Everything You Need to Know About BTUs and Air Conditioner Sizing

When the temperature soars, your air conditioner may be the only thing standing between you and a sleepless, sweaty night.

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However, if you're in the market for a new window unit or portable air conditioner, you'll need to understand some AC sizing basics.

BTU is the acronym for “British thermal unit,” which is a measurement used to express an energy source’s heat content. Essentially, it represents the amount of heat required to raise or lower the temperature of 1 pound of water by 1 degree Fahrenheit at a predetermined starting temperature. This standard measurement is commonly used in North America to show the heating or cooling capacity of stoves, furnaces and air conditioners. For ACs, BTUs typically reflect the unit’s cooling capacity per hour.

How Many BTUs Are Recommended Per Square Foot?

For effective cooling, Consumer Reports recommends 20 BTU per square foot of space. To calculate the square footage of a room, measure the length and width of the area and multiply these two numbers together. For example, a room that’s 25 feet long and 16 feet wide would be 400 square feet.

If adjacent rooms aren’t separated by doors, the air conditioner you purchase will also need to cool these spaces. Accordingly, the square footage of these attached areas should be added to the total to calculate the number of BTUs needed.

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What Other Factors Can Affect What BTU Rating Is Needed in a Space?

Although square footage may be the most important consideration when purchasing a window air conditioner, the following factors can also affect the BTU rating needed:

• Ceiling height
• Window and door size
• Window thickness
• Insulation
• How much sun and shade the room gets
• How many people regularly use the room
• What the room is used for

For example, a room with high ceilings or a space that’s used to entertain guests may need more powerful air conditioning units, while a heavily shaded room may not require as many BTUs to keep it cool. Certain types of rooms may also benefit from stronger AC units. For example, your kitchen may heat up excessively due to frequent use of a stove and oven, necessitating additional cooling.

How Do I Know What BTU Air Conditioner to Buy?

A room air conditioner’s size reflects its cooling capacity, so it’s measured in BTUs. Portable ACs typically range from about 5,000 to 12,500 BTUs per hour.

To calculate the minimum number of BTUs you’ll need to cool a room, multiply the square footage of the space by 20, with 20 representing the recommended number of BTUs per square foot. For example, using this calculation, the 400-square-foot room mentioned earlier would require at least an 8,000 BTU air conditioner. However, be sure to account for additional factors, such as the ones listed above, which can often add another 600 BTU or more to the required capacity.

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What Does It Mean When an Air Conditioner is Measured in Tons?

The cooling capacity of an air conditioner may also be measured in tons, particularly in larger units. However, that number doesn’t refer to its weight. Rather, it indicates the unit’s capacity for removing heat from a space. Essentially, a ton of refrigeration equals 12,000 BTUs of heat removed per hour, a number derived from the number of BTUs needed to melt a ton of ice over a 24-hour period. In other words, a 2-ton air conditioning unit has a cooling capacity of 24,000 BTU per hour.

How Can I Make the Best Purchasing Decision?

Although it may seem smart to purchase the strongest air conditioner available, a unit that’s too powerful can be as ineffective as one that isn’t powerful enough. By cooling the air too quickly, an oversize unit may not remove enough humidity from the space, causing the room to feel uncomfortably cold and clammy. Additionally, oversize or undersized units aren’t as energy efficient when it comes to cooling, and they can have an adverse impact on your electric bill. That’s why it’s so important to get the right size unit.

If you’re still having trouble choosing the right size unit for your space, many HVAC retailers and sites such as Calculator.net offer complimentary BTU calculators that can help you determine what size central or portable air conditioner you need to effectively cool your house. A local appliance store or HVAC company can also help you choose an appropriately sized and energy-efficient unit.