TanklesslyEndless hot water, sized to your fixtures.

Planner

Tankless water heater size planner

Your fixtures and your winter water in; the flow your heater must hold at that rise out, with every gas and electric unit we cover checked against it.

WaterSense showerheads use 2.0 GPM or less; standard heads 2.5. The GPM is printed on the head or in its listing.
Measure the cold water at a tap in winter, or read your zone off a maker's ground-water map: zones run from about 35°F in the far north to 77°F in the far south.
120°F is the usual factory setting.

How the math works

Tanklessly mathFlow you need = showers × shower flow + bathroom sinks × 1 GPM + kitchen sinks × 1.5 GPM + dishwasher 2 GPM + washing machine 2 GPM (Noritz's tankless sizing sheet). Temperature rise = set temperature − winter inlet water.
Tanklessly mathA gas heater holds about input BTU/h × efficiency ÷ (500 × rise) GPM: 0.95 for condensing units and 0.77 for non-condensing ones. With 0.95, a 199,000 BTU unit comes out at 4.4 GPM with 35°F water and 8.8 GPM with 77°F water, the same figures Rinnai publishes for its 199,000 BTU models. An electric unit holds kW × 3,412 ÷ (500 × rise) GPM, which matches Stiebel Eltron's DHX flow table. No estimate is allowed above the maker's stated maximum flow.
WinterSize for winter. Inlet water is coldest in January, and the same heater that runs three showers in August may run two. Noritz's sizing sheet puts it plainly: slightly oversizing a tankless system does little harm, undersizing leaves you short of hot water.
LimitsThese are estimates from each unit's input rating. Before buying, check the exact model's flow-versus-rise chart in its spec sheet, and the electrical or gas requirement.

Questions

What size tankless water heater do I need for a family of four?
It depends on what runs at once, not on head count. Two 2 GPM showers and a dishwasher need 6 GPM; with 47°F winter water and a 120°F set point that's a 73°F rise. A 199,000 BTU condensing gas unit holds about 5.2 GPM at that rise, so it covers two showers but not the dishwasher at the same time.
Can an electric tankless heater run a whole house?
In a mild climate, often yes. At a 50°F rise a 27 kW unit holds about 3.7 GPM, enough for a shower and a sink. At a 70°F winter rise it drops to about 2.6 GPM, barely one shower, and it draws about 112 amps at 240 volts.
Why does my tankless heater run lukewarm when two showers are on?
The combined flow is above what the unit can lift to your set point at today's inlet temperature. In winter the rise is larger, so the same flow gets cooler. Fewer fixtures at once, low-flow heads or a larger unit fix it.