Room Air Conditioner Size Calculator

Finding the floor square footage of the area to be cooled takes simple math. Just multiply the full length of the room by its width. ²

If your room has a more complex shape, ask for help. Ask the sales associate where you plan to buy your air conditioner. They can help you find the square footage. ²

ENERGY STAR shows a room air conditioner's cooling capacity in BTUs per hour, so a higher number means the unit can move more heat out of the room every hour. ²

ENERGY STAR publishes a chart. It starts at 5,000 BTUs per hour for the smallest qualifying rooms. It rises to 34,000 BTUs per hour for the largest rooms. ²

Square footage sets the starting point on this chart. Sunlight exposure, the number of occupants, and kitchen use then move that number up or down. The key facts below show how. ²

A unit that is too big will be less effective and offer reduced air conditioner efficiency. If the unit is too large, it will cool the room before it has a chance to remove the humidity. This can lead to discomfort because cool, moist air will make the room feel damp and clammy. ²

Make sure your unit is the right size. This calculator gives you a number. That number is the capacity ENERGY STAR's chart recommends for your room, with adjustments. ²

For example, take a 300 square foot room. It is neither shaded nor sunny. It has 2 occupants and no kitchen use. This room needs 8,000 BTU per hour. ²

Think about where you install the unit. Are you mounting the air conditioner near the corner of a room? Then look for a unit that can send airflow in the right direction. An improperly installed room air conditioner leaks air. It leaks as much air as a 6-square-inch hole. This raises energy costs. It also makes your home less comfortable. ²

On humid days, set the fan speed low. Slower air movement removes more moisture. Use an extra fan to spread the cooled air around. Follow the instructions that come with your unit. Also use the insulation materials included with your room air conditioner. ²

ENERGY STAR certified room air conditioners use better materials. They also come with clearer instructions. These improve the sealing and insulation around the unit. This reduces costly and uncomfortable air leaks. ²

Certified models are at least 19-36% more energy efficient than the minimum federal standards. Look for Energy Saver Mode. This mode limits fan operation when the compressor turns off. This saves more energy. You can override this default setting if you want. A Filter Reminder gives a visual signal too. It tells you when to check, clean, or replace the air filter. This keeps the unit running at high efficiency. ³

A dirty filter slows down air flow. This makes the system work harder to keep you warm or cool. This wastes energy. Improper installation can reduce system efficiency by up to 30 percent. This costs you more on your utility bills. It may also shorten the equipment's life. ⁴

The sources behind this page don't talk about sizing a unit that way. What they do say is that the size chart already builds in adjustments for your specific room: if it's heavily shaded, you reduce capacity by 10 percent, and if it's very sunny, you increase capacity by 10 percent. Beyond that, these sources don't say how to size for a specific number of degrees, so this page won't guess at it.