Every home is different, and with the range of AC units available it’s easy to end up with one that’s not powerful enough to keep your home cool or one that’s too powerful and runs up your energy bill unnecessarily. The most detailed way to figure out the size unit that you need is to have a Manual J calculation done for your home. Many utility companies can provide a free energy audit that will include this Manual J calculation, which takes into account many factors:
Your total square footage.
Your climate zone (based on where you live).
How much natural shade or sunlight you have.
The quality of your installation.
The number of people living in your home.
The type of windows you have.
If you aren’t concerned with going into that level of detail, there are a number of online calculators that can take your home’s square footage and climate zone into account to give you a ballpark size you’ll need.