Here is a quick guide for calculating the BTU per SQ Foot needed to heat or cool a space.
1.) Find the area of the space needed (L= Length, W= Width)
Rectangle = L x W
Triangle = L x W (divided by) 2
Circle= Measure distance from center then multiply by 3.14 (Pi)
2.) Determine the BTU Required
20 BTU x Area = BTU per square foot needed to heat/cool a space. 
-This does not take into account insulation, climate and other outside factors.
Generally, to make this simpler, use our guide below:
100-700 USE OUR 16,000 BTU
700-1000 18,000
1000-1200 21,000
1200-1400 23,000
1400-1500 24,000