The size of the radiator (or radiators) that you will need to heat your home is determined by a rather complex mathematical formula that takes into account the number of exterior walls in any given room, the type of insulation in those walls, the number of windows in a room, whether you have single or double glazing, the lowest exterior temperatures that are expected and even the type of flooring in your room. While all that needs to be taken into account to get an absolutely accurate size for the radiator you need, there is a rule of thumb that works rather well in most cases and is much simpler.
Find the number of cubic feet of air space in each room that needs to be heated. Measure the height, the width and the length of each room and then multiply the numbers together. For example, if a room is 20 feet long, 12 feet wide and 8 feet tall you would have a volume of 20X12X8 = 1,920 cubic feet.
Multiply the answer you got in Step 1 (1,920 cubic feet) by 148 if it is an upstairs room or by 185 if it is a downstairs room. This takes into account average heat loss through exterior walls and windows. In this example an upstairs room would have a value of 284,160 cubic feet (1920 X 148) and a downstairs room would have a value of 355,200 cubic feet (1920 X 185).
Convert your cubic feet into cubic meters by multiplying the number of cubic feet by 0.028317. In this example our upstairs room with a value of 284,160 cubic feet would have a value of (284,160 X .028317) 8,046.5587 cubic meters, which we could round off to 8,050 cubic meters. Our downstairs room would have a value of (355,200 X .028317) 10,058.198, which we could round off to 10,060 cubic meters.
Purchase a radiator for the upstairs that is rated to heat a room of 8,050 cubic meters and purchase a radiator for the downstairs which is rated to heat a room of 10,060 cubic meters. Your radiator should be rated to heat the volume of air to approximately -5.56 degrees C centigrade, or about 22.2 degrees Celsius.
While the formula used in this article is accurate for most situations, if you have poorly-insulated exterior walls or large single-pane windows or if the exterior temperature drops to below -20F during the winter, you may wish to consult more complex formulas found in Resources below.