Gas Tankless Water Heater Sizing Calculator
Determine the ideal gas tankless water heater size for your home by entering your fixture usage and climate conditions.
List of the Best Electric Tankless Water Heater:
Gas Tankless Water Heater Sizing Calculator: Find the Right Fit for Your Home
Our gas tankless water heater sizing calculator makes it easy to select the right unit for your household’s hot water needs. By calculating flow rate (GPM) and temperature rise, this tool ensures accurate sizing whether you’re installing a new system or upgrading an old tank heater. Proper sizing avoids cold water surprises and maximizes performance and energy efficiency.
Why Choose a Gas Tankless Water Heater?
Gas tankless water heaters provide a virtually endless supply of hot water on demand. They heat water only when needed, which saves energy compared to traditional tank models. Natural gas and propane models offer higher flow rates than electric ones, making them ideal for medium to large households or homes in colder climates.
How the Gas Tankless Water Heater Sizing Calculator Works
This calculator estimates the total hot water flow you need at one time (in GPM) and combines that with the required temperature rise to suggest an appropriately sized gas unit. It considers:
- Number of bathrooms
- Number of fixtures used simultaneously
- Incoming water temperature
- Desired output water temperature (typically 120°F)
Understanding Flow Rate (GPM)
Flow rate is the total volume of water you’ll use at once. Each appliance or fixture has its own flow rate, measured in gallons per minute:
| Fixture | Flow Rate (GPM) |
|---|---|
| Shower | 2.0–2.5 |
| Bathtub | 4.0 |
| Bathroom Sink | 0.5–1.0 |
| Kitchen Sink | 1.5 |
| Dishwasher | 1.5 |
| Washing Machine | 2.0–2.5 |
For example, if you're using a shower (2.5 GPM) and a washing machine (2.5 GPM) at the same time, your system must support 5 GPM.
What is Temperature Rise?
Temperature rise is the difference between the incoming cold water temperature and your desired hot water temperature. Most households set their output at 120°F. If your incoming temperature is 50°F, your heater needs to raise the water temperature by 70°F.
Typical Incoming Water Temperatures (U.S. Regions)
- Northern States: 40–50°F
- Midwestern States: 45–55°F
- Southern States: 60–70°F
Gas Tankless Heater Sizing Guidelines
Use this table as a reference when selecting your gas tankless water heater based on total flow rate and temperature rise:
| Flow Rate | Temperature Rise | Recommended Unit |
|---|---|---|
| 5 GPM | 70°F | 6–7 GPM Gas Heater (e.g., Rinnai V65iN) |
| 7 GPM | 70°F | 8–9 GPM Heater (e.g., Rheem RTGH-95DVLN) |
| 9+ GPM | 70°F | 11 GPM or dual-unit system (e.g., Rinnai RU199iN) |
Top-Rated Gas Tankless Water Heater Brands
- Rinnai: Industry-leading innovation, up to 11 GPM, Wi-Fi control, recirculation ready
- Rheem: Affordable high-performance models with indoor and outdoor options
- Noritz: Commercial-grade durability, ENERGY STAR® efficiency
- Takagi: Compact, reliable, and budget-friendly units
Frequently Asked Questions
Can one gas tankless heater support a whole house?
Yes, but it must be properly sized. For 2–3 bathrooms and multiple fixtures, an 8–11 GPM model is ideal. In very large homes, consider installing two units in parallel.
What size gas tankless water heater do I need for a family of 4?
Most families of four need a 7–9 GPM heater, depending on usage and climate. Use this gas tankless water heater sizing calculator for exact numbers.
Do gas tankless water heaters work in cold climates?
Yes, but you’ll need a more powerful unit to handle the larger temperature rise. Select a model with a high GPM rating at 70–80°F rise.
Should I oversize my tankless water heater?
A small buffer is good for future demand or larger families, but oversizing too much may reduce efficiency. It’s best to size based on realistic peak usage.
Is propane as effective as natural gas for tankless heaters?
Yes, propane units offer the same performance as natural gas models but are better for rural or off-grid homes. Always check if your selected model supports propane (LPG).
Installation Tips for Gas Tankless Units
- Install near your main water line or highest-demand zone
- Ensure proper venting (especially with condensing models)
- Use appropriate gas pipe sizing (consult manufacturer specs)
- Install a sediment filter or flush valve for long-term maintenance
Conclusion: Use This Calculator to Choose the Best Gas Tankless Heater
This gas tankless water heater sizing calculator gives you accurate recommendations based on your home's real-world demand. Whether you're upgrading from a tank system or installing in new construction, choosing the right size ensures endless hot water, better energy efficiency, and long-term satisfaction. Bookmark this tool and refer to it any time you or your customers need fast, reliable gas tankless water heater sizing.