Article: The radiator you choose has to pump out enough heat to warm the room. Think about the kind of climate you live in. For a climate with frigid winters, such as the area in the US from New England to the upper Midwest, your radiator needs to put out 40 BTU per square foot of floor area. To figure out the floor area, multiply the floor length by its width. For example, a room 25 feet (7.6m) long and 20 feet (6.1m) wide has an area of 500 square feet. You can convert meters to feet by multiplying the number of meters by 3.28. Pick a spot on the wall for the radiator. Take your tape measure and measure out the length and width of this area. Write it down so you can use it for later. Remember, more space means larger radiators, and larger radiators can give off more heat. Find your BTU per hour measurement from earlier. Divide it by the length of your wall space. If you are looking for a vertical radiator, divide the BTU per hour number by the height of the wall space. This gives you the BTU output you need per foot for every hour. Horizontal radiators are measured by length. Vertical radiators are measured by height. Check the packaging of a radiator before you buy it. It'll say how much heat the radiator creates. Try to match the BTU measurement you came up with in the last step. Most likely, you'll have to choose one within 10% of your heating needs. A stronger radiator can be turned down, but a weaker radiator cannot give off more heat.

What is a summary?
Determine the amount of heat you need per hour. Determine the amount of space you have available for a radiator. Calculate the heat output your radiator can provide. Find a radiator within 10% of your measurement.