What Size Tankless Water Heater Do I Need? (With Examples)
Size a tankless water heater in two steps: add up your flow and find your temperature rise. Includes simple formulas and examples for 1, 2 and 3 bathroom homes.
Tankless water heaters aren’t sized in gallons like tank heaters. They’re sized by how much hot water they can make at once (gallons per minute, or GPM) and how much they can heat it (the temperature rise). Get those two numbers right and the rest is simple.
Step 1: Add up your peak flow
Think about the most hot water your household uses at the same time, like a shower running while someone uses the kitchen sink. Then add up the flow of those fixtures.
| Fixture | Typical flow |
|---|---|
| Shower | 1.5 to 2.5 GPM |
| Bathroom faucet | 0.5 to 1.5 GPM |
| Kitchen faucet | 1.5 to 2.2 GPM |
| Dishwasher | 1 to 1.5 GPM |
| Washing machine | 1.5 to 3 GPM |
Check the flow rating stamped on your showerheads and faucets for exact numbers; newer fixtures often use less.
Example: two showers at 2 GPM each plus a kitchen faucet at 1.5 GPM = 5.5 GPM.
Step 2: Find your temperature rise
Temperature rise is how much the heater needs to warm the water:
Temperature rise = desired hot water temperature − incoming water temperature
Most people set hot water at about 105 to 120°F. Incoming groundwater temperature depends on where you live and the season: roughly 70°F or more in the Deep South and Southwest, around 50 to 60°F in the middle of the country, and 40°F or lower in northern states in winter.
Example: you want 115°F water and your winter inlet water is 45°F. Your rise is 70°F.
Always size for winter, when inlet water is coldest.
Step 3: Match it to a heater
Electric tankless
Electric heaters are rated in kilowatts (kW). A handy formula:
kW needed = GPM × temperature rise ÷ 6.83
Example: 4 GPM × 70°F rise ÷ 6.83 = about 41 kW, which is more than the largest common whole-house electric units. That’s why electric tankless works best in warm climates or for smaller loads.
In a warm climate with a 35°F rise, the same 4 GPM needs only about 20 kW.
See our picks for the best electric tankless water heaters.
Gas tankless
Gas heaters are rated in BTU per hour. Roughly:
BTU needed = GPM × temperature rise × 500 ÷ efficiency
Example: 5.5 GPM × 70°F × 500 = 192,500, divided by 0.95 efficiency = about 203,000 BTU. Many whole-house gas tankless units top out near 199,000 BTU, so this household would be right at the limit.
Gas tankless units also come with flow charts that show GPM at different temperature rises. Check the chart for your rise rather than the headline GPM number.
Quick sizing guide by home size
These are rough starting points for a cold-winter climate with a 70°F rise. Warmer climates need less.
| Home | Typical peak use | Peak flow | Gas | Electric |
|---|---|---|---|---|
| Apartment or 1 bathroom | 1 shower + 1 faucet | About 3 to 4 GPM | 120,000 to 160,000 BTU | 27 to 36 kW (may fall short in winter) |
| 2 bathrooms | 2 showers + 1 faucet | About 5 to 6 GPM | 180,000 to 199,000 BTU | Usually too much for electric in cold climates |
| 3 bathrooms | 2 to 3 showers + appliances | About 7 to 8 GPM | 199,000 BTU, or two units | Not practical in cold climates |
Common mistakes
- Sizing for summer. Inlet water can be 20 to 30 degrees colder in winter.
- Using the headline GPM. That number is usually at a small temperature rise.
- Ignoring your electrical panel or gas line. Large electric units often need a 200-amp panel; gas units need the right gas line size and venting.
- Forgetting simultaneous use. Size for your busiest morning, not average use.
Still deciding whether tankless is right for you? Read our honest tankless vs tank cost comparison.
Still wondering?
What size tankless water heater for a family of 4? Usually one that can deliver about 5 to 6 GPM at your winter temperature rise. In cold climates that typically means a large gas unit.
Can one tankless heater serve a whole house? Yes, if it’s sized properly. Large homes sometimes use two units together, or a tankless heater plus a small point-of-use unit at a far sink.
What happens if I undersize it? The water won’t get as hot when several fixtures run at once, or the flow will drop.