Boiler Size Calculator
Estimate preliminary boiler heating capacity, input power, daily energy and operating cost from floor area, ceiling height and adjustable load factors.
Default project preview
87,285 BTU/h
Space-heating load
20.22 kW
Required input power
28.42 kW
Daily input energy
227.4 kWh
Daily energy cost
$27.29
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Heated floor area
Enter this project assumption in ft². The current imperial default is 2000 ft².
Average ceiling height
Enter this project assumption in ft. The current imperial default is 8 ft.
Base heating load rate
Enter this project assumption in BTU/h per ft². The current imperial default is 30 BTU/h per ft².
Insulation adjustment
Enter this project assumption in factor. The current imperial default is 1 factor.
Climate adjustment
Enter this project assumption in factor. The current imperial default is 1.15 factor.
Domestic hot-water allowance
Enter this project assumption in %. The current imperial default is 15 %. Minimum: 0. Maximum: 100.
Capacity reserve
Enter this project assumption in %. The current imperial default is 10 %. Minimum: 0. Maximum: 30.
Boiler efficiency
Enter this project assumption in %. The current imperial default is 90 %. Minimum: 1. Maximum: 100.
Equivalent full-load hours
Enter this project assumption in hours/day. The current imperial default is 8 hours/day. Minimum: 0. Maximum: 24.
Energy price per kWh input
Enter this project assumption in a unitless value. The current imperial default is 0.12. Minimum: 0.
How to read the outputs
Space-heating load
before hot-water allowance. Default preview: 20.22 kW.
Required input power
90% efficiency. Default preview: 28.42 kW.
Daily input energy
8 full-load hours. Default preview: 227.4 kWh.
Daily energy cost
entered energy rate. Default preview: $27.29.
Hot-water allowance
added to space heat. Default preview: 15%.
Combined design factor
height × insulation × climate. Default preview: 1.15.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Base load uses area × entered load rate. Ceiling, insulation and climate factors adjust space heat. Hot-water allowance and reserve are added before dividing by efficiency.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 29, 2026
This calculator compares an entered design heat load with boiler output and efficiency assumptions. It does not replace a building load calculation, radiation survey, combustion design or hydronic-system assessment.
Calculation steps
- 1Establish the building design heat loss.
- 2Add only justified distribution or pickup allowance.
- 3Compare required output—not input—with manufacturer ratings.
- 4Check minimum firing rate, water temperature and emitter capacity.
Worked numerical example
A verified 72,000 BTU/h design load with 10% pickup allowance requires about 79,200 BTU/h net output. At 90% efficiency, the corresponding input is roughly 88,000 BTU/h.
Purchasing and rounding
Boilers are selected by certified input and output ratings plus system compatibility. Excessive oversizing can increase cycling and reduce seasonal efficiency.
Interpret the result
The required output is the useful heat delivered to the system. Input is higher because efficiency is less than 100%.
Page-specific assumptions
- Design heat load is credible.
- Efficiency matches operating conditions.
- Distribution losses are represented once.
- Domestic hot-water priority is assessed separately.
Common mistakes
- Replacing the old boiler size automatically.
- Comparing load to input instead of output.
- Adding multiple safety factors.
- Ignoring minimum firing rate and emitter capacity.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
FAQ
Boiler Size Calculator questions
Why can an oversized boiler be inefficient?+
Oversized boilers may cycle frequently and spend less time operating near efficient steady conditions.
Does this calculate radiator size?+
No. Emitters must deliver the room load at the intended water temperature.
Can domestic hot water require a larger boiler?+
Yes. Indirect tanks and combi systems can impose a different peak demand than space heating.