Heating & Cooling
Furnace Size Calculator
Estimate the BTU input rating for a gas furnace from square footage, climate zone and efficiency.
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Heating & Cooling
Boiler Size Calculator
Get a starting estimate for boiler output in BTU per hour and kilowatts based on heated floor area, your climate and how well the house holds heat.
What is a boiler size calculator?
Boilers are rated by the heat they deliver per hour, in BTU/h or kW. An undersized boiler cannot keep up on the coldest days; an oversized one short-cycles and wastes fuel. This tool gives a sensible starting range for a conversation with a heating contractor.
How does it work?
It multiplies heated floor area by a heat-load factor for your climate, then adjusts for insulation quality. The same method is used by the Furnace Size Calculator for forced-air systems.
What information is needed?
- Total heated floor area in square feet
- Your general climate zone
- A rough idea of how well insulated the house is
How to use the Boiler Size Calculator
- Add up the floor area of every heated room.
- Choose the climate that matches your winters.
- Pick the insulation level.
- Select Calculate and compare the result with your current boiler’s rating plate.
The formula
In plain terms, here is the calculation the tool runs:
Output (BTU/h) = Area × Climate factor × Insulation factor
kW = BTU/h ÷ 3,412
- Area
- Heated floor area in sq ft
- Climate factor
- 30–60 BTU per sq ft
- Insulation factor
- 0.85 good, 1.0 average, 1.2 poor
Worked example
Example
An 1,800 sq ft house in a cold climate with average insulation.
- 1,800 × 50 = 90,000
- 90,000 × 1.0 = 90,000 BTU/h
- 90,000 ÷ 3,412 = 26.4 kW
Look at boilers rated around 80,000–100,000 BTU/h output.
Frequently asked questions
Is boiler size input or output?
Compare output (sometimes called DOE heating capacity or net rating). Input is the fuel burned; output is the heat delivered after efficiency losses.
Should I match my old boiler’s size?
Not automatically. Older boilers were often oversized, and insulation or window upgrades since then reduce the load.
What about radiators?
Radiator output must also match each room’s heat loss, especially with low-temperature condensing boilers.
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