A tankless water heater is sized by flow rate (gallons per minute) at the temperature rise your home needs — not by tank gallons. You total the fixtures you want running at once, then pick a unit that delivers that combined GPM at the required temperature rise. Undersizing is the #1 cause of lukewarm tankless water, so the sizing math matters more than the brand on the box.
How is tankless sizing different from a tank?
A tank stores a fixed volume of pre-heated water; size it by gallons and recovery rate. A tankless unit heats water instantly as it flows, so it's sized by two things: how much hot water you draw at peak (GPM) and how far it must raise the incoming water temperature (temperature rise). Both have to be satisfied at the same time, or the output temperature drops. Choosing between types first? See tank vs. tankless vs. heat-pump.
How do you calculate the right size?
- Add up simultaneous flow. List the fixtures you want to run at once (e.g., two showers + a sink) and sum their typical GPM.
- Determine temperature rise. Subtract your incoming groundwater temperature from your target hot-water temperature. Colder incoming water means a larger unit for the same flow.
- Match a unit that delivers your peak GPM at that temperature rise — manufacturers publish flow-vs-rise curves for exactly this.
- Plan for growth. A growing household or a future addition argues for a little headroom — or planning for a second unit.
What about gas vs. electric?
| Factor | Gas tankless | Electric tankless |
|---|---|---|
| Typical flow capacity | Higher GPM | Lower per-unit GPM |
| Infrastructure needs | Adequate gas line + venting | Substantial electrical (often a panel upgrade) |
| Best for | Whole-home, higher demand | Point-of-use, smaller homes, electrification |
Which fits depends on your home's gas and electrical service — confirmed during the estimate.
Hard water shortens the gap between maintenance. A correctly sized tankless still needs periodic descaling in hard-water areas to hold its flow rate — factor that into ownership, and see water-heater lifespan.
Why size matters for cost and comfort
Oversize and you overpay; undersize and you get lukewarm peaks — neither is acceptable. Right-sizing delivers endless hot water at the temperature you set, which is the entire promise of tankless. Budget ranges live in the water-heater cost guide, and install must meet permitted code.
Why trust Air Synergy to size it?
A family-owned team with decades of combined technician experience in the Bay Area and a 4.9-star rating across 189 reviews, we size tankless units to your real simultaneous demand and your home's gas/electrical capacity — not a rule of thumb — and install to code. Explore water heating.
Your next step
Thinking tankless? Book a free in-home estimate. We'll calculate your peak demand and temperature rise and recommend a properly sized unit in writing.
Sizing guidance reflects standard tankless practice (flow rate at temperature rise); your figures depend on fixtures and incoming water temperature. Terms in the glossary. Company facts per Air Synergy.
Common questions
What size tankless water heater do I need?
Tankless units are sized by flow rate (gallons per minute) at the temperature rise your climate requires, not by tank gallons. You add up the fixtures you want to run at once, then choose a unit that delivers that combined GPM at the needed temperature rise. An undersized unit runs lukewarm during peak demand, so correct sizing is the whole game.
Why does my tankless water heater run lukewarm sometimes?
Almost always because demand exceeded its capacity — too many hot-water fixtures running at once for the unit's GPM — or because incoming water is colder than the unit was sized for. The fix is correct sizing for your real simultaneous demand, and sometimes a larger unit or a second one for big households.
Do gas and electric tankless units size differently?
Yes. Gas tankless units generally deliver higher flow rates and need adequate gas supply and venting; electric tankless units need substantial electrical capacity and may require a panel upgrade. Which is right depends on your home's gas and electrical service, which we confirm during the estimate.