Measure your room's width. Make the measurement in inches for the most precision.
Measure your room's length, in inches.
Multiply the length in inches by the width in inches to get the total area. For example, if your room is 180 inches by 240 inches, the overall area square inches is 43,200.
Divide the area in square inches by 144 to get the area in square feet. In this example, 43,200 translates to 300 square feet.
Multiply the number of square feet by 20 to get the needed BTUs for the room. If your room is 300 square feet in area, you need an air conditioner rated at 6,000 BTUs.
Things You Will Need
- Tape measure
- If you are in an especially warm area, or if your house is exposed to a lot of direct sunlight, adjust upward the 20 BTU per square foot figure in your calculation. Conversely, if your house is shaded by trees or other objects that block direct light, lower the 20 BTU figure.