Most households need a tankless water heater rated somewhere between 5 and 10 GPM, but the right number for your home depends on two things: how many fixtures might run hot water at the same time, and how cold your incoming water gets. Get either one wrong and you’ll end up with a unit that can’t keep up, even if the brand and price were right.
This guide walks through both calculations in plain terms. For the full picture on tankless systems generally, see our Complete Guide to Tankless Water Heaters.
Quick answer:
GPM, gallons per minute, is the rate at which a tankless unit can heat water to your target temperature. It’s not a measure of total capacity the way tank size is; a tankless unit doesn’t run out, it just can’t exceed its rated flow. Ask for more hot water than the unit’s GPM rating supports, by running too many fixtures at once, and water temperature drops rather than the unit simply working harder.
The method is straightforward: add up the flow rate of every fixture you’d realistically run at the same time, not every fixture in your house.
A common example: a shower running at the same time as a bathroom faucet. If the shower uses 2.0 GPM and the faucet uses 1.5 GPM, your target is at least 3.5 GPM. A household where two showers might run simultaneously needs to size for both combined, not either one alone. The goal isn’t your home’s maximum theoretical draw, it’s your realistic peak.
Fixture flow rates are federally regulated, which makes this part of the calculation more reliable than most people expect.
Fixture | Typical Flow Rate |
Showerhead | Up to 2.5 GPM (federal maximum under the Energy Policy Act of 1992); WaterSense-certified models are capped at 2.0 GPM |
Bathroom faucet | Up to 2.2 GPM federal standard; WaterSense-certified models are capped at 1.5 GPM, some as low as 1.0-1.2 GPM |
Kitchen faucet | Generally up to 2.2 GPM under the same federal standard |
Bathtub faucet | Not federally restricted the same way; commonly runs higher, often cited around 4 to 6 GPM depending on the fixture |
Dishwasher | Measured in gallons per cycle rather than continuous GPM in most modern units, so it draws less predictably than a running fixture |
Washing machine | Same as above; modern machines meter total fill volume rather than sustaining a fixed flow rate |
If your fixtures are older or non-WaterSense, lean toward the higher end of these ranges.
Flow rate alone doesn’t finish the job. A tankless unit’s maximum GPM output changes depending on how much it has to raise the water’s temperature, called temperature rise, the gap between your incoming groundwater temperature and your target hot water temperature.
Colder incoming water demands a bigger rise, which lowers the unit’s effective GPM output. A unit rated for 8 GPM at a modest rise might only deliver 5 or 6 GPM when the rise required is larger. This is exactly why the same model performs differently installed in different parts of the country.
For homeowners in Tacoma, WA, this matters more than average. Pacific Northwest groundwater runs cooler than in warmer regions, and it drops further in winter. That colder starting point increases the temperature rise a tankless unit has to deliver, which means Pierce County and South Sound homeowners often need a higher-GPM unit than a homeowner in a warmer climate would, even with identical fixtures and identical usage habits. This is one of the most common sizing mistakes we see in Washington homes: a unit sized using a national average rise, rather than a Tacoma winter one, that underperforms the first cold snap it faces.
Gas and electric units aren’t sized the same way, because they don’t generate heat the same way.
Gas tankless water heaters draw on burner capacity, measured in BTU input, to heat water quickly, which generally allows for higher GPM output at a given temperature rise. Most whole-house gas units are built to cover higher simultaneous demand, which is why larger households and colder climates lean gas more often.
Electric tankless water heaters are limited by the heating capacity their elements can deliver relative to your home’s electrical service. Electric units generally top out at lower GPM than gas units at the same temperature rise, and higher-output electric models require higher amperage, sometimes more than a standard panel comfortably supports.
Neither is universally “better.” The right choice depends on your fuel access, your electrical capacity, and the GPM your household actually needs, covered in more depth in our Tankless vs. Tank Water Heater Cost Guide.
These are starting points based on typical simultaneous fixture use, not a guarantee for your specific home.
Household | Common Scenario | Typical GPM Range |
Family of 2, 1-2 bathrooms | One shower plus occasional faucet use | Roughly 4-6 GPM |
Family of 3-4, 2 bathrooms | Two fixtures running together during busy mornings | Roughly 6-8 GPM |
Family of 5+, 3+ bathrooms | Multiple showers or a shower plus laundry overlapping | Roughly 8-10 GPM |
Larger homes, 5+ bedrooms or multiple simultaneous bathrooms | High simultaneous demand | 10 GPM or higher, sometimes multiple units |
Colder-climate households, including most of Washington, should treat these as a starting floor rather than a ceiling.
A rough calculation gets you close. It doesn’t account for your actual water pressure, your specific fixtures’ real-world flow, or your home’s true winter groundwater temperature. Undersizing is the single most common reason homeowners end up unhappy with a new tankless system, and it’s almost always avoidable with an accurate assessment before installation, not after.
If you’re in Tacoma, Pierce County, or the greater South Sound region, reach out toYour Local Plumbing for a sizing assessment based on your actual home, not a national average. We factor in your real winter groundwater temperature, not a generic U.S. average, before recommending a unit, which is exactly the step a national online calculator can’t do for you. We size and install both gas and electric tankless systems and can tell you honestly whether your existing gas line or electrical service supports the unit you’re considering, covered further in our Tankless Water Heater Conversion Guide.
Most families of four land somewhere around 6 to 8 GPM, assuming two bathrooms and typical morning overlap between a shower and a faucet or laundry. Households with more simultaneous fixture use, or colder incoming water, should size toward the higher end.
For most homes, yes, 10 GPM covers even fairly heavy simultaneous use. Larger homes with multiple bathrooms running at once, especially in colder climates with a higher required temperature rise, may still need to confirm against their specific fixture combination rather than assuming 10 GPM is automatically sufficient.
Often, yes. Colder incoming groundwater requires a larger temperature rise, which reduces a given unit's effective GPM output. A unit that performs well in a warm climate can underdeliver in a colder one at the exact same GPM rating, which is why regional groundwater temperature belongs in the calculation, not just fixture count.
Demand exceeds the unit's rated output, and water temperature drops rather than the unit simply straining to keep up. This typically shows up as lukewarm water when two fixtures run together, even though either fixture alone works fine.
Usually on the energy bill, though the size varies by usage and local energy prices, and can be modest for a household with already-high, constant demand. Weigh that against periodic professional maintenance, a real recurring cost tankless carries that most tank systems don't.
Sizing comes down to two numbers: your peak simultaneous demand and your incoming water temperature. Get those right and the GPM rating picks itself.
Most households need 5 to 10 GPM, sized by adding the flow rate of every fixture likely to run at once and adjusting upward for colder groundwater, colder climates like Washington should size above the national average, not to it.
Your Local Plumbing is Tacoma’s best choice for residential and commercial plumbing solutions you can always count on.
