Calculators
Boiler Sizing Calculator
Estimate boiler output needed for space heating from heat loss, and for instant hot water from flow and temperature rise.
- Runs in your browser
- Nothing uploaded
The tool
Enter your numbers below. Results update as you type, and nothing leaves your browser.
Everything you enter stays on your own machine. Nothing is sent to us or to anyone else, which you can confirm in your browser network tab.
What people use it for
- Choosing a combi boiler size
- Replacing an old boiler
- Checking an installer's recommendation
- Planning radiant heating
About this tool
What the Boiler Sizing Calculator does
A boiler must cover the building’s peak heat loss, and a combination boiler must also heat mains water on demand. Hot water usually sets the size of a combi: heating water flow by a temperature rise takes about 500 BTU per hour for each gallon per minute per °F. This calculator compares both and recommends the larger.
It also works as a boiler output sizing calculator or a combi boiler size calculator.
Formula
- Heating load = area × design heat loss per sq ft
- Hot water load = gallons per minute × temperature rise × 500
- Required output = the larger of the two
Worked example
Using the starting values in the calculator above:
| Input | Value |
|---|---|
| Heated area | 1,800 sq ft |
| Design heat loss | 30 BTU/h per sq ft |
| Combination boiler (instant hot water) | Yes |
| Hot water flow wanted | 3 gal/min |
| Temperature rise | 70 °F |
| Boiler efficiency | 95% |
| Result | Value |
|---|---|
| Required output | 105,000 |
| In kilowatts | 30.8 |
| Size set by | Hot water demand |
| Space heating load | 54,000 |
| Hot water load | 105,000 |
| Gas input at this efficiency | 110,526 |
How to read the result
Oversized boilers cycle on and off and run less efficiently; condensing boilers work best with lower water temperatures.
A proper room-by-room heat loss calculation is the right basis for sizing heating.
For planning only. Electrical, gas and refrigerant work must follow current codes and be carried out or checked by a licensed professional.
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Questions
Questions about this tool
The things clients actually want to know — costs, timelines, ownership and what happens after launch.
One that covers your peak heat loss, and for a combi, the hot water flow you want at your incoming water temperature.
Heating mains water instantly usually needs more power than heating the home.
Divide BTU per hour by about 3,412.
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