Heat loss can be calculated manually or by using an automatic calculator or an online heat loss calculator. The transfer of heat occurs from a high temperature area to a lower temperature area. Heat can be lost through doors, windows, floors, walls and ceilings. Heat loss is measured using British thermal units (BTUs). A BTU is the amount of heat needed to raise 0.454kg. of water by 1 degree Fahrenheit (F). An R-value or R-factor is a conductivity measure of resistance to heat loss.

Calculate wall heat loss rate in BTUs per hour. The equation is "Q" over "t," where "Q" is the area of the wall multiplied by the inside Fahrenheit temperature minus the outside Fahrenheit temperature; "t" is the thermal resistance of the wall. Thermal resistance is calculated by the wall's square feet multiplied by the Fahrenheit divided by BTUs per hour. When calculating, do a separate equation for each wall, ceiling and floor, making adjustments for doors and windows.

Calculate heat loss per day. Do that by multiplying the heat loss rate you found for BTUs per hour by 24 hours. The result is just a calculation of the loss of heat through the walls.

Calculate heat loss per degree day, which is the heat loss per day with a 1 degree difference between inside and outside Fahrenheit temperature. It is calculated by multiplying feet squared by 1 degree divided by the R-factor. This figure is then multiplied by 24 hours per day.

Calculate heat loss for an entire heating season. To begin, estimate the typical heating season, for instance September through April, and calculate the amount of days in the heating season. This information may be obtained from the National Weather Service. Multiply the figure from heat loss per degree day by the number of days in the heating season. The result is a calculation of heat loss for an entire heating season.