The proper furnace size can be determined by a contractor using industry-standard calculations, but an estimate can be determined by using a British thermal unit, or BTU, calculator. Free BTU calculators can be found online.
BTUs are the amount of energy needed to heat 1 pound of water by 1 degree Fahrenheit. The correct furnace size will depend on how many BTUs per hour are needed to generate sufficient heat for the home. The online calculators provide an accurate estimate but do not take into account additional factors, such as the size, shape, climate and insulation of the area being heated.