Tankless Water Heater Size Calculator
Use this tankless water heater size calculator to estimate required GPM, temperature rise, gas BTU/h, electric kW, simultaneous fixture demand and whether your home needs small, medium or large whole-house capacity.
The correct tankless size is not based only on bathrooms. It depends on how many hot-water fixtures run at the same time and how cold your incoming water is.
Quick answer: what size tankless water heater do I need?
To size a tankless water heater, add the hot-water flow rate of fixtures that may run at the same time. Then calculate temperature rise by subtracting incoming water temperature from target hot water temperature.
Example: if two showers and one sink may run together at 5.5 GPM, and your incoming water is 50°F while your target is 120°F, you need a tankless unit rated for about 5.5 GPM at a 70°F temperature rise.
Tankless Water Heater Size Calculator
Select the hot-water fixtures that may run at the same time. Then choose incoming water temperature and target output temperature.
Formula notes: BTU/h ≈ GPM × temperature rise × 500 ÷ efficiency. kW ≈ BTU/h ÷ 3,412.
Sizing attention score
Review required: Compare with manufacturer chart at your temperature rise.
How tankless sizing works
Tankless water heaters heat water as it flows. That means size depends on the flow you need right now and how much the heater must raise water temperature.
| Sizing factor | Meaning | Why it matters |
|---|---|---|
| GPM | Gallons per minute of hot water demand | Two showers and a sink need more capacity than one sink. |
| Temperature rise | Target hot water minus incoming water temperature | Cold water requires more heat and lowers available GPM. |
| Fuel type | Gas, propane or electric | Gas often supports higher whole-house GPM; electric may need major electrical capacity. |
| Fixture pattern | What runs at the same time | Size for realistic peak use, not every fixture in the house running forever. |
| Manufacturer chart | Model output at each temperature rise | The same unit may deliver different GPM in Florida than in Minnesota. |
Fixture GPM guide for tankless sizing
Use actual fixture ratings when possible. These are planning estimates only.
| Fixture | Typical planning GPM | Sizing tip |
|---|---|---|
| Standard shower | 2.0–2.5 GPM | Use the showerhead label if available. |
| Low-flow shower | 1.5–2.0 GPM | Good for reducing required tankless size. |
| Bathroom sink | 0.5–1.0 GPM | Usually not the main sizing driver. |
| Kitchen sink | 1.5–2.2 GPM | Can matter if used during shower time. |
| Tub fill | 4–8+ GPM | Large tubs often need bigger units or slower filling. |
| Dishwasher | Varies | Many modern dishwashers heat water internally, but plumbing demand can still matter. |
| Washing machine | Varies | Hot wash cycles can overlap with showers in real homes. |
Tankless water heater size examples
About 3 GPM demand. In a warm climate, a smaller tankless may work. In a cold climate, temperature rise may still require a stronger unit.
Often around 5 to 6 GPM before margin. This is a common whole-house sizing scenario.
May require 8 to 11+ GPM, multiple units, recirculation planning or usage scheduling.
Installation checks before buying
A tankless water heater is not just a box swap. The right model also depends on utility service and code requirements.
| Check | Why it matters | Ask before buying |
|---|---|---|
| Gas line capacity | Large gas tankless units can need high BTU input. | Can my gas line and meter support this model? |
| Venting | Tankless units may need special vent materials and clearances. | What vent type and length does the manual require? |
| Electrical service | Whole-house electric tankless units can require large amperage. | Does my panel have enough capacity? |
| Minimum flow | Some units will not fire below minimum flow. | Will low-flow fixtures activate the heater? |
| Water hardness | Hard water can scale heat exchangers and reduce performance. | Do I need treatment or scheduled descaling? |
| Recirculation | Long pipe runs can delay hot water even with a tankless unit. | Do I need a recirculation pump or built-in recirculation model? |
Trusted sizing references
Use these references to verify tankless sizing basics and manufacturer-specific details.
Energy.gov: Sizing a New Water Heater Rinnai: Tankless Water Heater FAQs Rinnai: Tankless Solution Selector Rheem: Residential Tankless Water Heaters Bradley: US Groundwater Temperature MapTankless Water Heater Size Calculator FAQs
How do I size a tankless water heater?
Add the hot-water GPM of fixtures that may run together. Then calculate temperature rise by subtracting incoming water temperature from target hot water temperature. Compare the required GPM at that rise with the model chart.
What is temperature rise?
Temperature rise is the number of degrees the heater must raise incoming water. Example: 120°F target water minus 50°F incoming water equals 70°F temperature rise.
How many GPM do I need for two showers?
Two standard showers often need about 4 to 5 GPM together. Add sinks, tub fill, dishwasher or washer if they may run at the same time.
Can an electric tankless run a whole house?
Sometimes, especially in warm climates or smaller homes. But whole-house electric tankless units can need major electrical capacity, so panel and circuit requirements must be checked.
Is gas tankless better for large homes?
Gas tankless units often handle higher whole-house flow at large temperature rise. Large homes may still need a high-output model, multiple units, recirculation or usage planning.
Why does cold groundwater reduce GPM?
Colder inlet water requires more heat to reach the target temperature. The higher the temperature rise, the lower the available hot-water flow for many tankless units.
What target hot water temperature should I use?
Many homes use around 120°F, but local code, scald protection, dishwasher needs and manufacturer instructions may affect the target.
Can this calculator replace manufacturer sizing?
No. Use this calculator for planning, then verify with the exact model performance chart, installer review and local code requirements.
Last editorial check: June 2026. Model ratings, flow rates, efficiency, activation flow and installation requirements vary by manufacturer and local conditions.
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