Someone’s in the shower. Someone else turns on the kitchen faucet to rinse a pan. And just like that, the water goes lukewarm. If that sounds familiar, the tankless unit isn’t broken; it’s undersized.
What size tankless water heater do I need? It’s the question that decides if your new system actually solves the cold-water problem or just moves it somewhere else. The answer isn’t a single number pulled from a chart. It comes from two numbers working together, and once you know them, sizing stops being a guess.
Here’s how to work it out, using the same method plumbers use, confirmed by This Old House’s own sizing guide.
GPM and Temperature Rise
Every tankless water heater is rated by two things: how many gallons per minute (GPM) it can heat, and how much it can raise the water temperature while doing it. Get both numbers right for your household, and the unit performs. Get either one wrong, and you’re back to lukewarm showers.
| Household Profile | Typical Simultaneous Demand | Suggested Unit Size |
|---|---|---|
| 1–2 people, 1 bathroom | 2.5 – 3.5 GPM | 4 – 6 GPM |
| 3–4 people, 2 bathrooms | 4 – 5.5 GPM | 6 – 8 GPM |
| 5+ people, 3+ bathrooms | 6 – 9 GPM | 9 – 11 GPM |
Those ranges build in a safety margin, because the real math depends on your specific fixtures, not just how many people live in the house. Two four-person households can land on two different unit sizes once you account for how many bathrooms they’re running at once, if the dishwasher tends to overlap with shower time, and how far the unit sits from the fixtures it’s serving.
That’s the gap most online calculators miss. They ask for your household size and hand back a single number, without ever asking what actually happens in your bathroom at 7 a.m.
The Real Formula: GPM Plus Temperature Rise
Tankless units don’t work like tanks. A storage tank just needs enough gallons to outlast your shower. Tankless water heater installation for LA homes requires enough capacity to heat water continuously at whatever rate your fixtures are pulling it, every single time.
That’s why water heater size for a tankless system comes down to two calculations instead of one number on a label:
- Flow rate. Add up the GPM of every fixture you’ll realistically run at the same time.
- Temperature rise. Subtract your incoming water temperature from your target output temperature.
Nail both, and you’ve got your answer. Skip one, and you’ll either underperform or overpay for capacity you’ll never use.
Step 1: Add Up Your Peak Flow Rate
This is where how to size a tankless hot water heater actually starts, not with your house size, but with what’s running at once.
Typical fixture flow rates:
- Bathroom sink: 0.75 – 1.5 GPM
- Shower: 2.0 – 2.5 GPM
- Kitchen sink: 1.5 – 2.2 GPM
- Dishwasher: 1.0 – 1.5 GPM
- Washing machine: 1.5 – 2.5 GPM
Say two people shower back to back while someone runs the kitchen sink. That’s 2.5 + 2.5 + 1.8, roughly 6.8 GPM. A unit rated for 5 GPM will fall behind the moment that third fixture opens, even though it handled two showers just fine on its own.
This is also where most homeowners calculate the size of a tankless water heater wrong. They size for an average day instead of the busiest ten minutes of a Monday morning.
Try a second example: a family of four with two bathrooms. One shower runs at 2.5 GPM while a second person brushes their teeth at 1 GPM, and the dishwasher kicks on mid-cycle at 1.3 GPM. That’s 4.8 GPM, before anyone even starts laundry. Round up for the appliance you forgot about, and a 6 GPM-rated unit is a safer floor than the 5 GPM most people default to.
What Happens When You Get the Size Wrong
Undersizing shows up immediately: the “cold water sandwich” effect, where the temperature dips or swings the moment a second fixture opens. It’s not a defect, it’s physics, the unit simply can’t heat more water than it’s rated for.
Oversizing is quieter but still costly. A unit rated well beyond your household’s peak demand costs more upfront, and some models struggle to modulate down for small draws like a single sink, which shortens component life over time. Getting water heater size right the first time avoids both problems, and it’s cheaper than replacing a mismatched unit two years in.
Step 2: Figure Out Your Temperature Rise
Temperature rise is the number most cost calculators skip, and it’s the one that changes everything depending on where you live.
The industry-standard assumption is a 50°F incoming water temperature, which is realistic for colder parts of the country. To reach a standard 120°F output, that means a 70°F temperature rise.
Los Angeles doesn’t work off that number. Groundwater here typically runs in the mid-to-upper 60s°F, well above the national baseline. That usually puts your required rise closer to 50°F rather than 70°F, which matters more than it might seem.
Here’s why: a tankless unit’s GPM rating is tied to a specific temperature rise. A unit that only manages 5 GPM in a 70°F-rise climate can often deliver closer to 7–8 GPM at the smaller rise LA homes actually need. Shop with a spec sheet built for a colder state, and you can end up buying more unit than your house requires, or worse, sizing off the wrong number entirely.
How Many Fixtures Will Realistically Run at Once?
Tankless water heater size should match your household’s actual habits, not a worst-case fantasy where every faucet in the house runs simultaneously.
Walk through a typical morning:
- Small household, one bathroom: One shower at a time, maybe a sink running briefly. Low simultaneous demand.
- Family with two bathrooms: Two showers overlapping is common, especially before school or work. Plan for that overlap, not just one shower.
- Larger household, multiple bathrooms: Showers, a dishwasher cycle, and laundry can all land in the same 20-minute window. Size for that window, not the daily average.
If your family’s routine genuinely stacks three or four fixtures at once, don’t round down to save on the unit price. That’s exactly how a new tankless system ends up with the same complaint as the old tank it replaced.
Common Sizing Mistakes
- Sizing for people instead of fixtures. Two people can still need a bigger unit than four, if their schedules overlap more.
- Ignoring winter groundwater dips. Even in Los Angeles, incoming water runs a few degrees cooler in January than in August. Size for the coldest month, not the average one.
- Buying based on tank-heater habits. A 50-gallon tank mindset doesn’t translate to tankless. GPM and temperature rise are the only numbers that matter here.
- Copying a neighbor’s unit. Two houses on the same block can have different pipe runs, fixture counts, and gas line capacity. What worked next door doesn’t guarantee it’ll work for you.
This Isn’t How You’d Size a Tank
If you’ve shopped for a tank water heater before, the sizing method probably involved picking a gallon capacity, 40, 50, or 80 gallons, based on family size. Tanks are sized by a “first hour rating”, essentially how many gallons of hot water they can deliver in a single busy hour before running out.
Tankless throws that model out entirely. There’s no reserve to run out of, and no gallon capacity to compare. The only numbers that matter are the two you’ve already calculated: flow rate and temperature rise. Bringing tank-shopping habits into a tankless decision is one of the most common ways homeowners end up with a mismatched unit.
Gas vs. Electric: Sizing Works Differently
Fuel type changes what’s realistic. Manufacturer spec sheets typically show a demand water heater delivering a 70°F temperature rise at about 5 GPM for gas-fired units, but only around 2 GPM for electric ones.
That gap matters a lot for larger households. Most whole-home installs in Los Angeles lean gas specifically because it can hit higher flow rates at a given temperature rise without the panel upgrades electric units often require.
Why Professional Sizing Still Beats a Chart
Charts like the one above get you close. They don’t account for your actual fixtures, your home’s existing gas or electrical capacity, or if your family’s real routine matches the “typical” household it’s built around.
When we handle tankless water heater installation in Los Angeles, we walk the house, count the fixtures that actually run together, and check what your gas line or electrical panel can support before recommending a model. We also check your gas meter’s capacity and how far the unit will sit from your farthest fixture, both of which affect real-world performance in ways no online chart accounts for.
That’s the difference between a unit that’s sized correctly on paper and one that performs correctly every morning, and it’s why the answer to what size tankless water heater do I need should come from someone standing in your house, not just a table on a page.
So, back to the original question. What size tankless water heater do I need? Add up your peak GPM, figure your temperature rise using LA’s warmer groundwater, and match that against a unit rated to deliver both, together, not just one or the other.
Frequently Asked Questions
Can a tankless water heater be too big for my home?
Yes. Oversized units can short-cycle on small draws, like a bathroom sink, which reduces efficiency and adds wear over time. Bigger isn’t automatically better; matched is better, which is the whole point of asking what size tankless water heater do i need instead of just buying the largest model available.
Do I need a larger unit if I have a soaking tub or spa-style shower?
Often, yes. Soaking tubs can pull 4 to 8 GPM to fill in a reasonable time, more than a standard shower. If your home has one, factor its fill rate into your peak demand calculation separately.
Does water pressure affect tankless sizing?
Indirectly. Low incoming pressure can reduce the effective flow rate reaching the unit, which may make a correctly sized heater feel underpowered. A plumber can test your home’s pressure as part of a proper sizing assessment.
Can I install two tankless units instead of one larger one?
Yes, this is common in larger homes. Running two smaller units in parallel, or dedicating one to a specific zone like a primary suite, can be more efficient than one oversized unit trying to cover the whole house.
Does a water softener or whole-house filter change what size I need?
It can. Softeners and filters add a small amount of flow restriction, which may reduce the effective GPM reaching your fixtures. It’s worth mentioning any water treatment systems when a plumber calculates your household’s sizing.
