How Big of a Tankless Water Heater Do I Need

If you’re thinking about upgrading to a tankless water heater, you’re not alone. More and more homeowners are ditching traditional tank models for the promise of endless hot water, energy savings, and compact design. But there’s one big question that can make or break your experience: How big of a tankless water heater do I need?

Unlike tank water heaters, where you simply buy a 40- or 50-gallon tank, sizing a tankless model requires a little calculation. If you get one that’s too small, you’ll end up with lukewarm showers when multiple appliances run. If you go too big, you’ll overspend on upfront costs and possibly waste energy.

The good news? Finding the right size isn’t complicated once you understand two key factors: flow rate and temperature rise. In this guide, we’ll break down everything step by step so you can choose the perfect tankless water heater for your household.

How Big of a Tankless Water Heater Do I Need

How Tankless Water Heaters Work

Traditional tank heaters store hot water in a tank, ready for use. When you turn on the faucet, the hot water flows out until the tank runs low, and then it needs time to heat back up.

Tankless water heaters work differently. Instead of storing hot water, they heat water on demand as it flows through the unit. This means you’ll never technically “run out” of hot water as long as the unit is properly sized for your needs.

Key features of tankless heaters:

  • Compact and wall-mounted
  • Energy-efficient (no standby heating loss)
  • Provide continuous hot water supply

But here’s the catch: tankless heaters have a maximum output capacity, measured in gallons per minute (GPM). If your household demand exceeds this limit, the water temperature drops. That’s why choosing the right size matters so much.

READ MORE: What Does a Water Heater Expansion Tank Do?

Understanding Sizing Basics

When it comes to tankless water heater sizing, you’ll need to know two things:

  1. Flow Rate (GPM): This measures how many gallons of hot water per minute your household needs at peak usage. Showers, sinks, washing machines, and dishwashers all contribute to the total demand.
  2. Temperature Rise: This measures how much the heater needs to raise the incoming water temperature to reach your desired hot water temperature (usually 120°F).

Here’s a simple example:

  • If your incoming water is 50°F and you want 120°F water, you need a 70°F temperature rise.
  • If two showers (2.5 GPM each) and a sink (1.0 GPM) run at once, you need a 6.0 GPM capacity.

So, the perfect heater for this household must provide at least 6.0 GPM at a 70°F temperature rise.

Calculating Flow Rate Needs

To calculate your household’s flow rate, make a list of all fixtures and appliances that could be used simultaneously. Here are some averages:

Fixture/ApplianceAverage Flow Rate (GPM)
Shower2.0 – 2.5
Bathroom Sink1.0
Kitchen Faucet1.5
Dishwasher1.5 – 2.0
Washing Machine2.0 – 2.5

Now, think realistically. Do two showers often run at the same time? Do you run the dishwasher while laundry is going?

Example scenario for a family of four:

  • 2 showers running: 5.0 GPM
  • Kitchen faucet: 1.5 GPM
  • Washing machine: 2.0 GPM
    Total Peak Demand: 8.5 GPM

This means the household should choose a tankless water heater that provides 8.5 GPM at the required temperature rise.

Determining Required Temperature Rise

The other half of the equation is temperature rise. This depends on where you live.

  • In colder regions, incoming groundwater can be as low as 35–40°F.
  • In warmer climates, it may be closer to 60–70°F.

Since most people prefer hot water around 120°F, the difference between incoming water temperature and desired output is the required rise.

RegionAverage Groundwater TempTemp Rise Needed (to 120°F)
Northern U.S.40°F80°F rise
Midwest50°F70°F rise
Southern U.S.60°F60°F rise
Southwest70°F50°F rise

So, if you live in Chicago where incoming water is 50°F, and you want 120°F water, your heater must handle a 70°F rise. That means you’ll need a stronger unit than someone living in Florida, where only a 50°F rise is required.

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Small Household Needs

If you live alone or with one other person, your hot water needs will be relatively low. In most cases, a small tankless water heater will be more than enough to keep up.

Let’s say your typical usage includes:

  • One shower running at a time (2.0–2.5 GPM)
  • Occasional bathroom sink use (1.0 GPM)
  • Maybe a dishwasher or washing machine (2.0 GPM)

Even if two of these appliances overlap, you’re looking at a total demand of 4–5 GPM at peak use. For such a household, a tankless water heater that can handle 5 GPM at your region’s required temperature rise will do the job.

For example:

  • A couple in Texas, where incoming water averages 65°F, only needs a 55°F rise. A 5 GPM electric tankless heater would be sufficient.
  • A single homeowner in Minnesota, however, may need closer to 6 GPM gas-powered capacity due to the colder groundwater requiring a higher rise.

Recommended sizes for small households:

  • Electric: 18–24 kW unit
  • Gas: 120,000–140,000 BTUs

This setup ensures one person can shower while another washes dishes—without the water suddenly turning cold.

Medium Household Needs

A family of 3–4 people will almost certainly need a larger unit. With more people comes more overlap in showers, laundry, and kitchen usage.

Typical usage scenario:

  • Two showers at the same time (5 GPM)
  • One bathroom sink (1 GPM)
  • Dishwasher (1.5 GPM)

That’s 7.5 GPM total. And if the washing machine gets added, you’re looking at 9–10 GPM during peak demand.

For a medium house, it is intelligent to the size on the high side. Even if you do not always use that water together, having additional capacity ensures comfort and avoids overwork the system.

Recommended sizes for medium households:

  • Electric: 24–32 kW unit (best for warmer regions)
  • Gas: 160,000–190,000 BTUs (providing 7–8 GPM at moderate rise)

For families in cold areas, gas tankless water heaters are usually better options because they maintain high GPM at steep temperatures.

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Large Household Needs

If you have 5 or more people at home, you will need severe hot water capacity. Many rain, laundry and all the dishwashing demands at one time can push the demand in double digits. Example landscape:

  • Three showers (7.5 GPM)
  • Kitchen faucet (1.5 GPM)
  • Washing machine (2.5 GPM)
  • Dishwasher (1.5 GPM)

That’s 13 GPM total demand and not unrealistic in a busy household.

For this situation, you’ll need either a high-capacity tankless water heater or multiple units working together. Many families choose to install two tankless heaters in parallel, ensuring one unit doesn’t carry the full burden.

Recommended sizes for large households:

  • Electric: Not ideal—would require very large amperage
  • Gas: 199,000 BTUs or higher (often 9–11 GPM capacity)
  • Multiple Units: Two 160,000 BTU units can provide enough capacity for very large families

In large households, gas tankless systems almost always outperform electric options due to higher heating capacity and efficiency at heavy loads.

Whole-House vs. Point-of-Use Units

Not every house requires a single, whole-home tankless heater. In fact, sometimes small point-use units are more understandable.

  • Whole-House Units: Designed to handle the combined demand of the entire household. Perfect for medium to large families.
  • Point-of-Use Units: Installed near individual fixtures, such as a bathroom sink or guest shower. These small electric units are ideal for cabins, tiny homes, or supplementing a main water heater.

When to choose point-of-use heaters:

  • If you live in a small apartment or tiny home
  • If you only need hot water at one location (e.g., bathroom or kitchen)
  • If your main water heater is far from a fixture and hot water takes too long to arrive

Sometimes, the best solution is a hybrid approach: a large gas tankless unit for the main house and a small point-of-use electric heater for a large gas tankless unit and a distant bathroom or basement kitchenette.

READ MORE: Water Heater Price Comparison

Gas vs. Electric Tankless Sizing Differences

When sizing, it’s important to understand that gas and electric tankless heaters perform differently.

  • Gas Tankless Water Heaters:
    • Higher flow rate capacity (up to 10–11 GPM)
    • Better for large households or cold climates
    • Require venting and a gas supply line
    • Sizes measured in BTUs (British Thermal Units)
  • Electric Tankless Water Heaters:
    • Best suited for small to medium households
    • Easier and cheaper to install
    • Limited to about 4–7 GPM, depending on model
    • Sizes measured in kW (kilowatts)

For example, an electric unit may advertise 6 GPM but that’s usually at a 40–50°F rise. In a northern climate where you need a 70–80°F rise, output may drop to only 3–4 GPM. Gas units, with much higher heating capacity, handle those situations more effectively.

Efficiency and Energy Savings

One of the most common reasons people switch to tankless water heaters is efficiency. But here’s the catch: efficiency isn’t just about the technology itself it’s also about getting the right size for your needs.

If you oversize your tankless water heater, you’ll pay more upfront and possibly lose efficiency because the unit will cycle on and off unnecessarily when demand is low. Think of it like driving a giant truck when all you really need is a compact car. The truck will still get you where you’re going, but it’ll cost you more in fuel.

On the flip side, if you undersize your heater, you’ll constantly push it to its maximum limit. This not only makes showers less comfortable but also puts strain on the unit, potentially shortening its lifespan. An undersized system can also cause temperature fluctuations when multiple faucets are running at once.

When properly sized, tankless water heaters can:

  • Cut energy bills by up to 30–40% compared to tank models
  • Reduce water waste since you get hot water faster at fixtures
  • Last longer (20+ years, compared to 10–15 for tank heaters)

So, the key to saving money long-term isn’t just buying a tankless heater, it’s buying the right size for your home.

Common Mistakes When Choosing a Size

Choosing the right tankless water heater isn’t always straightforward, and many homeowners make mistakes that lead to frustration later. Here are the top errors to avoid:

  1. Ignoring Flow Rate: Some people buy a heater without calculating their household’s GPM needs. If your heater can’t keep up with demand, you’ll get lukewarm showers when multiple appliances are running.
  2. Not Considering Temperature Rise: A unit that works fine in Florida may be completely inadequate in Michigan. Always factor in your local groundwater temperature.
  3. Basing Choice on Price Alone: The cheapest option may not meet your needs. Spending a little more upfront can save money in energy costs and prevent early replacement.
  4. Overestimating “Unlimited Hot Water”: While tankless heaters can provide endless hot water, they can only heat a certain amount at once. That’s why sizing is critical.
  5. Not Accounting for Future Growth: Planning to add another bathroom or a large soaking tub? Size your unit with future needs in mind.

Avoiding these mistakes ensures you won’t regret your purchase after installation.

READ MORE: Is a Water Heater an Appliance?

Professional Sizing Assistance

When you can calculate an increase in flow rate and temperature, many homeowners help consult a professional plumber or installer before taking a final decision.

Why? Because professionals:

  • Measure your exact groundwater temperature
  • Estimate peak demand based on your home’s fixtures
  • Recommend the best unit size for your climate and lifestyle
  • Ensure your home’s electrical or gas system can support the heater

Some manufacturers also offer online sizing calculators where you enter your household size, location, and fixtures, and they recommend the ideal model. While these are useful, a plumber’s inspection is still the most accurate method.

If you’re investing in a system that could last 20 years, a little professional guidance is well worth it.

Maintenance Considerations for Longevity

Properly shaping your tankless water heater is only half a fight. Maintenance is equally important to get the most from your investment.

  • Regular Flushing: Tankless units should be flushed annually (or more often in areas with hard water) to remove mineral buildups from heat exchangers.
  • Descaling: Hard water causes limescale buildup, which reduces efficiency and can overwork the system. Installing a water softener may extend your heater’s life.
  • Filter Cleaning: Many tankless systems include inlet filters to catch debris. Cleaning these regularly prevents clogs.
  • Monitor Performance: If you notice water taking longer to heat or fluctuating in temperature, it may be a sign the unit needs servicing.

Undersized or oversized units can also impact longevity. An undersized heater is forced to run at full capacity constantly, while an oversized one cycles too often. Both situations wear out the heater prematurely. Proper sizing, combined with regular maintenance, ensures your unit lasts 20 years or more.

Conclusion

So, how big of a tankless water heater do you need? The answer depends on two key factors: your household’s flow rate (GPM) and your region’s required temperature rise.

  • Small households (1–2 people) typically need 4–6 GPM capacity.
  • Medium households (3–4 people) usually need 7–9 GPM.
  • Large households (5+ people) may require 10+ GPM or multiple units.

Electric models work well for small to medium homes in warmer climates, while gas models are better suited for large households or cold regions. Choosing the right size not only guarantees endless hot water but also maximizes energy savings and extends the life of your system.

Before purchasing, take time out to calculate your real needs, factor in your climate, and consult a professional if you are uncertain. With the right size tankless water heater, you will enjoy comfort, efficiency and peace of mind for the years to come.

FAQs

1. Can I install two smaller tankless water heaters instead of one big one?
Yes, many household owners install two small units in parallel. This can be more efficient, especially for large homes, and a unit provides excessive surrenders when required maintenance.

2. Do tankless water heaters work in cold climates?
Yes, but you’ll need a higher-capacity unit to handle the greater temperature rise. Gas models are typically recommended for northern regions.

3. How long do tankless water heaters typically last?
With proper maintenance, tankless water heaters can last 20 years or more, compared to 10–15 years for traditional tank models.

4. What size is best for a two-bathroom home?
Most two-bathroom homes need a tankless unit that provides at least 6–8 GPM, depending on climate and usage habits.

5. Can a tankless water heater really supply unlimited hot water?
Yes, as long as the unit is properly sized. The limitation is not total hot water, but how many fixtures can run at the same time without the water cooling down.

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