | 6 GPM |
| Gulf coast and Florida | 72°F | 48°F | 6.8 GPM |
Questions people ask
What size tankless water heater does a family of 4 need?
Plan around 6.5 GPM of simultaneous draw (two showers plus the kitchen sink) at your winter temperature rise. In the south that is one 199,000 BTU unit with a little margin; in the north the same unit delivers only about 4.7 GPM at a 70°F rise, so you either keep the kitchen sink out of shower hour or install two units. The family-of-4 page has the table by region.
Why does my 9.8 GPM unit run out during two showers?
The brochure number is measured at a 35°F temperature rise, which no US home sees in winter. At a northern 70°F rise the same burner physically delivers half the flow, about 4.7 GPM, and two 2.5 GPM showers already ask for 5. The unit is fine; the marketing rise was doing the heavy lifting.
Gas or electric tankless?
For a whole house in a cold climate, gas, and it is not close. Two showers at a 70°F rise take about 51 kW of electric heating, which is more than most homes’ entire service. Electric tankless earns its keep at a single fixture, or as a whole-house unit in the Gulf south where the rise is under 50°F and a 27 to 36 kW unit can cover a couple of fixtures.
What happens if I undersize a tankless?
It protects the temperature by throttling the flow. You will not get cold water; you will get a shower that drops to a trickle when the dishwasher calls for hot. That is why sizing runs on the worst simultaneous moment, not the average one.