If you’re adding an air conditioner, size is the first question to answer. A professional installer will calculate the number of BTUs necessary if you’re adding a central air system. But if you just want to purchase a window unit, you can find BTU calculators online to make your own estimate. Here’s the information you’ll need for most of them.
First, determine the number of cubic feet in the room by multiplying the length times the width times the height. Then make note of what direction it faces—north, south, east, or west—as well as the number of windows on that wall. Insulation is also a factor, so be ready to give it a rating—something like poor, fair, or good.
Once the calculator determines the number of BTUs needed, resist the urge to go bigger than necessary. It makes the unit cycle on and off more often, which is much less efficient.