Yesterday, I got a call from a homeowner who had just installed a new tankless unit. Nice system. Expensive, too. But there was one big problem.
Two showers ran at the same time… and the hot water turned cold.
Sound familiar?
The issue wasn’t the heater quality. It was the size. The unit was simply too small for the home. I see this mistake all the time in my repair work.
So today I’ll answer the big question homeowners ask me:
“what size tankless water heater do i need?”
Let’s break it down in simple terms.
What Size Tankless Water Heater Do I Need?
| Household Size | Bathrooms | Recommended Flow Rate |
|---|---|---|
| 1–2 people | 1 bathroom | 5–6 GPM |
| 3–4 people | 2 bathrooms | 7–9 GPM |
| 5+ people | 3 bathrooms | 9–11 GPM |
Most homes need a tankless heater between 7–10 GPM.
But the real answer depends on two things:
- Hot water flow rate (GPM)
- Temperature rise
I’ll show you how to figure it out.
Pro Tip: Always size your heater for the busiest time of day, like mornings when everyone showers.
Why Size Matters
- Yesterday I worked on a house with three kids. Morning chaos.
- Mom was showering. The dishwasher was running. The washing machine started.
- The tankless heater was rated for 5 GPM.
- But the home needed almost 9 GPM.
- That’s why the hot water kept cutting out.
- Sizing a tankless unit right the first time saves money, stress, and cold showers.
Pro Tip: Never guess heater size. Always calculate demand first.
Step-by-Step: How to Size a Tankless Water Heater
Here’s the simple method I use on every install.
Step 1: Count the Fixtures
Think about how many things use hot water at once.
Typical flow rates:
| Fixture | Average Flow |
|---|---|
| Shower | 2.5 GPM |
| Kitchen faucet | 1.5 GPM |
| Dishwasher | 2 GPM |
| Washing machine | 3 GPM |
Example:
Shower + dishwasher + washer
2.5 + 2 + 3 = 7.5 GPM
So the heater must support at least 8 GPM.
Pro Tip: If two showers run at once, count both.
Step 2: Check Temperature Rise
Temperature rise means how much the heater must warm the water.
Here’s the simple math.
- Hot water setting = 120°F
- Incoming cold water = 55°F
- Temperature rise = 65°F
- Cold states need bigger heaters because the water starts colder.
- In most US homes the rise is 60–70°F.
Pro Tip: Always size using the coldest winter water temp, not summer.
Tankless Water Heater Size Chart
Here’s a quick guide I share with customers.
| Simultaneous Use | Needed Size |
|---|---|
| 1 shower | 4–5 GPM |
| 2 showers | 6–7 GPM |
| 2 showers + washer | 8–9 GPM |
| Large family home | 10–11 GPM |
If your home has three bathrooms, aim for 9–11 GPM.
Pro Tip: Bigger isn’t always better. Oversized units cost more and waste energy.
Tank vs Tankless: Why Sizing Is Different
Many people replace a 40 or 50 gallon tank heater and assume size works the same.
It doesn’t.
- Tank heaters store hot water.
- Tankless heaters heat water on demand.
- So instead of gallons, tankless systems are rated in GPM (gallons per minute).
- That’s why the question “what size tankless water heater do i need” depends on flow rate, not tank size.
Pro Tip: A 50-gallon tank often converts to about 7–8 GPM tankless capacity.
When It’s Time to Buy a New Tankless Heater
Sometimes sizing isn’t the only problem. The unit may simply be old.
You may need a new one if:
- Water runs cold often
- Heater is 10–15 years old
- Repair costs keep rising
- You added another bathroom
- New tankless heaters are far more efficient.
- Modern models can reach 95% energy efficiency.
- They also last 20 years or more.
If you’re upgrading, look for:
- 8–10 GPM capacity
- Energy Star rating
- Built-in scale protection
Pro Tip: Always choose a unit slightly above your minimum GPM need.
Safety First
Tankless water heaters use gas or high power electricity.
Never install one without:
- proper gas line size
- good venting
- correct electrical setup
Bad installs cause poor heating and can be dangerous.
If you’re unsure, call a licensed plumber.
Pro Tip: Annual flushing prevents mineral buildup and keeps flow strong.
Quick Summary
Let’s keep it simple.
To answer “what size tankless water heater do i need”, check two things:
- Total GPM demand
- Required temperature rise
Most homes need:
- Small homes → 5–6 GPM
- Average families → 7–9 GPM
- Large homes → 9–11 GPM
Size it right and you’ll enjoy endless hot water.
No cold showers. No stress.
FAQ
Most families of four need 7–9 GPM depending on how many showers run at once.
Yes. But the unit must support 6–7 GPM or higher.
Not always. Oversized units cost more and may run less efficiently.
Homes with three bathrooms usually need 9–11 GPM.
Most last 15–20 years with yearly cleaning and maintenance.
