Calculations of the number of British Thermal Units necessary to heat or cool each square foot of a room must also account for the climate zone and the room's insulation, sun exposure and primary function. Some calculations also account for the ceiling height. Divide the final BTU calculation by the total square footage to get the number of BTUs per square foot.
Calculate the square footage
For square rooms, calculate the square footage by multiplying the room's width by its length. For irregular shapes, divide the room into a combination of squares and triangular shapes. Calculate the square footage of each shape and the add them together to determine the total for the room.
Check the climate zone for BTU calculation
Check the climate zone maps provided by the U.S. Energy Information Administration to determine the necessary BTUs per square foot for your specific climate.
Adjust BTUs to account for shade
For heavily shaded rooms, reduce the amount of BTUs by 10 percent. Increase the amount by 10 percent for rooms with heavy sun exposure.
Increase BTUs for multiple occupants
For rooms expected to regularly hold more than two people, increase the number of BTUs by 600 for each additional occupant.
Add BTUs for kitchens
For kitchens, add 4,000 BTUs to compensate for the energy from appliances.
Account for insulation quality
Add BTUs for rooms with poor insulation, and subtract for rooms with good insulation.