Picking the right size tankless water heater is the most important decision you will make on the project. If the unit is too small, the shower runs lukewarm whenever someone starts the dishwasher. If it is too big, you pay for capacity (and sometimes a bigger gas line) you will never use. The good news: sizing a tankless heater comes down to two numbers you can work out in a few minutes. Those are flow rate (GPM) and temperature rise.
This guide walks through both, explains how gas and electric units differ, and ends with a simple sizing table you can use to narrow your choices.
How tankless sizing works
A tank heater stores hot water. A tankless heater heats water as it flows through, so its "size" isn't measured in gallons. It's measured by how many gallons per minute it can heat by a given number of degrees. Every tankless spec sheet lists this as a GPM figure at a certain temperature rise, for example "5.6 GPM at a 67°F rise."
Sizing therefore comes down to two questions:
- How much hot water will you use at the same time? This is your peak flow rate in GPM.
- How much does the water need to be heated? This is your temperature rise: target outlet temperature minus incoming water temperature.
Choose a heater that can deliver your peak GPM at your temperature rise, and you are sized correctly.
Step 1: Add up your peak flow rate (fixtures × GPM)
List the hot water fixtures that are likely to run at the same time during your busiest moment. That's usually a weekday morning or after dinner. Then add up their flow rates. You'll find the rating on the fixture, in its manual, or on the manufacturer's website. If you don't know it, use the typical ranges below for planning:
| Fixture | Typical flow rate |
|---|---|
| Showerhead | 1.5 – 2.5 GPM (2.5 GPM is the federal maximum) |
| Bathroom faucet | 1.0 – 1.5 GPM |
| Kitchen faucet | 1.5 – 2.2 GPM |
| Dishwasher | 1.0 – 1.5 GPM |
| Clothes washer (hot or warm cycle) | 1.5 – 2.0 GPM |
| Bathtub filler | 4.0 – 6.0 GPM or more |
Example: Two showers (2.0 GPM each) plus the kitchen faucet (1.5 GPM) gives a peak demand of 5.5 GPM.
A few practical tips:
- Be realistic. Most homes rarely run more than two or three hot water fixtures at once.
- Showers and faucets usually mix hot and cold water, so they don't draw 100% hot water. Counting the full fixture rate builds in a safety margin, which is a good idea for tankless.
- Deep soaking tubs and body-spray shower systems can need much more flow. Size for them on their own if you have them.
Step 2: Find your temperature rise by region
Temperature rise is the difference between your incoming (groundwater) temperature and the temperature you want at the tap. Most homes set the outlet at about 120°F. Incoming water is coldest in winter, so always size for winter conditions.
The figures below are general approximations for winter inlet temperatures in the continental US. Your local water utility can give you a more exact figure.
| Region (approximate) | Winter inlet temp | Rise to 120°F |
|---|---|---|
| Northern states (Upper Midwest, Northern Plains, New England, Mountain West) | 35 – 45°F | 75 – 85°F |
| Central states (Mid-Atlantic, Ohio Valley, Lower Midwest, Pacific Northwest) | 45 – 55°F | 65 – 75°F |
| Upper South (Carolinas, Tennessee, Oklahoma, Northern California) | 55 – 65°F | 55 – 65°F |
| Deep South (Gulf Coast, Florida, Southern Texas, Desert Southwest) | 65 – 75°F | 45 – 55°F |
As a rule of thumb, a home in Minnesota needs a far more powerful heater than an identical home in Florida to deliver the same GPM.
Step 3: Convert BTU or kW to GPM
Gas tankless heaters are rated by input in BTU per hour (BTU/h). Electric models are rated in kilowatts (kW). You can estimate output GPM with these formulas:
- Gas: GPM ≈ (BTU/h input × efficiency) ÷ (500 × temperature rise)
- Electric: GPM ≈ (kW × 3,412) ÷ (500 × temperature rise)
The constant 500 comes from the weight of water (about 8.33 lb per gallon) times 60 minutes. For planning purposes, assume about 95% efficiency for a condensing gas unit, about 80–85% for a non-condensing gas unit, and close to 100% for electric.
Check against a real spec sheet: Navien's published QuickFacts for the 199,900 BTU/h Navien NPE-240A2 list 5.6 GPM at a 67°F rise. The formula gives (199,900 × 0.95) ÷ (500 × 67) ≈ 5.7 GPM, which is close enough for planning. Always use the manufacturer's flow chart for the final decision.
Tankless water heater sizing table
This table shows estimated maximum flow for common heater sizes at different temperature rises. Gas figures assume a condensing unit at about 95% efficiency. Find your temperature rise from Step 2, then pick the smallest heater whose number meets or beats your peak GPM from Step 1.
| Heater input | 45°F rise | 60°F rise | 70°F rise | 80°F rise |
|---|---|---|---|---|
| Gas 120,000 BTU/h | 5.1 GPM | 3.8 GPM | 3.3 GPM | 2.9 GPM |
| Gas 150,000 BTU/h | 6.3 GPM | 4.8 GPM | 4.1 GPM | 3.6 GPM |
| Gas 180,000 BTU/h | 7.6 GPM | 5.7 GPM | 4.9 GPM | 4.3 GPM |
| Gas 199,900 BTU/h | 8.4 GPM | 6.3 GPM | 5.4 GPM | 4.7 GPM |
| Electric 18 kW | 2.7 GPM | 2.0 GPM | 1.8 GPM | 1.5 GPM |
| Electric 27 kW | 4.1 GPM | 3.1 GPM | 2.6 GPM | 2.3 GPM |
| Electric 36 kW | 5.5 GPM | 4.1 GPM | 3.5 GPM | 3.1 GPM |
Quick read of the table:
- 1 – 2 bathrooms in a warm climate (about 4–5 GPM at a 45–55°F rise): a 120,000–150,000 BTU/h gas unit is usually enough.
- 2 – 3 bathrooms in a central climate (about 5 GPM at a 65–75°F rise): look at 180,000–199,900 BTU/h.
- 3+ bathrooms or a cold northern climate (5.5+ GPM at a 75°F+ rise): a single 199,900 BTU/h unit may fall short. Consider two units installed in cascade.
Gas vs electric tankless: which fits your home?
Gas tankless (natural gas or propane)
Gas units deliver far more hot water per unit, which is why most whole-house installs use gas. Things to plan for:
- Gas supply: A 199,900 BTU/h unit needs a gas line and meter big enough for that load on top of your furnace, range and dryer. Undersized gas piping is one of the most common causes of poor tankless performance.
- Venting: Condensing units (such as the Navien NPE-2 series) have a second heat exchanger and vent with plastic pipe. Navien lists 2" PVC for runs of up to 75 feet on the NPE-2. They also produce acidic condensate that needs a drain, usually through a neutralizer. Non-condensing units (such as the Navien NHW-SU) cost less up front but use a different vent system. Navien lists a 3" x 5" concentric vent for the NHW-SU.
- Efficiency: Navien rates the NPE-S2 models at 0.96 UEF and the NPE-A2 models at 0.95 UEF, versus 0.81 UEF for the non-condensing NHW models.
Browse condensing and non-condensing options in our Navien tankless water heaters collection. The NPE-S2 is the standard model. The NPE-A2 adds Navien's ComfortFlow buffer tank and built-in recirculation pump. The NHW series is the compact non-condensing line.
Electric tankless
Electric units need no venting or gas line, and they are very efficient at the point of use. The trade-off is electrical capacity. Whole-house electric models usually need 240V service and several dedicated high-amperage breakers. Even a large unit only delivers about 3–4 GPM at a northern-climate temperature rise. Electric tankless works best in warm climates, in smaller homes, or as a point-of-use heater for a single bathroom or kitchen. Have an electrician confirm your panel can handle the load before you buy.
Common sizing mistakes to avoid
- Sizing for summer water temperatures. Always use your coldest winter inlet temperature.
- Ignoring the gas line. The heater can only deliver its rated output if it gets its full rated gas supply.
- Forgetting high-flow fixtures. Rain heads, multiple body sprays and large tubs can double your peak demand.
- Using "max GPM" from marketing copy. The headline flow figure is often quoted at a small temperature rise. Use the GPM at your temperature rise.
- Skipping recirculation planning. If your fixtures are far from the heater, a unit with built-in recirculation (such as the Navien NPE-A2) or a separate recirculation setup can cut the wait for hot water.
Quick sizing checklist
- List the hot water fixtures you run at the same time and add their GPM.
- Find your winter inlet temperature and subtract it from 120°F.
- Use the table above, or the manufacturer's flow chart, to find a unit that meets your GPM at that rise.
- Confirm your gas line, venting route and condensate drain (gas), or your electrical service (electric).
- Have a licensed plumber or gas fitter install the unit. Gas, venting and electrical work must meet local code.
Ready to compare models? Shop all gas tankless water heaters, with free shipping on qualifying orders. If you aren't sure which size fits your home, contact our team with your fixture list and ZIP code, and we'll help you narrow it down.