Here’s a handy chart based on information from the cool people over at EnergyStar. It’s not a precise measurement: according to our breezy and cool colleagues over at Consumer Reports, the type of appliance you might need will also depend on what you use the room for, and how many people are usually in it. For example, if you’re buying a unit for a kitchen or a an open-plan room that includes a kitchen, you’ll need about 4,000 additional BTUs to compensate for that whole “cooking” thing that happens in kitchens.
Consumer Reports also offers suggested models for different types of sizes of rooms, and broad ranges of capacities that might work for different room types ranging from tiny bedrooms to huge living rooms.
You’ll need 10% more capacity for rooms that are very sunny, and 10% less if there’s no sunlight coming in the windows at all.