Choosing the right size tankless water heater is the most important step for non-stop comfort and energy savings. Go too small, and you’ll run out of hot water with every extra shower or load of laundry. Go too big, and you’ll waste money up front. This guide will show you exactly how to size a tankless water heater for your family, with simple charts, examples, and top product recommendations for any household.
Quick Picks: Best Tankless Water Heaters by Size
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Best for 1-2 Bathrooms:
EcoSmart ECO 27 Electric Tankless Water Heater – Great for smaller homes or apartments, up to 6.5 GPM in warm climates. -
Best for 3-4 Bathrooms:
Rinnai V94iN Natural Gas Tankless Water Heater – Reliable, high-flow, and ideal for most families. -
Best for 5+ Bathrooms:
Navien NPE-240A2 Condensing Tankless Water Heater – Massive 11.2 GPM output for even the busiest homes.
How to Size a Tankless Water Heater (Step-by-Step)
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Count Simultaneous Hot Water Uses:
How many showers, sinks, appliances, or tubs will run at the same time during peak use? -
Add Up GPM Requirements:
Use this quick reference:- Shower: 2.0–2.5 GPM
- Kitchen sink: 1.5 GPM
- Dishwasher: 1.5 GPM
- Washing machine: 2.0 GPM
- Bathtub: 4.0+ GPM
Example: Two showers (2.0 GPM each) + one sink (1.5 GPM) = 5.5 GPM needed at once.
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Check Your Incoming Water Temperature:
Colder climates mean a bigger heater is needed for the same GPM, since the unit must heat water more.- South/Southwest: 70°F incoming = best GPM
- North/Northeast: 40°F incoming = lower GPM at same heater size
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Pick a Unit with Enough GPM at Your Temp Rise:
Look for the “GPM @ temp rise” spec on your model (for example, 7.5 GPM @ 45°F rise).
Rule of thumb: Most families need 6–8 GPM for 2–3 baths, and 9–11+ GPM for 4 or more baths or big bathtubs.
Tankless Water Heater Sizing Chart (By Region & Bathrooms)
| Bathrooms | Southern US (Warm Inlet, 70°F+) | Northern US (Cold Inlet, 40°F) |
|---|---|---|
| 1 | 4–5 GPM | 6–7 GPM |
| 2 | 5–7 GPM | 7–9 GPM |
| 3 | 7–9 GPM | 9–11 GPM |
| 4+ | 9–11 GPM | 11+ GPM |
For extra-large bathtubs, jacuzzis, or simultaneous multi-appliance use, size up to the next GPM range.
Buyer’s Guide: Choosing the Right Size Tankless Water Heater
- GPM Rating: Always pick a heater that covers your max expected use during peak times.
- Fuel Type: Gas/propane models deliver the highest GPM for large families. Electric units are best for smaller homes or warmer regions.
- Temperature Rise: This is how much the heater can increase incoming water temp. Know your region’s typical groundwater temperature.
- Efficiency (UEF): Higher efficiency means lower operating costs. Look for 0.82 UEF or higher for gas, 0.90+ for electric.
- Warranty: Reliable brands offer at least 10 years on the heat exchanger.
- Smart Features: Digital controls, recirculation, and WiFi/app support are a plus for families.
- Professional Sizing: For complex homes (multi-zone, radiant heating), ask your plumber for a sizing consultation.
Detailed Reviews: Best Tankless Water Heaters by Size
EcoSmart ECO 27 Electric Tankless Water Heater
- Up to 6.5 GPM in warm climates, 3–4 GPM in colder areas
- Perfect for 1–2 bathrooms or smaller homes
- Digital controls, compact design, lifetime warranty (residential)
Final Verdict: Best electric model for apartments, condos, or small houses.
See it on Amazon
Rinnai V94iN Natural Gas Tankless Water Heater
- 9.8 GPM max flow (ideal for 3–4 baths, or 2–3 in cold climates)
- Energy Star rated, reliable, and super-quiet
- 10-year heat exchanger warranty
Final Verdict: The best all-around gas unit for most families.
See details on Amazon
Navien NPE-240A2 Condensing Tankless Water Heater
- 11.2 GPM (suitable for 4+ bathrooms or highest-demand homes)
- Ultra-high efficiency, built-in recirculation, WiFi-ready
- 15-year heat exchanger warranty
Final Verdict: The ultimate choice for large families and busy homes.
Check price on Amazon
Comparison Table: Tankless Water Heater Sizing & Top Picks
| Model | Max GPM | Type | Efficiency | Warranty | Best For | Price Link |
|---|---|---|---|---|---|---|
| EcoSmart ECO 27 | 6.5 | Electric | 0.90 UEF | Lifetime | 1–2 baths | View |
| Rinnai V94iN | 9.8 | Natural Gas | 0.82 UEF | 10 yr HEX | 3–4 baths | View |
| Navien NPE-240A2 | 11.2 | Natural Gas | 0.95 UEF | 15 yr HEX | 5+ baths | View |
FAQ: Sizing a Tankless Water Heater
- Can I go smaller to save money?
Not recommended—an undersized heater means cold showers and unhappy family members. Always size for peak demand. - What if my home has hard water?
Consider a scale filter or water softener to maintain full GPM and extend heater life. - Gas vs. electric for big homes?
Gas models are best for large families—higher flow at higher temp rise. Electric is good for small homes or warm climates. - How can I get instant hot water?
Look for models with recirculation or add a recirc pump for near-instant delivery at any faucet. - Should I install it myself?
Professional installation is strongly recommended, especially for gas or high-amp electric units, to ensure safety and code compliance.
Conclusion
Getting the size right is the secret to endless hot water and happy mornings! Use the steps and chart above to find your peak demand, then pick a unit that fits your climate and family size. Go with a trusted brand and pro install for best results—and enjoy worry-free hot water for years to come. Need personalized help sizing your system or comparing models? Just ask!