Radiator Size for a Room

• 1). Calculate the volume of the room you want to heat by multiplying the length times the width times the height. Those dimensions should be in feet, and the resulting volume will be expressed in cubic feet.
  
  
• 2). Adjust the numbers for the use of the room. Multiply the cubic footage by 5 if the room will be used as a dining room or lounge. Multiply by 4 if it will be used as a bedroom. Multiply by 3 if it will be used as a common area or kitchen.
  
  
• 3). Adjust for the construction of the room. If the windows are double-glazed, subtract 10 percent. If the room faces north, and therefore receives minimal sunlight, add 15 percent. If the room has French windows, add 20 percent.
  
  
• 4). Determine the size of radiator you need. The number you produced by doing Steps 2 and 3 is the necessary Btu-per-hour output of the radiator.
  
  

Radiator Size for a House

• 1). Repeat the calculations from the first section for each room in the house.
  
  
• 2). Add the heating requirements of all the rooms together.
  
  
• 3). Add 30 percent to the total calculated in Step 2 -- 20 percent for the central water heater and 10 percent for general heat losses -- to determine the total capacity a central radiator would need.