What size tankless water heater for a family of 3?
Size to 5 GPM: two showers running at once. At that draw, a single 199,000 BTU gas unit covers it in the 3 warmest regions, and northern homes need two units or a smaller simultaneous moment. The table below runs the number against every US region's winter water.
The size by region, at 5 GPM
Delivered heat is 500 × 5 GPM × the rise from your winter inlet water to 120°F. Gas classes assume 82% efficiency; "two units" means the draw is past a single 199,000 BTU residential unit.
| Region | Winter rise | Heat needed | Gas unit (input) |
|---|---|---|---|
| Far north (MN, ND, MT, ME, VT) | 78°F | 195,000 BTU/hr | two units |
| North (NY, MA, MI, PA, WA, CO) | 70°F | 175,000 BTU/hr | two units |
| Middle band (VA, KY, MO, KS, NV) | 62°F | 155,000 BTU/hr | 199,000 BTU |
| South (GA, AL, TX, AZ, most of CA) | 54°F | 135,000 BTU/hr | 180,000 BTU |
| Gulf coast and Florida | 48°F | 120,000 BTU/hr | 160,000 BTU |
Here is the honest tension in this household size. The scenario draw of 5 GPM needs 175,000 BTU/hr of delivered heat at the DOE-default 70°F rise, and the biggest residential unit delivers about 163,180 BTU/hr. The fix is rarely a second unit, though that works. It is usually behavioral: the third shower moves fifteen minutes, and the draw drops inside one unit's range. Decide whether your household will actually do that before you buy the architecture that assumes it.
Our scenario is an assumption, stated plainly: two showers running at once is the worst moment we consider realistic for 3 people. If your household's collisions are bigger (a garden tub plus two showers, say), run the calculator with your own moment; a tub filler alone is 4 GPM.
Five GPM is exactly where the residential catalogue ends
Two showers at once is the number the entire tankless industry sizes around, and a family of three is the smallest household that reliably produces it with no appliance involved. It is also, to within a rounding error, the draw that finishes off the biggest unit made. At the DOE-default 70°F rise those two showers ask for 175,000 BTU/hr of delivered heat, and 199,000 BTU of gas at 82% efficiency delivers about 163,180. The table above is that one fact restated per region.
So three people in a cold climate is the smallest household in America that can outrun the whole residential catalogue on a single unit, which lands badly on anyone who assumed the ceiling was a problem for big houses. The same two showers in the middle band and the south fit one unit with margin. Geography is doing all of the work here, not the household.
If you want one unit and you live north, the lever is the second shower rather than the equipment. Fifteen minutes of stagger takes the draw to 2.5 GPM, which a mid-size unit covers anywhere in the country, in January.
Questions people ask
How many GPM does a family of 3 need from a tankless?
About 5 GPM: two showers running at once, at 2.5 GPM per shower head. If your peaks are calmer than that, size to your real worst moment instead; the calculator takes any combination.
Does the brochure GPM rating apply to my house?
Only if your winter inlet water is 85°F, because ratings are quoted at a 35°F rise. Real winter inlets run 42 to 72°F depending on region, so real capacity is roughly half to three-quarters of the brochure number.