Size a tankless water heater using peak hot-water flow in GPM and the temperature rise (outlet minus coldest incoming water temperature).
For the full breakdown, see our best Tankless Water Heaters guide.
Figuring out how to size tankless water heaters calculator starts with two numbers, not a five-page manual. Find your peak hot-water flow, find your temperature rise, and any sizing tool takes it from there. Get the two numbers right and the heater will keep up on the coldest morning of the year; get them wrong and you’ll be standing in a cold shower in January.
Two Numbers Decide Every Tankless Sizing Decision
Peak simultaneous hot-water flow is the total gallons per minute (GPM) from every fixture and appliance that could run at the same time. For a typical family home, that means two showers running with a dishwasher or washing machine. Lochinvar’s tankless sizing sheet says to list the faucets and appliances or measure the flow directly rather than guessing at it.
Temperature rise is the second number, usually written as ΔT. It’s your desired outlet temperature minus the coldest expected incoming groundwater temperature. Most U.S. sizing guides use a 120°F delivery target, and you should size to the coldest month of the year, not the annual average groundwater temperature. That matters most in northern states where the winter supply can sit well below 50°F.
What Size Tankless Water Heater Do I Need?
The size you need is the model whose flow-versus-rise performance meets or beats your peak flow at your required temperature rise. Capacity charts list that as BTU/hr for gas heaters and kilowatts or amps for electric heaters.
For gas heaters, the core sizing formula is BTU/hr = GPM × 500 × ΔT. A home that needs 5 GPM at a 60°F rise calls for 5 × 500 × 60 = 150,000 BTU/hr. Electric sizing uses the same flow and rise but converts the result to kilowatts, and the calculator output usually includes amps because the electrical load is often the real limit.
| Scenario | Peak Flow + Rise | Minimum Gas Size |
|---|---|---|
| Small house or apartment | 3.0 GPM at 45°F rise | ≈68,000 BTU/hr |
| Two-bath family home | 4.5 GPM at 55°F rise | ≈124,000 BTU/hr |
| Three-bath home with dishwasher | 6.5 GPM at 60°F rise | ≈195,000 BTU/hr |
| Large home in a cold climate | 8.0 GPM at 70°F rise | ≈280,000 BTU/hr |
Once you know the minimum capacity, compare actual models before buying. If you would rather let a tool do the math, Navien’s tankless sizing guide walks through the same inputs and applies them to its product line.
Electric models have one extra catch that is often missed: sizing guides normally assume 240V service. If the home has 208V service, heating power drops and the guide recommends choosing the next-larger model. Gas models need attention on gas-line capacity and venting, so check those notes in the sizing results before you spec the unit.
Sizing Mistakes That Lead To Cold Showers
Most sizing errors come from using the wrong incoming water temperature, sizing by household size alone, or ignoring simultaneous flow. Avoid these:
- Using the annual average groundwater temperature instead of the coldest-month temperature. That understates the rise the heater has to overcome.
- Sizing by bedrooms or square footage. The fixtures that run at the same time decide the GPM, and fixture flow rates vary by model and efficiency.
- Choosing an electric unit without checking the available voltage. 208V service needs a larger model than 240V service.
- Assuming a tankless unit stores hot water. It only heats on demand, so peak simultaneous flow really is the peak it has to cover.
- Forcing one unit to handle a demand it cannot meet. Two units in parallel, or simply staggering when fixtures run, fixes a high-peak home.
FAQs
What incoming water temperature should I use?
Use the coldest-groundwater temperature your location sees in winter, not the average for the year. In northern states that can be 40–50°F; in warmer climates it may be closer to 60–70°F. Sizing tools with regional presets handle this automatically when you enter your location or ZIP code.
Can I install two tankless water heaters in parallel?
Yes. When peak demand exceeds what a single unit can deliver at the required rise, two units in parallel are a common solution. The setup requires proper gas and vent planning, and it provides some redundancy if one unit needs service, so it is usually best coordinated by a licensed installer.
Is a bigger tankless water heater always better?
No. Oversizing adds cost and can cause problems at low flow, since many models need a minimum flow rate to turn on. Match the heater to your calculated peak flow and temperature rise instead of buying the largest model in the lineup.
References & Sources
- Navien. “Sizing Tankless Water Heaters.” Explains the two sizing inputs and manufacturer-specific sizing logic.
- Lochinvar. “Tankless Sizing Sheet.” Detail on listing fixtures or measuring flow for the GPM side of sizing.
