Gas Tankless Water Heater Sizing Calculator
Use our calculator to determine the ideal gas tankless water heater size for your home based on simultaneous hot water usage and temperature requirements.
List of the Best Gas Tankless Water Heater:
Why Use a Gas Tankless Water Heater Sizing Calculator?
Installing a gas tankless water heater is one of the best ways to achieve energy-efficient, on-demand hot water for your home. But to experience its full benefits, proper sizing is essential. That’s where our gas tankless water heater sizing calculator comes in. This tool helps you determine the right size unit based on how much hot water you use and the temperature rise required in your region. Whether you're upgrading your current system or designing a new home, using this calculator ensures your heater delivers consistent performance — without overpaying for capacity you don't need.
How the Sizing Calculator Works
Our gas tankless water heater sizing calculator analyzes two key variables:
- Total Flow Rate (GPM): The gallons per minute you expect to use simultaneously — such as a shower and dishwasher running at the same time.
- Temperature Rise: The difference between the incoming cold water and your desired hot water temperature, typically 120°F.
The calculator uses these inputs to determine the BTU or GPM output needed from a gas tankless unit. Then, it recommends models that match or exceed that requirement.
Why Accurate Sizing is So Important
Oversizing leads to wasted fuel, higher costs, and inefficient cycling. Undersizing results in frustrating cold water moments during peak use. With gas models — especially those powered by natural gas or propane — accuracy is key to maximizing efficiency and comfort. Our gas tankless water heater sizing calculator helps prevent common sizing mistakes by providing clear, model-based recommendations.
Common Flow Rates of Household Fixtures
To calculate total GPM, add the expected flow of all fixtures that may run at the same time:
- Showerhead: 2.0–2.5 GPM
- Bathroom faucet: 0.5–1.0 GPM
- Kitchen faucet: 1.0–2.2 GPM
- Dishwasher: 1.5–2.0 GPM
- Washing machine: 2.0–2.5 GPM
- Bathtub: up to 4.0 GPM
Temperature Rise by Region
The colder your climate, the more energy your heater needs to bring water up to usable temperatures:
- Southern U.S. (e.g., Florida, Texas): Groundwater ~65°F → Temp rise = 55°F
- Midwest (e.g., Illinois, Ohio): Groundwater ~50°F → Temp rise = 70°F
- Northern U.S./Canada: Groundwater ~40°F → Temp rise = 80°F or more
Our calculator adjusts recommendations based on your inlet temperature so you don’t have to guess.
Popular Gas Tankless Water Heater Models
| Model | Max GPM | Temp Rise at Max GPM | Fuel | Application |
|---|---|---|---|---|
| Noritz NRC66DV | 6.6 | 35°F | Natural Gas | Small homes, 1-2 bathrooms |
| Rinnai RU199iN | 9.8 | 60°F | Natural Gas | Whole-home, cold climate |
| Takagi T-H3-DV-N | 10.0 | 60°F | Natural Gas | Multi-bathroom homes |
| Navien NPE-240A2 | 11.2 | 45°F | Natural Gas / Propane | High GPM demand homes |
| Bosch Greentherm C 1050 | 10.5 | 63°F | Natural Gas / Propane | Large homes, high-efficiency |
Tips to Maximize Efficiency
- Install low-flow fixtures to reduce peak GPM demand.
- Insulate hot water pipes to maintain temperature during distribution.
- Use recirculation systems to minimize water waste and wait times.
- Install the unit close to the main points of use for faster delivery.
- Have a professional install your unit to ensure safe gas and venting connections.
FAQs About Gas Tankless Water Heater Sizing
Can I oversize a gas tankless water heater?
Technically yes, but it may lead to more frequent cycling and fuel waste. Proper sizing ensures the heater operates efficiently within its optimal range.
Is gas better than electric for tankless systems?
Gas heaters typically provide higher GPM and heat water faster, making them ideal for larger households or colder climates. Electric units are more suitable for smaller homes or point-of-use setups.
Can I use propane instead of natural gas?
Yes, many gas tankless water heaters are compatible with propane. Always verify the model supports propane and ensure proper regulator and venting setup.
What happens if my heater is undersized?
You may notice temperature drops when multiple fixtures run, and the heater may operate near full capacity constantly, reducing its lifespan.
Conclusion: Make the Smart Choice With Our Sizing Calculator
Choosing the correct gas tankless water heater is about more than just buying the biggest unit — it’s about matching performance to your real needs. Our gas tankless water heater sizing calculator helps you do just that. Whether you're looking for efficiency, power, or both, this tool ensures your next heater is the right one for long-term comfort and performance.