A top air conditioner is no small investment, so it’s worth doing your research to ensure you get the right one. The first step is figuring out what size air conditioner you need. An undersized unit will struggle to cool your space and run up your energy bills.

To help you ensure optimal comfort and efficiency, we researched what buyers need to know about air conditioners. We’ll walk you through how to determine the right unit size for your home, other factors to consider as you shop, and how to save money on your purchase.

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How To Size Your Air Conditioner

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), British thermal units per hour (BTUh), or tons. One ton of air conditioning is equivalent to 12,000 BTUs because that is how much heat it removes in one hour.

An air conditioner generally needs about 20 BTUs per square foot of living space to keep your home comfortably cool.

Why Does AC Size Matter?

AC size matters for three reasons:

• Comfort: An appropriately sized AC effectively cools your space, maintaining your desired temperature for optimal comfort.
• Efficiency: A properly sized AC uses less energy to achieve your desired temperature, saving you money on electricity bills.
• Life span: Operating within its ideal capacity minimizes the stress on an AC unit, ensuring a longer life span and fewer breakdowns.

An air conditioner that’s too small may not be able to cool your entire home. The overall temperature will likely be uncomfortable in warmer seasons, and depending on your home’s ductwork, some rooms may be much hotter than others. The unit will run constantly to keep up with cooling the space, which wears out your system and increases your energy bills. 

However, bigger isn’t always better. A unit that’s too big for your space will turn off and on more frequently, cooling in quick jolts rather than a steady pace. This short cycling can lead to uneven cooling, drafts, humidity issues.. The AC will cool your home too quickly without removing enough humidity from the air. Short cycling also wastes energy, increases wear on the unit and shortens its life span.

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What Size Air Conditioner Do I Need?

The size of the air conditioner you need depends primarily on your home’s square footage.

• Measure the square footage of each room. Measure the length and width of each room in your home. Multiply those numbers together to determine the room’s square footage.
• Calculate your home’s total square footage. Add the square footage of each room to determine the total square footage of your home.
• Determine the number of BTUs needed. Multiply the total square footage by 20 to determine the number of BTUs needed to cool your entire home.

If you need to know the proper ton rating, divide the BTUs by 12,000.

Central Air Conditioner Sizing by Home Size

If you already know your home’s square footage, here’s a quick guide to central AC sizing:

Home Size	AC Size (BTUs)	AC Size (Tons)
700–1,000 square feet	18,000	1.5
1,000–1,200 square feet	21,000	1.75
1,200–1,400 square feet	23,000	1.92
1,400–1,600 square feet	24,000	2
1,600–1,800 square feet	27,000	2.25
1,800–2,000 square feet	30,000	2.5
2,000–2,200 square feet	33,000	2.75

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 and have a cooling capacity between 5,000 and 12,500 BTUs. As with central AC, you’ll need to calculate the square footage and multiply by 20 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 those areas as well. 

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

• Reduce capacity by 10% if 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.

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

Room Size	Square Footage	AC Size (BTUs)
Small	150–250	5,000–6,500
Medium	250–350	7,000–8,500
Large	350–550	9,800–12,500

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Other Factors That Affect Air Conditioner Size Requirements

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. Higher ceilings increase the volume of air that needs cooling, potentially requiring a higher BTU AC.
• Climate: Air conditioning units in warmer climates run more frequently. They turn on more, run more hours per day, and operate during more months in the year. A unit with a higher SEER rating is beneficial in these areas. Milder climates might require less cooling power, meaning you can opt for a lower BTU AC.
• Ductwork: Leaky ductwork wastes energy and reduces cooling efficiency, potentially requiring a more powerful AC to compensate.
• Heat-generating appliances: Appliances in rooms such as kitchens and laundry rooms regularly generate heat, making it harder to keep them cool.
• 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. Trees, awnings, and Trees, awnings, and
• Insulation: Efficient insulation keeps heat out in summer and in during winter, potentially allowing you to size down your AC. A poorly insulated home needs a larger unit to maintain comfortable temperatures.
• Occupancy: Humans generate their own body heat. More people means more BTUs to offset the extra heat.
• Sun exposure: Homes that face south receive more direct sunlight, which means they absorb more heat and take longer to cool than north-facing homes. Less sun exposure translates to a smaller AC.
• Windows: The more windows you have—and the larger and less energy-efficient they arethe more tonnage you’ll need to keep your space cool. South- and west-facing windows have the biggest impact.

Because so many factors can affect the size of your AC, we highly recommend getting a professional opinion. A qualified HVAC technician can perform a Manual J calculation to determine the perfect size for comfort, efficiency, and cost-effectiveness.

To learn more about what goes into properly sizing heating and cooling equipment, check out this video with heating expert Richard Trethewey. Using a model house, Richard explains the effect of insulation, sun exposure, wind, and ventilation on heat loss, heat gain, and HVAC sizing.

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What Are BTUs?

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.

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What Is a SEER Rating?

A SEER rating, which stands for Seasonal Energy Efficiency Ratio, is a measurement of how efficiently an air conditioner cools a space over a typical cooling season. It’s the ratio of the cooling output (in BTUs) to energy input (in watts) over the course of the cooling season. The less energy the unit uses to produce the right amount of cooling, the higher the SEER rating, and vice versa.

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

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How To Save Money on Air Conditioning

Today’s federal energy standards require higher energy efficiency than in the past, making air conditioners cheaper to run. Look for an Energy Star label with an energy-efficiency ratio (EER) of 10 or higher —or a SEER rating of 15 or higher—to ensure an efficient unit. The higher that number, the lower your operating costs. The upfront cost for an AC unit may be high, but you can save money in the long run.

Use the following tips to improve or prolong your unit’s life span and efficiency:

• Regularly clean or replace the 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. 
• Program a schedule or use a smart thermostat. Your unit doesn’t need to run at the same level if no one’s home. While you don’t want to let your home get so hot that your unit has to work hard to cool it back down, a smart or programmable thermostat allows you to lower AC costs by keeping the temperature a little higher while you’re gone. You can set a schedule so that your AC runs less while you are away but cools your home back down before you return.
• Minimize heat gain and air leaks. Opt for light-colored, energy-efficient curtains or blinds to reflect sunlight and reduce heat gain. Reduce the amount of heat generated indoors by turning off lights, appliances, and electronics not in use and using exhaust fans. Seal windows, doors, and any gaps around pipes and wires to prevent cool air from escaping and hot air from entering.
• Raise the temperature and use fans. Ceiling fans create a wind chill effect, allowing you to raise the thermostat setting while feeling cooler. Consider smart fans that integrate with your smart home system. Aim for a comfortable yet slightly higher temperature. The smaller the difference between outdoor and indoor temperatures, the more money you will save.
• Maintain your system. Like all of your appliances, your air conditioning system needs routine maintenance to work 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.

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How To Hire a Professional

There are a few things homeowners should know before calling an HVAC pro. Follow these tips to hire the right HVAC contractor: 

• Require proof of insurance and licensure from your state’s contractor license board.
• Ask about experience and training, particularly with the type of unit you have or want.
• Request a home evaluation first to get an itemized estimate.
• Get references or read online customer reviews.

We recommend getting quotes from at least three reputable HVAC companies before committing to a purchase but don’t choose by price alone. Experienced contractors tend to charge higher rates, so the cheapest option could also be the least qualified. However, a higher price point is not a guarantee of quality. An expensive contractor can give you a positive or negative experience, as can a more affordable competitor..

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Our Conclusion

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. Calculate your home’s square footage and multiply by 20 to get a rough idea of what size air conditioner you need. Then, consult a professional for a more accurate assessment. Check reviews and credentials, shop around to ensure you’re getting the best deal, and compare SEER ratings and Energy Star labels to choose an efficient system. Taking a little extra time to do the prep work will help you make a better investment.

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FAQ About Air Conditioner Sizing

How do I calculate what size air conditioner I need?

To calculate what size air conditioner you need, calculate the square footage of your home and multiply it by 20 to get the BTUs needed. Every 12,000 BTUs equals 1 ton. 

What size room will a 12,000 BTU air conditioner cool?

A 12,000 BTU unit will cool a 400- to 600-square-foot space. 

How many tons AC do I need for 1,500 square feet?

An air conditioner size of 2 tons will cool a space of 1,500 square feet. 

How big of an AC unit do I need for 2,000 square feet?

A 2,000-square-foot space needs an AC unit of approximately 3 tons. 

Our Rating Methodology

We are 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.