Quick Answer: Size a tankless water heater with one formula: BTU/hr = GPM × 500 × temperature rise, where rise is your target output temperature (usually 105°F) minus your coldest incoming water temperature. A typical 2–3 bathroom home needs 5,000–7,500 total GPM demand from simultaneous fixtures, which translates to roughly 150,000–199,000 BTU for gas or 24–36 kW for electric once you factor in a cold-climate rise. The single biggest sizing mistake is using a manufacturer’s headline GPM number, which is almost always measured at a 60°F rise — in a cold-winter climate your real usable flow can drop by 30–40% below that number.
Every tankless brand publishes a peak GPM figure, and almost every buyer sizes off that number — which is exactly backward. GPM on a tankless heater isn’t fixed; it’s a function of how much your incoming water has to be heated, and that changes by region and by season. This guide walks through the actual formula, the groundwater temperatures that drive it, how much flow your fixtures really demand, and three worked examples so you can size correctly the first time instead of returning an undersized unit.
The sizing formula
Every tankless water heater sizing decision comes down to one equation:
BTU/hr required = GPM demand × 500 × temperature rise (°F)
- GPM demand is the total flow rate of every fixture you expect to run at the same time.
- Temperature rise is your desired output temperature (105–120°F is typical) minus your coldest incoming groundwater temperature.
- 500 is a constant (8.34 lbs/gallon × 60 minutes) that converts gallons and degrees into BTU/hr.
For electric units, convert BTU/hr to kilowatts by dividing by 3,412 (1 kW = 3,412 BTU/hr). A unit that needs 150,000 BTU/hr of gas input needs roughly 44 kW of electric input to match it — which is why whole-home electric tankless heaters top out around 36 kW: matching a large gas unit’s output on electric alone would require amperage most residential panels can’t carry.
Groundwater temperature: the variable everyone skips
Temperature rise is where sizing goes wrong, because incoming water temperature varies enormously by region and by season. Per manufacturer groundwater-temperature charts (Rinnai and A.O. Smith both publish regional maps used industry-wide for sizing), winter groundwater ranges from about 77°F in southern Florida to as low as 37°F across the northern tier states.
| Region example | Winter groundwater | Rise to 105°F output | BTU/hr for 5 GPM demand |
|---|---|---|---|
| Southern Florida, South Texas | ~77°F | 28°F | ~70,000 |
| Gulf Coast, Southern California | ~67°F | 38°F | ~95,000 |
| Mid-Atlantic, Pacific Northwest | ~50°F | 55°F | ~137,500 |
| Upper Midwest, New England, Northern Plains | ~37°F | 68°F | ~170,000 |
That’s the entire reason two homeowners with identical fixture counts end up buying different-sized heaters. A household in Minneapolis needs roughly 2.4x the BTU input of an identical household in Miami to deliver the same 5 GPM at 105°F, purely because of winter groundwater temperature. Size to your coldest month, not your annual average — undersizing shows up as lukewarm showers every winter, not as a one-time inconvenience.
Check your ZIP code's groundwater temperature first
- Before comparing any specific model, run your ZIP code through a manufacturer sizing tool to get an estimated winter inlet temperature — it's the single input every other calculation depends on.
- Cross-check the result against the regional table above; if your area sees hard freezes, size toward the colder end even if the calculator suggests otherwise.
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How much flow your fixtures actually demand
The other half of the equation is GPM demand — what you’ll realistically run at once, not what your house could theoretically run if every faucet were open.
| Fixture | Typical flow | Notes |
|---|---|---|
| Shower | 1.5–2.5 GPM | Federal max 2.5 GPM (EPAct 1992); WaterSense-labeled heads cap at 2.0 GPM |
| Kitchen faucet | 1.5–2.2 GPM | Higher on pot-fillers and older fixtures |
| Bathroom faucet | 1.0–1.5 GPM | WaterSense-labeled faucets run as low as 1.0 GPM |
| Dishwasher | 1.0–2.5 GPM | Intermittent draw, not continuous through the cycle |
| Washing machine | 1.5–3.0 GPM | Hot-fill cycles only; many modern cycles use cold or warm |
| Soaking tub | 4.0–6.0 GPM | The single highest-demand fixture in most homes |
The practical exercise: list the fixtures most likely to run at the same time in your household’s real routine — a shower plus the kitchen faucet in the morning rush, or two showers back-to-back in a multi-teen household — and add their GPM. That sum is your design GPM, not the sum of every fixture in the house.
Three worked examples
2-bathroom home, mild climate (67°F groundwater, 38°F rise). Design demand: one shower (2.0 GPM) + kitchen faucet (1.5 GPM) = 3.5 GPM. BTU needed: 3.5 × 500 × 38 = 66,500 BTU/hr. This is comfortably covered by almost any whole-home gas unit or a mid-tier electric unit like a Rheem RETEX-24 sized for warm-climate service.
3-bathroom home, moderate-cold climate (50°F groundwater, 55°F rise). Design demand: two showers running together (4.0 GPM) + dishwasher (1.5 GPM) = 5.5 GPM. BTU needed: 5.5 × 500 × 55 = 151,250 BTU/hr. This lands in the 160,000–199,000 BTU whole-home gas tier — a unit like the Rinnai Sensei RX199iN or Noritz NRC98-DV-NG — or a 36 kW electric unit if the panel has spare 150A capacity.
4-bathroom home, cold climate (37°F groundwater, 68°F rise). Design demand: two showers (4.0 GPM) + washing machine (2.0 GPM) = 6.0 GPM. BTU needed: 6.0 × 500 × 68 = 204,000 BTU/hr — this exceeds what a single whole-home unit reliably delivers at that rise, which is exactly the scenario where installers recommend two smaller units running in parallel (a “twin” or “manifold” setup) rather than one oversized flagship straining at its rated ceiling.
Gas vs. electric: the constraint that decides the format
The BTU math is identical for both fuels, but what actually caps your options differs:
- Gas is limited by BTU input and gas-line diameter. A whole-home unit needing 160,000–199,000 BTU/hr often requires upsizing the gas line from 1/2 inch to 3/4 inch — our tankless water heater cost guide breaks down that $500–$1,500 line item in detail.
- Electric is limited by amperage and panel capacity. A 24 kW unit needs roughly 100A of dedicated service; a 36 kW unit needs roughly 150A. If your main panel is 100A or 150A total, there may not be room without an upgrade — get an electrician’s load calculation before buying, not after.
Neither constraint shows up in a manufacturer’s GPM spec sheet, which is why the practical sizing process has to include a look at your actual gas line and electrical panel, not just the BTU/GPM/rise math on paper.
The mistake that sends people back to the store
Buyers who size off a retailer’s headline “11 GPM!” number and skip the rise calculation are the ones who end up with a tankless heater that can’t keep two showers hot in January. The fix costs nothing extra at purchase time — it’s just doing the arithmetic with your winter groundwater temperature and your real simultaneous demand instead of the manufacturer’s best-case 60°F-rise number. Compare that against our best gas tankless and best electric tankless roundups, which list each unit’s GPM at multiple rise points specifically so you can size correctly before you buy.
The bottom line
Multiply your realistic simultaneous fixture demand by 500, then by the gap between your coldest groundwater temperature and your target output temperature — that BTU/hr number, not the sticker GPM, is what actually tells you which tier of heater to buy. Cold-climate homes need meaningfully more BTU or kW than warm-climate homes with identical fixture counts, and running two smaller units in parallel often beats forcing one oversized flagship to its rated ceiling. If you’re still deciding between fuel types once you know your target size, our electric vs. gas comparison covers which constraint — gas line or electrical panel — is cheaper to solve in your home.