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.
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.