What Size Fire Do I Need (Calculating BTU)

 fireplace.co.uk What Size Fire Do I Need (Calculating BTU)

Calculating Amount of Heat required (BTU)

You will need 1 kW of heat for every 15 cubic meter of space to be heated. To calculate what size of fire or stove you will need, measure the width, length, and height of the room or space and multiply the three figures together to obtain the cubic area of the space to be heated. For example, if the width of the room is 4m, the length 8m and the room is 2.8m in height the cubic area or total volume of the room is:

Cubic area = Volume m3 = Length x Width x Height = 4m x 8m x 2.8m = 89.6 cubic meters.


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Consideration of different levels of insulation

Once you have calculated the cubic area of the space to be heated you then need to determine the level of insulation within the house and room.  If the house is a new build, built to modern building regulations, with good insulation and only prone to minimum heat loss and is fitted with double glazing, insulated cavity walls, a minimum of 100mm of roofing insulation, good draught proofing, and floor insulation then divide the volume of the space by 25. If the house and room has average insulation, some double glazing and some insulated cavity walls etc then divide the volume by 15. If the home is poorly insulated with little or no double glazing then the volume of the space to be heated needs to be divided by 10 or use the calculator provided below.       

Level of Insulation


Volume Meter Squared/25 = kW Output


Volume Meter Squared/15 = kW output


Volume Meter Squared/10 =kW Output 

For example one well-insulated 1,000-square-foot home in England needs around 24,000 BTUs to heat in winter.


The British Thermal Unit, or BTU, is an energy unit. It is approximately the energy needed to heat one pound of water for 1 Fahrenheit. 1 BTU = 1,055 joules. 1BTU/hour = 0.293 watt. 3412.14163 BTU's = 1kWh.

 Please note if the fire is to be used as a secondary heat source to central heating etc you can subtract the heat output of the radiator from the total kW for the space or room. For example if the room requires a heat output of 7 kW and the radiator heat output is 2 kW you will only need a fire with a heat output of 5 kW. 


By Phil Cleaver


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