What Size Air Conditioner Do I Need? (2023 Guide)

A new air conditioner is no small investment, so it’s important to get the right one. The wrong size can create expensive issues that will quickly make you regret your purchase. 

We at the This Old House Reviews Team researched what buyers need to know about air conditioners, including how to determine the right unit size for your home, how to save money, and when to hire a professional.

Air conditioner size doesn’t refer to the unit’s measurements, but rather its cooling capacity. Air conditioners are measured in British thermal units (BTUs) or British thermal units per Hour (BTUh). One ton of air conditioning removes 12,000 BTUs of heat per hour. Put another way, an air conditioner needs 20 BTU to control each square foot of living space. 

Why Does AC Size Matter?

AC size matters for a few reasons. An air conditioner that’s too small will struggle to cool your entire home. The overall temperature will likely be uncomfortable in warmer seasons, and some rooms may be much hotter than others depending on your home’s ductwork. The unit will run constantly to keep up with cooling the space, which wears out your system and increases your energy bills. 

A unit that’s too big will also wear down quicker because it will frequently turn on and off. It will turn on briefly to cool the space, then back off when the temperature is quickly reached. Operating in quick jolts instead of a steady pace causes unit stress. A unit that’s too large for your home will cool the space too quickly without removing enough humidity from the air, making the atmosphere too cold and clammy and potentially causing mold.

The size of air conditioner you need depends on your home’s square footage. The exception is if you’re purchasing a window air conditioner; see below for those instructions.

For a central air conditioner, you’ll first find your home’s square footage by multiplying the length of each room by its width, then adding them all together. Next, multiply the total square footage by 20 to see how many BTUhs are needed to cool your space. 

Energy Star recommends making the following adjustments to get the best size:

• Reduce capacity by 10% of the room is heavily shaded
• Increase capacity by 10% if the room is very sunny
• Increase capacity by 4,000 BTUs if the unit is used in a kitchen
• If more than two people regularly occupy the room, add 600 BTUs for each additional person.

Air Conditioner Sizing by Home Size

Window AC Sizing by Room Type

Most window AC units have a cooling capacity between 5,000 and 12,500 BTUs. You’ll need to calculate the square footage of the room the window AC will be in to determine what size you need. Don’t forget to include the square footage of rooms that are joined without a door, since the window unit will be cooling that area as well. 

Multiply the length of the room by the width to get the square footage, and do the same for adjoining rooms not separated by a door. Add the square footage up. The total is the square footage your window unit will need to accommodate. 

Here’s how many BTUs you’ll need for different room sizes:

• Small (150–250 square feet): 5,000–6,000 BTUs
• Medium (250–350 square feet): 7,000–8,500 BTUs
• Large (350–550 square feet): 9,800–12,500 BTUs

Square footage isn’t the only thing that affects AC unit size. The following factors can vary the numbers slightly:

• Ceiling height: AC size calculations are generally designed for ceilings that are 9 feet tall, which is the average for homes built today. Add another one or two BTUs if you have a vaulted or high ceiling exceeding 9 feet. 
• Climate: Air conditioning units in warmer climates run more frequently. They turn on more, run more hours in a day, and run more months in a year. A unit with a higher SEER rating is beneficial in these areas.
• Heat-generating appliances: Rooms such as kitchens and laundry rooms have appliances that regularly generate heat, making it harder to keep the room cool. Add another one or two BTUs for these rooms as well. 
• Home facade: Your home’s color and siding play a part in how cool it naturally is. Darker paint colors absorb more sun, as do dense materials such as brick. 
• Insulation: Similarly, the less insulation you have in your home, the more the air in your home will escape, so you may need a slightly higher tonnage.
• Sun exposure: Homes with little shade or that face south or west absorb more sun and take longer to cool, putting more pressure on the AC unit. 
• Windows: The more windows you have in your home and the less energy-efficient they’re designed to be, the more tonnage you’ll need to contain cool air in the space.

BTU stands for British thermal unit. This is the amount of energy that’s required to increase the temperature of a pound of water by one degree Fahrenheit. BTUh refers to British thermal unit per hour, which measures an air conditioning unit’s cooling ability in one hour, also known as tons. You don’t need to be an expert in the science of thermal units, but you will need to calculate these measurements to find the proper AC unit for your space.

SEER stands for Seasonal Energy Efficiency Ratio. A SEER rating is the ratio of the cooling output of an AC system over an average cooling season divided by the total energy used. More simply, a SEER rating represents how much energy and money the unit requires to operate effectively over one year. The less energy the unit uses to produce the right amount of cooling, the higher the SEER rating, and vice versa. 

The U.S, Department of Energy requires a SEER of no less than 14 in northern states and 15 in southern states.

Today’s federal energy standards require higher energy efficiency than in the past, making them cheaper to run. Look for an Energy Star label with an energy-efficiency ratio (EER) of 10 or higher to ensure an efficient unit. The higher that number, the lower your operating costs. 

Aside from that, use the following tips to improve or prolong you unit’s life span and efficiency:

• Regularly clean your filter: Do this roughly every four months for central air filters unless your unit’s instructions suggest otherwise. Window units need to be cleaned monthly. Vacuum any heavy buildup off the filter, then wash it in warm soapy water and let it dry fully before reinstalling it. 
• Use a timer if you have one: Your unit doesn’t need to run at the same level if no one’s home. While you don’t want to let the temperature in your home get so hot your unit has to work hard to cool it back down, timing your home to be a couple degrees warmer when no one’s home and then begin cooling again before you arrive is a good way to lower AC costs.
• Maintain your system: Like all of your appliances, your air conditioning system needs maintenance to perform well and last longer. Keep it clean, store window units in a dry area in winter months, and have a professional AC technician inspect the unit every couple of years to make sure it’s running at peak performance.

There are a few things homeowners should know when choosing the right HVAC contractor. 

• Require proof of insurance and licensure from the Contractors State License Board.
• Ask about experience and training, particularly with the type of unit you have.
• Request a home evaluation first to get an itemized estimate.
• Get references or read online customer reviews.

Lastly, don’t choose by price alone. An expensive contractor can give you a positive or negative experience, as can the cheapest option.

Not getting the right size air conditioner can lead to an uncomfortable home, higher energy costs, mold, or a cooling system that doesn’t last as long as it should. Take your time when shopping for a new air conditioner. Measure your space’s square footage accurately and calculate BTUs carefully. Look at the SEER rating and Energy Star evaluation. If hiring a professional, check their credentials. Taking a little extra time to do the prep work will help you make a better investment.

The This Old House Reviews Team is committed to providing comprehensive and unbiased reviews to our readers. This means earning your trust through transparent reviews and data to support our ratings and recommendations. Our rating system for HVAC providers is on a 100-point system based on seven factors:

• System cost (20 points): We determine each brand’s average price range for a full-size AC or furnace. Most units cost between $3,100 and $8,500. The lower the average price range, the more points a company receives. 
• Efficiency rating (20 points): We consider the company’s best available energy efficiency ratings, namely its highest Seasonal Energy Efficiency Ratio (SEER) and Annual Fuel Utilization Efficiency (AFUE) ratings. The higher the rating, the higher the score.
• Sound rating (10 points): We analyze a company’s lowest possible sound rating in decibels. Companies with lower sound ratings are rated higher.
• Warranty options (10 points): We award the most points to companies with warranties above the average five- or 10-year warranties. We deduct points from companies with warranties that cover three or fewer parts.
• Energy Star certification (10 points): We award more points to companies with Energy Star-certified products.
• Reputation (10 points): We review how each company is scored on the Business Better Bureau (BBB). We deduct points for any alerts or pending lawsuits.
• Customer service and resources (20): We consider each company’s available customer support and resources. This includes whether the company offers a user-friendly product search tool, FAQ section, pricing guides, online manuals, and a mobile app. 

To share feedback or ask a question about this article, send a note to our Reviews team at reviews@thisoldhousereviews.com.