How to Calculate BTU for a Garage
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 10foot 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.
The Drip Cap
 BTU, or British Thermal Unit, is a unit used to measure energy requirements for thermal regulation.
 If your ceiling is lower than that, you may not need as many BTUs as this formula will suggest.
 However, if you live up north in colder areas, you might want to use 10.
 In the example, you would multiply 300 square feet by 8.3, which equals a total wattage of 2,490.
References
Writer Bio
C. Taylor embarked on a professional writing career in 2009 and frequently writes about technology, science, business, finance, martial arts and the great outdoors. He writes for both online and offline publications, including the Journal of Asian Martial Arts, Samsung, Radio Shack, Motley Fool, Chron, Synonym and more. He received a Master of Science degree in wildlife biology from Clemson University and a Bachelor of Arts in biological sciences at College of Charleston. He also holds minors in statistics, physics and visual arts.
Photo Credits
 propane heater image by FXTW from Fotolia.com
 propane heater image by FXTW from Fotolia.com
More Articles
 How to Convert From SHGC to Shading Coefficients
 How to Convert RValue to UFactor in Insulation
 How to Calculate Wind & Snow Loads for New Jersey Building Codes
 How to Convert 2X4 Ceiling Tiles to 2X2 Ceiling Tiles
 Government Grants for Insulating Older Homes
 How to Calculate the Size of an Air Conditioner for a Server Room