What size tankless water heater for a family of 4?
Size to 6.5 GPM: two showers running while the kitchen sink is on. At that draw, a single 199,000 BTU gas unit covers it in the warmest region, 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 6.5 GPM
Delivered heat is 500 × 6.5 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 | 253,500 BTU/hr | two units |
| North (NY, MA, MI, PA, WA, CO) | 70°F | 227,500 BTU/hr | two units |
| Middle band (VA, KY, MO, KS, NV) | 62°F | 201,500 BTU/hr | two units |
| South (GA, AL, TX, AZ, most of CA) | 54°F | 175,500 BTU/hr | two units |
| Gulf coast and Florida | 48°F | 156,000 BTU/hr | 199,000 BTU |
Here is the honest tension in this household size. The scenario draw of 6.5 GPM needs 227,500 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 non-shower fixture in the scenario 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 while the kitchen sink is on is the worst moment we consider realistic for 4 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.
The kitchen tap is the most expensive fixture on this page
What separates this page from the family-of-three page is one kitchen faucet at 1.5 GPM, and it costs more than anything else you could add. At 6.5 GPM a single 199,000 BTU unit covers the Gulf coast and nowhere else. Keep the kitchen sink out of shower hour and the same house at 5 GPM sits inside one unit through the middle band as well. Two climate zones, bought back by not rinsing a pan at 7:40.
That is worth knowing before a quote arrives, because the installer's honest answer at 6.5 GPM is two units manifolded, and two units is about twice the equipment plus gas line and venting sized for both. Ask what the proposal looks like at 5 GPM first, then decide whether your household will actually live that way.
Dishwashers cause more confusion at this household size than anything else. A dishwasher does draw hot water on the fill, so it belongs in a simultaneous moment if it runs during one. It fills for about two minutes, and most current machines raise the temperature themselves afterwards. Run it after everyone is out of the shower and it never enters this arithmetic at all.
Questions people ask
How many GPM does a family of 4 need from a tankless?
About 6.5 GPM: two showers running while the kitchen sink is on, 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.