The most common complaint about tankless heaters is not the technology; it is the wrong size. Pick a unit that is too small and the shower turns lukewarm when the dishwasher starts. Pick one too large and you may pay for capacity you never use. Correct tankless water heater sizing comes down to two numbers: how much hot water you need at once (flow rate) and how much the unit must warm the incoming water (temperature rise). This guide shows how to find both and how to read a manufacturer’s capacity chart.
General guidance only. Fixture flow rates, unit capacities and local codes vary. Confirm with the manufacturer’s sizing tools and a licensed professional.
The Two Numbers That Matter
1. Peak flow rate (GPM)
GPM stands for gallons per minute. It is the total hot water flow you expect when several fixtures run together at the busiest time of day.
2. Temperature rise
This is the difference between the temperature of the water entering the heater and the temperature you want at the tap.
Temperature rise = desired hot water temperature − incoming water temperature
A heater has a maximum flow it can deliver at a given temperature rise. As the rise increases, the achievable flow goes down. That is why the same model can handle more fixtures in Florida than in Minnesota.
Step 1: Estimate Peak Flow Rate
List the fixtures you might use at the same time, then add their flow rates. Typical figures are approximate, and your fixtures may be different. Check labels or product specs.
| Fixture | Typical hot water flow (approximate) |
|---|---|
| Bathroom faucet | 0.5 to 1.5 GPM |
| Kitchen faucet | 1.5 to 2.2 GPM |
| Shower (standard head) | 1.5 to 2.5 GPM |
| Dishwasher | 1 to 2 GPM |
| Washing machine | 1.5 to 3 GPM |
| Bathtub filler | 4 or more GPM (varies widely) |
Note that a shower’s flow is a mix of hot and cold water. If you set the shower at a warm temperature, only part of that flow comes from the heater. For safe sizing, many installers use the full flow as a conservative estimate.
Example scenario:
– Two showers at 2.0 GPM each = 4.0 GPM
– Dishwasher running = 1.5 GPM
– Total = 5.5 GPM
If you rarely use all of those at once, you can plan for a lower peak. The question to ask is, “What is the worst realistic morning?”
Step 2: Find Your Incoming Water Temperature
Cold water temperature varies by region and season. Groundwater in warm southern areas may be well above 60°F year-round, while northern areas can see much colder water in winter. Your options:
- Measure it: run a cold tap for a few minutes and check with a kitchen thermometer.
- Use regional or seasonal maps from a manufacturer or your local utility.
- Ask a local plumber, who will know typical winter conditions.
Always size for the coldest time of year. If a unit works in January, it will work in July.
Step 3: Decide on Your Target Temperature
Many households set their heaters at or near 120°F, which is commonly recommended as a balance between comfort and scald safety. Some people want slightly higher or lower. Check local guidelines and consider scald risk, especially with children or older adults. Dishwashers often have their own booster heaters, so check the appliance manual.
Step 4: Calculate Temperature Rise
Using the formula:
Example: Desired temperature 120°F, incoming winter water 50°F.
Temperature rise = 120 − 50 = 70°F
In a milder climate with 70°F incoming water, the rise would be only 50°F, so the same unit could deliver more flow.
Step 5: Match to a Unit’s Capacity Chart
Manufacturers publish tables or graphs showing the maximum GPM at different temperature rises. To use them:
- Find the row or line for your temperature rise (for instance 70°F).
- Read the maximum GPM for the model.
- Check that it is higher than your peak demand.
If you need 5.5 GPM at a 70°F rise, you need a unit whose chart shows at least that at 70°F. If a candidate gives only 4.5 GPM at that rise, either choose a larger unit, install two units, or reduce expected simultaneous use.
Do not compare units by maximum GPM alone. Maximum GPM is usually quoted at a small temperature rise. At a bigger rise, the real capacity may be much lower.
BTU Input: A Quick Cross-Check
Gas tankless heaters are rated by BTU input, which is the heat energy they can burn per hour. As a very rough relationship, heating one gallon of water by one degree Fahrenheit takes about 8.3 BTU, adjusted for efficiency. A common approximate formula is:
Required BTU/hour ≈ GPM × 500 × temperature rise ÷ efficiency
where 500 is approximately 60 minutes times 8.33 lb per gallon. This is an approximation intended only as a cross-check; manufacturer data is more reliable.
Example: 5.5 GPM × 500 × 70 ÷ 0.90 ≈ 214,000 BTU/hour.
That figure may exceed what is available in some single units, which is one reason very large homes sometimes use multiple heaters.
Electric Tankless Sizing: Power Instead of BTU
Electric heaters are sized in kilowatts (kW). The required power is roughly:
kW ≈ GPM × temperature rise ÷ 6.8 (approximate, assuming near-100% electric efficiency; it comes from dividing the BTU figure above by 3,412 BTU per kWh)
For a 70°F rise at 3 GPM, that works out to roughly 31 kW, which is a very large electrical load. In practice, whole-home electric units in cold climates may require very high amperage, and many homes cannot support it without an upgrade. Treat this as a rough check and confirm with manufacturer data. Check your electrical panel capacity and breaker requirements with an electrician. For more on this, see gas vs electric tankless requirements.
Practical Sizing Tips
- Size for realistic peak use, not every fixture at once.
- Consider future changes: a planned bathroom addition, a bigger family or new appliances.
- Think about distance: long pipe runs mean longer waits for hot water regardless of heater size.
- Use low-flow fixtures: reducing GPM at the tap lowers the required capacity.
- Consider two smaller units in large homes, one for each zone.
- Check minimum activation flow: some units need a minimum flow, often a fraction of a GPM, to start heating. Very low flows may stay cold.
Common Sizing Mistakes
- Using summer water temperature
- Reading maximum GPM without checking temperature rise
- Forgetting that gas line size and venting must support a bigger unit
- Ignoring the dishwasher or washing machine
- Letting the installer guess without calculation
Quick Reference Worksheet
- List fixtures used together and add up GPM.
- Choose desired temperature (for example 120°F).
- Identify winter incoming temperature.
- Calculate rise = desired − incoming.
- Check the manufacturer’s capacity chart for GPM at that rise.
- Choose a unit with a comfortable margin.
- Verify gas, venting or electrical requirements.
Gas vs Electric Tankless: Installation Requirements
Troubleshooting Lukewarm Water from an On-Demand Heater
Frequently Asked Questions
What size tankless water heater do I need for a family of four?
It depends on peak simultaneous use and your coldest incoming water, not just household size. Follow the steps above or ask a licensed plumber to calculate.
Is bigger always better?
No. An oversized unit costs more and may need larger gas lines or electrical service. Match capacity to realistic peak demand.
Does cold weather change my required size?
Yes. Colder incoming water raises the temperature rise and lowers the flow a unit can deliver.
Can two tankless heaters work together?
Many installations connect units in series or parallel to meet higher demand. Follow the manufacturer’s instructions.
Conclusion
Tankless water heater sizing is straightforward when you break it down: add up peak flow, calculate the temperature rise for your coldest water, and compare it with the capacity chart. Build in some margin, confirm your gas or electrical supply can handle the unit, and have a licensed professional verify the plan.
