What size tankless water heater for a family of 2?
Size to 3.5 GPM: one shower running while someone uses a bathroom sink. At that draw, one gas unit handles it anywhere in the country; the question is only which size class. The table below runs the number against every US region's winter water.
The size by region, at 3.5 GPM
Delivered heat is 500 × 3.5 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 | 136,500 BTU/hr | 180,000 BTU |
| North (NY, MA, MI, PA, WA, CO) | 70°F | 122,500 BTU/hr | 160,000 BTU |
| Middle band (VA, KY, MO, KS, NV) | 62°F | 108,500 BTU/hr | 140,000 BTU |
| South (GA, AL, TX, AZ, most of CA) | 54°F | 94,500 BTU/hr | 140,000 BTU |
| Gulf coast and Florida | 48°F | 84,000 BTU/hr | 140,000 BTU |
This is the easy end of tankless. At 3.5 GPM the draw sits inside a single unit's range even at a far-north 78°F rise, so the shopping decision is a size class, not an architecture. Buy the class from the table, not the biggest unit on the shelf: an oversized burner short-cycles on small draws like a bathroom sink.
Our scenario is an assumption, stated plainly: one shower running while someone uses a bathroom sink is the worst moment we consider realistic for 2 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 2 need from a tankless?
About 3.5 GPM: one shower running while someone uses a bathroom sink, 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.