Plumbing
Tankless Water Heater Size Calculator
Size a tankless unit from temperature rise and simultaneous flow, which is where most undersizing happens.
Result
Tankless capacity falls in winter because the incoming water is colder and the required rise is larger. Size on the coldest month, not an annual average. Gas units usually need a larger gas line and a different vent; electric units often need service capacity a typical house does not have.
Get a Free Business AuditWhat moves the number
- Temperature rise required, which is the gap between incoming and delivered temperature.
- Simultaneous flow in gallons per minute, not total fixture count.
- Incoming water temperature in the coldest month, which is the design case.
Where this one goes wrong
Sizing on a summer temperature rise. The unit that delivers two showers in August may manage one in February, and the customer calls in winter. Size to the coldest incoming temperature.
Before you quote
- Whether the gas line and meter can supply the input, which is often far larger than a tank unit.
- Whether the venting required is available or achievable.
- For electric units, whether the electrical service can carry the load at all.
Questions
About this calculator
Why do tankless units disappoint in winter?
Because capacity is expressed as flow at a given temperature rise. Colder incoming water means a larger rise, and the same unit delivers considerably less flow at the temperature the customer wants.
Does a tankless unit need a bigger gas line?
Very often. The input rating is much higher than a storage heater, and an existing line sized for a tank frequently cannot supply it. That check belongs in the quote, not in the installation.
Is electric tankless a straight swap?
Rarely. Whole-house electric tankless draws heavily and commonly needs a service upgrade, which can cost more than the unit. Confirm the electrical capacity before quoting.
The formula
BTU/h = GPM x temperature rise x 500, divided by efficiency
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How to use this calculator
- Flow needed at once (GPM). Add the fixtures that realistically run together.
- Incoming water temperature (F) (F). Enter your own figure; the page starts at 50 F.
- Hot water temperature (F) (F). Enter your own figure; the page starts at 120 F.
- Fuel. Enter your own figure; the page starts at Gas.
- Efficiency (%). Enter your own figure; the page starts at 92%.
The result updates as you type, so you can try a figure, see what it does to temperature rise needed, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Flow needed at once (GPM) | 4.5 |
| Incoming water temperature (F) | 50 F |
| Hot water temperature (F) | 120 F |
| Fuel | Gas |
| Efficiency | 92% |
What comes out
| Result | Value |
|---|---|
| Breaker size needed | 0 |
| Gas input needed | 171,196 |
| Temperature rise needed main result | 70.0 |
| Input required | 171,196 |
| If electric | 46.2 |
| Electric current at 240 V | 0.0 |
Raising 4.5 GPM by 70.0 degrees needs about 171,196 BTU/h, which usually means a dedicated gas line larger than the one feeding a storage heater.
How sensitive the answer is
Holding everything else steady and changing only flow needed at once (gpm), this is what happens to temperature rise needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Flow needed at once (GPM) | Temperature rise needed |
|---|---|
| 2.25 | 70.0 |
| 3.38 | 70.0 |
| 4.5 example above | 70.0 |
| 6.75 | 70.0 |
| 9 | 70.0 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Flow needed at once (GPM)
- Add the fixtures that realistically run together.
- Incoming water temperature (F) (F)
- Measured in F, between 33 and 80, starting at 50 F.
- Hot water temperature (F) (F)
- Measured in F, between 90 and 160, starting at 120 F.
- Fuel
- Choose from Gas, Electric.
- Efficiency (%)
- A percentage, between 0 and 100, starting at 92%.
What the results tell you
| Result | What it is |
|---|---|
| Breaker size needed | A number. |
| Gas input needed | A number. |
| Temperature rise needed | The headline result. A decimal figure. |
| Input required | A number. |
| If electric | A decimal figure. |
| Electric current at 240 V | A decimal figure. |
Before you rely on this number
Pipe sizing, flow and pressure figures depend on the actual fittings, the run, the material and local code. Code requirements vary by jurisdiction and override any general calculation, so check what applies where the work is happening.
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