Read time: ~8–10 minutes
Tools you can use now: Water Heater Size Calculator | Drip Water Waste Calculator
Picking the right tankless water heater size matters. If the unit is too small, the water turns lukewarm. If it is too big, you may spend more than you need. This guide makes it easy. We use simple words and clear steps. You will learn how much hot water you need, how warm you want it, and what size fits best.
Want quick answers? Try our Water Heater Size Calculator. It does the math for you.
What Is a Tankless Water Heater?
A tankless water heater is also called an “on-demand” water heater. It does not store hot water in a big tank. Instead, it heats water as it flows. Turn on a faucet or shower, and the unit heats the water right then. When you turn it off, it stops heating. That is it. Simple.
- Tank style: stores 30–80 gallons and keeps it hot all day.
- Tankless style: no tank. It heats only what you use.
Tankless can give you steady hot water and save space. But you must size it right.
Why Sizing Matters
Two things set the size you need:
- Flow rate (GPM): how much water you use at one time. GPM means “gallons per minute.”
- Temperature rise: how many degrees the unit must heat the water.
If the unit is too small, water can be cool, or the heater may shut off when many fixtures run at once. If it is too large, you pay more than needed. The right size gives comfort, steady heat, and better bills.
Step 1: List the Fixtures You Use at the Same Time
Each fixture has a typical flow rate. Add up the ones you use together during your busy time (your “peak”).
| Fixture | GPM |
|---|---|
| Standard shower | 2.0–2.5 |
| Low-flow shower | 1.5 |
| Bathroom faucet | 1.0–1.5 |
| Kitchen faucet | 1.5–2.2 |
| Dishwasher | 1.5–2.0 |
| Washing machine | 2.0–2.5 |
Example: One shower (2.0 GPM) + kitchen sink (2.0 GPM) = 4.0 GPM.
Not sure how to add it up? Use our Water Heater Size Calculator. It asks a few simple questions and gives you a GPM number to aim for.
Step 2: Find Your Temperature Rise
Next, figure out how much the unit must heat the water. That is the “temperature rise.”
- Find your incoming water temperature (cold water coming into your home).
- Pick your desired hot water temperature (most people like 110–120°F).
- Subtract: hot temp minus incoming temp = temperature rise.
Example: Incoming 60°F, desired 120°F → rise is 60°F.
In warmer places, incoming water may be 65–75°F. In colder places, it can be 40–50°F. Colder water means a bigger rise and often a bigger unit.
Step 3: Match the Two Numbers
Tankless models list how many GPM they can heat at different rises. A small unit may give 5 GPM at a 35°F rise, but only 2.5 GPM at a 70°F rise. So you must use both numbers together: your peak GPM and your temperature rise.
The easiest way to check this is our Water Heater Size Calculator. It guides you with simple steps.
Quick Guide by Home Size (Approximate)
Assumes about a 60°F rise. Your needs may differ.
- 3–4 GPM: small home or condo
- 5–7 GPM: medium family home
- 8–10 GPM: large home or light commercial
- 11+ GPM: large commercial or many fixtures at once
Real-World Examples
Small Family Home
Peak: 1 shower (2.0 GPM) + 1 sink (1.5 GPM) = 3.5 GPM. In a warm area (rise ~50–60°F), a 4–5 GPM unit may work.
Large Family
Peak: 2 showers (4.0–5.0 GPM) + dishwasher (1.5–2.0 GPM) = 6.0–7.0 GPM. In a cooler area (rise ~65–75°F), look at 8–10+ GPM.
Small Office
Peak: Break sink (1.5 GPM) + 2 bath sinks (3.0 GPM) = 4.5 GPM. A mid-size unit can fit well.
Gas vs. Electric: Does It Change Sizing?
Both can work well. Many gas units can reach higher GPM at big temperature rises. Electric units can be great for smaller loads or warmer climates. Electric may need a panel upgrade. Gas may need the right venting and gas line size. A licensed pro can check your setup.
What About Multiple Units?
Some homes or businesses use more than one tankless unit. You can run them in parallel to meet high demand. This is common for gyms, salons, restaurants, or large homes that need many showers at once.
Recirculation Loops and Sizing
A recirculation loop brings hot water to fixtures faster. It can cut the wait at far sinks. It does not change the basic sizing math much, but it can improve comfort and reduce waste. If you are fighting slow hot water, ask about this add-on.
Watch Out for These Common Mistakes
- Guessing: Do not buy “whole-home” on the box alone. Do the math.
- Ignoring peak use: Mornings with two showers can strain a small unit.
- Forgetting cold water: Colder regions need more power for the same GPM.
- Not planning ahead: Adding a bath later? Size with that in mind.
- Skipping a pro check: A quick visit can confirm venting, gas, or electric limits.
Use These Free Tools
- Water Heater Size Calculator — Find your GPM needs fast.
- Drip Water Waste Calculator — See how small leaks waste water, energy, and money.
Leaks raise your hot water use without you knowing it. If a faucet drips, your heater may run more than it should. Check your home with the drip tool above.
Tankless vs. Tank: How Sizing Is Different
- Tank sizing: based on storage gallons and recovery rate.
- Tankless sizing: based on flow rate (GPM) and temperature rise.
Tankless matches the real way we use hot water. You heat only what you use, when you use it.
Simple 5-Step Checklist
- List fixtures you use at the same time.
- Add their GPM numbers to find your peak.
- Estimate your incoming water temperature.
- Pick your desired hot temperature (often 110–120°F).
- Use the calculator and compare units by GPM at your rise.
Need help? Contact Finish Line MEP. We size, install, and service tankless systems.
FAQ: Tankless Water Heater Sizing
How many GPM do I need for a family of four?
Many families of four need about 6–8 GPM, but it depends on how many fixtures run at once and how cold your water is. Use the calculator to get a better number for your home.
Can a tankless run two showers at the same time?
Yes, if the unit is sized for it. Two standard showers can use 4–5 GPM. Add a little extra for sinks or a dishwasher that may run, too.
What temperature should I set?
Many people set 120°F for comfort and safety. Some go a bit lower to save energy. If you have a mixing valve, your plumber can help fine-tune it.
Do I need gas, or can I use electric?
Both work. Gas can handle high GPM and big rises. Electric can be great for smaller loads or warm areas. Your panel or gas line size may set your choice. A pro can check.
Does hard water change the size I need?
Hard water does not change the GPM you need, but it can affect long-term performance. Ask about filters or treatments to keep the unit clean.
Can I add a second tankless later?
Yes. Many systems can link two or more units. This helps when you add a bath or want more hot water in the future.
Will a recirculation pump change my sizing?
Not much. It improves wait times but does not change the main sizing math. It is a comfort upgrade.
Putting It All Together
Right size = happy showers, happy sinks, and happy bills. List your fixtures. Add up the GPM. Find your temperature rise. Then match those numbers to a unit. Our Water Heater Size Calculator makes it quick.
Fix leaks, too. A tiny drip adds up fast. Try the Drip Water Waste Calculator to see how much water and energy you could save.
Want a pro to handle it? Contact Finish Line MEP. We will size, install, and support your system the right way.
