BTU, or British Thermal Unit, is a unit used to measure energy requirements for thermal regulation. If you took one pound of water and raised its temperature by one degree Fahrenheit, you would have used one BTU of energy. Air conditioners and heaters use units of BTU to describe their cooling or heating capacity. Calculating the required BTU for heating or cooling your garage will ensure you purchase an adequate unit to address your needs.
Calculate the total square footage of your garage by measuring the length and width with a tape measure and multiplying the figures. As an example, a 15 x 20 foot garage would have an area of 300 square feet.
For this calculation, the formula assumes an average garage ceiling height of approximately eight feet. If your ceiling is lower than that, you may not need as many BTUs as this formula will suggest. Likewise, if your ceiling is higher than eight feet, you might need more. You can adjust the final estimate by adding or subtracting nine percent per one foot of ceiling height. For example, if you had a 10-foot ceiling, you would increase the estimated BTUs by approximately 18 percent. You can also use an online calculator for greater precision.
Multiply the square footage by the approximate wattage required per square foot. A good average is 8.3. However, if you live up north in colder areas, you might want to use 10. Warmer areas down south may only require 7.0. These figures assume FHA Standard insulation. In the example, you would multiply 300 square feet by 8.3, which equals a total wattage of 2,490.
Convert the total wattage into BTUs by multiplying by 3.412. In the example, your total BTU requirements would be 8,496.