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Use our Boiler Size Calculator to estimate heat load, boiler output, pipe loss, and fuel input with simple formulas and clear steps.
Need help with boiler sizing? Our Boiler Size Calculator makes it simple. It helps you find the heat output your building needs. You can use it for a home, room, or small building.
Enter your heat loss or basic building data. The tool then works out the heat load. You can also add pipe loss and warm-up needs. This helps you plan for a boiler that fits your space.
A Boiler Size Calculator helps you estimate the heat output your building needs. Think of it like picking the right size coat. A small coat won’t keep you warm. A coat that’s too big may cost more than you need.
The right boiler should meet your heat load on cold days. The answer depends on heat loss, air flow, and indoor comfort needs. Floor area alone may not give a good result.
The calculator offers two ways to find boiler size.
If you know your total heat loss, use this formula:
Boiler heat load (W) = Total building heat loss (W)
To convert kilowatts to watts, multiply by 1,000.
If you know your building data, use these formulas:
Fabric heat loss (W) = UA × Temperature difference
Air heat loss (W) = 0.33 × Air changes per hour × Room volume × Temperature difference
Total heat loss (W) = Fabric heat loss + Air heat loss
Here, UA is the total heat-loss value for walls, windows, roof, and floor. Room volume is in cubic meters (m³). The temperature gap uses degrees Celsius.
For the final boiler output, add any valid extra loads:
Required boiler output (W) = Total heat loss + Pipe loss + Warm-up load
If you also need fuel input, use this formula:
Fuel input (kW) = Boiler output (kW) ÷ Efficiency as a decimal
For example, 90% efficiency is 0.90. Only use extra loads when they apply to your system.
Choose a method. Select known heat loss if you have a heat-load figure. Choose the building-data method if you want to work it out.
Enter your values. Add heat loss, or enter the UA value, room volume, air changes, and indoor and outdoor temperatures.
Add extra loads. Enter pipe loss or warm-up load if needed. You can use a value or a percent.
Add boiler efficiency. Choose the fuel-input option if you want to estimate the input capacity.
Calculate your result. Check the required boiler output. Use it as a guide when you compare boiler models.
Suppose a room has a UA value of 180 W/K. Its volume is 250 m³. Air changes are 0.5 per hour. The indoor temperature is 20°C, while the outdoor design temperature is −5°C.
First, find the temperature gap:
20 − (−5) = 25°C
Next, find fabric heat loss:
180 × 25 = 4,500 W
Now find air heat loss:
0.33 × 0.5 × 250 × 25 = 1,031.25 W
Add both values:
4,500 + 1,031.25 = 5,531.25 W
The room needs about 5.53 kW of heat output under these sample conditions. This is a room heat load, not the size for an entire building.
Our Boiler Size Calculator gives you a quick way to estimate heating needs. It works with known heat loss or basic building data. It can also include extra loads and estimate fuel input.
For the best result, use sound heat-loss data and local design temperatures. A heating expert can check the final size before you buy a boiler.
Find the building heat loss first. Then add any extra loads that apply. The result gives the required useful heat output.
Floor area can give a rough guide. But insulation, windows, air leaks, and local weather all affect heat loss.
Boiler output is the useful heat the boiler can supply. Fuel input is the energy it uses to make that heat.
Yes, when you work out fuel input. Divide the required output by the efficiency as a decimal.
It can help with basic heating estimates. Some systems need a separate check for hot water, steam, or industrial loads.