BTU sizing method
The simplified BTU method multiplies your heated square footage by a climate factor that represents the heat loss per square foot in your region. Cold climates use 40 to 60 BTU per square foot, moderate climates use 25 to 40, and warm climates use 20 to 25. Adjustments are made for ceiling height, insulation quality, and window area.
The result is the net heating load. To find the required furnace input BTU rating, divide the net load by the furnace efficiency (AFUE). A 96-percent-efficient furnace delivers 96 percent of its input BTU as usable heat.
Net load = Area x Climate factor x Adjustments
Climate factor is in BTU per square foot per hour and varies by heating zone.
Furnace input = Net load / AFUE
AFUE is the annual fuel utilisation efficiency, expressed as a decimal.