The size of a radiator will determine its capacity to produce heat, which is measured in British thermal units (Btu) per hour. The amount of heat you need will depend on the structure of the house, the size of the room and how you are going to use the room. You do not want to get a radiator that is too small, lest you not have enough heat, nor one that is too large, lest you waste money on excess capacity. You can size a radiator for either one room or an entire house.
Things You'll Need
 Measuring tape
Radiator Size for a Room

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.

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.

Adjust for the construction of the room. If the windows are doubleglazed, 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.

Determine the size of radiator you need. The number you produced by doing Steps 2 and 3 is the necessary Btuperhour output of the radiator.
Radiator Size for a House

Repeat the calculations from the first section for each room in the house.

Add the heating requirements of all the rooms together.

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.