How to Size a Tankless Water Heater

Undersizing a tankless water heater is the single most common reason people end up disappointed with a switch away from a traditional tank. Unlike a storage tank, which just needs to be big enough to hold what you'll use before it runs out, a tankless unit has to heat water fast enough to keep up with demand in real time. Get that sizing wrong, and you get lukewarm water at the second fixture — not a defective unit, just an undersized one.

Why Tankless Sizing Works Differently

A storage tank water heater's capacity is measured in gallons — how much hot water it has stockpiled and ready to go. A tankless unit doesn't store anything; it heats water on demand as it flows through the unit. That means tankless sizing isn't about volume at all — it's about flow rate and temperature rise, measured in GPM (gallons per minute) and the temperature increase needed between incoming cold water and target hot water temperature.

The Two Numbers That Actually Matter

• Peak simultaneous flow rate (GPM) — add up the flow rate of every fixture that might realistically run at the same time (a shower, a dishwasher, a sink), not just the single biggest fixture in the building. This is where undersizing usually happens: sizing for one shower instead of the realistic worst case of a shower plus a sink running together.
• Temperature rise (ΔT) — the gap between your incoming groundwater temperature and your target output temperature (typically 120°F). This varies significantly by climate: incoming water in the northern half of the country can be 40°F colder than in the deep South, which directly reduces how much flow a given unit can support at the same target output temperature.

Every tankless unit's rated GPM capacity is only valid at a specific ΔT — the same unit that comfortably handles 8 GPM at a mild 35°F rise might only manage 4-5 GPM if the incoming water is 30°F colder. Manufacturer spec sheets publish a flow-vs-temperature-rise chart for exactly this reason, and it's the piece people skip most often when sizing off a single advertised GPM number. Our Tankless Water Heater Sizing Calculator walks through both flow rate and temperature rise together rather than either alone.

Gas vs. Electric: Different Constraints

• Gas tankless units generally support higher flow rates and are the standard choice for whole-building or multi-fixture commercial applications, but require adequate gas line sizing and proper venting — an undersized gas supply line can bottleneck a unit's output just as much as an undersized water line.
• Electric tankless units are simpler to install (no venting) but are limited by available electrical service — high-GPM electric units draw significant amperage, and older buildings often don't have spare panel capacity without an upgrade.

Single Large Unit vs. Multiple Smaller Units

For commercial applications with high or variable demand, running two or more smaller tankless units in a parallel/manifold configuration is often a better approach than one large unit: it adds redundancy (one unit failing doesn't mean zero hot water), and units can modulate together to match actual demand more efficiently than a single oversized unit running well below its rated capacity most of the time.

Common Sizing Mistakes

• Sizing for one fixture instead of realistic simultaneous demand — the most common cause of "not enough hot water" complaints after a tankless install.
• Ignoring regional groundwater temperature — a unit sized correctly for Florida's incoming water temperature can be meaningfully undersized in a colder climate at the same rated GPM.
• Undersizing the gas line on gas units, which throttles output regardless of how well the unit itself is sized.
• Not accounting for future fixture additions — a bathroom or unit added later increases peak simultaneous demand beyond the original calculation.

Bottom Line

Tankless water heater sizing comes down to matching real peak simultaneous flow demand against a unit's actual GPM capacity at your specific incoming water temperature — not the single largest advertised GPM number on the spec sheet. Getting this right up front avoids the most common complaint with tankless systems: adequate hot water for one fixture, but not enough when a second one runs at the same time.

National Boiler Supply carries commercial and residential tankless water heaters and parts. Run your numbers through our Tankless Water Heater Sizing Calculator, or reach out with your fixture count and region and we'll help you size the right unit.