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How to calculate radiator size for a room

Updated February 21, 2017

When you calculate the radiator size for each room you need to take into account several different factors. The usage of the room, type of windows and location will all effect the heating requirements of the space. These factors, plus the volume of the room, are what you will need to figure the BTUs (British Thermal Units) of heat that will be needed to keep you comfortable. Once you know the BTU requirements you can shop for the right sized radiator for each of your rooms.

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  1. Find the volume in cubic feet of the room. Measure the length, width and height of the room. Multiply the measurements on your calculator. The total is the volume in cubic feet. For instance, a room that is 10 feet long by 12 feet wide by 8 feet high will have a volume of 960 cubic feet (27 cubic metres).

  2. Convert the volume of the room to BTU according to the room type. If you have measured a bedroom, multiply the volume by 4 (960_4 = 3,840 BTU). If this is a living room or dining room, multiply the volume by 5 (960_5 = 4,800 BTU). If the room is a family room or kitchen, then multiply by 3 (960*3 = 2,880 BTU).

  3. Adjust for special instances to find a more accurate BTU requirement. If the room faces north then add 15 per cent. If you have french windows in the room, add 20 per cent. If the windows in the room are double glazed, deduct 10 per cent from the total BTU as the heat retention in such a room will be greater than normal.

  4. Write down the total BTUs required to heat that room on your pad and give the room a name (ie. Master Bedroom) and repeat these steps for each room. When you are done, you will have a list of radiator sizes to shop for and know where each will go.

  5. Tip

    If the room you have measured has an excessive amount of furniture or is oddly shaped, purchase two or more radiators. Place the radiators around the room so they can radiate heat without being blocked by furniture.


    Not all radiators use the same types of fuel (gas, oil, water). Make sure the radiator you select has a fuel that is safe to use in the room you will be placing it in. A radiator in a child's bedroom that can be knocked over and spill fuel can cause serious injury.

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Things You'll Need

  • Measuring tape
  • Pen and paper
  • Calculator

About the Author

Cassandra Tribe

Cassandra Tribe has worked in the construction field for over 17 years and has experience in a variety of mechanical, scientific, automotive and mathematical forms. She has been writing and editing for over 10 years. Her areas of interest include culture and society, automotive, computers, business, the Internet, science and structural engineering and implementation.

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