How to use an AC Sizing Chart for Window Air Conditioner

How to use an AC Sizing Chart for Window Air Conditioner


When picking a window air conditioner, you might think that it’s always better to go with a more powerful machine. But once you’ve figured out the dimensions of your window and which features you need in a unit, there’s one more measurement to take: BTU.

What is BTU? Though it might sound like a K-Pop act, BTU is short for British Thermal Units, an old timey measurement for heat. A single BTU is defined as the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit. Since the invention of the BTU, scientists have realized that heat is a measurement of energy, so in science class the BTU has been replaced by the all-purpose energy unit of measurement, the Joule. One BTU is equivalent to about 1055 joules. 

But in the AC business, we’re all about tradition, so we’ve stuck with the BTU. When you look at AC units, the BTU they produce will be listed among other specifications. So to return to our initial assumption, why isn’t more BTU always better?

If your AC unit produces too many BTU, it will run on short cycles. Short cycles mean that while your room will cool, it won’t properly dehumidify, leaving you with a clammy, uncomfortable home. Short cycles also constantly repeat the energy spike when your unit turns on, which will actually increase your energy bill. In addition, constantly firing up the compressor for short cycles will burn your AC unit out faster.

Okay! So if too many BTUs are bad, what happens if you pick a machine with not enough? Well, your home will never cool down. Without enough kick, the AC can only cool the area immediately in front of its vents. You might as well not be running anything at all.


Now that we’ve told you the BTU horror stories, let’s go over how to pick the correct amount of BTUs for your home. First, figure out the square footage of the room or rooms you want the machine to cool. Remember, opening and closing doors will affect the functional square footage, so you can modify your floor plan a little bit to optimize your AC unit. If you’ve ever looked up a BTU sizing chart, you’ll see a direct conversion between square footage and the amount of BTUs required.

If only it were simple! Once you’ve found out the BTU equivalent for your square footage, you still need to fine tune the calculation.

The first refinement is ceiling height. Intuitively, with tall ceilings, you have more total area that needs to be cooled. The standard BTU calculation presumes ceilings that are between eight and nine feet. So if your ceilings are higher than nine feet, you need to increase the BTU accordingly. Exact increase amount

After you’ve checked the ceilings, consider what the room is used for. A kitchen is going to have a lot of ambient heat, so you’ll need more BTUs to keep it cool. If you’re cooling a kitchen, machine shop, or home gym, you’ll need to make sure you’ve got the BTUs to counteract that ambient heat. Exact increase amount

The next refinement is figuring out how high the room is off the ground. Ground-level, garden unit and second-floor apartments are all part of the standard BTU calculation. But for every floor past the second, you’ll need to add more BTUs. So while a penthouse might be “cooler” than a garden unit, it’s going to take more power to keep it cool.

Now it’s time to think about sun exposure. Got huge, southern-facing windows? Does your home have “excellent natural light?” As lovely as sunshine can feel, it’s also radiating heat energy. So in order to counteract constant direct sunlight, you’ll need to Exact increase amount

The last bit of fine-tuning in our BTU calculation is occupancy. How many people are in the room you’re cooling? Humans produce a lot of heat. If more than X people are in the room, you’ll need to add X BTUs per person. If you’re having company over, you don’t need to go out and buy a stronger AC unit. But if you’ve got a large family and you all pile into the living room for movie night, then you’ll want an AC that can keep up.


So, it turns out there’s a fair amount of fine-tuning necessary to calculate how many BTUs you need. If you have more questions, our support team is available seven days a week to help you. Remember, when it comes to BTUs, more isn’t always better. Much like Goldilocks, you’re looking for the AC unit that’s just right.