What Size Central Air Conditioner Do I Need?

Buying a new central air conditioner is something most homeowners will have to deal with at least once or twice after they purchase a home. How large of a central air conditioner you choose to install in your home is the most important decision you’ll make next to the overall quality of the unit itself.

If you choose a system that’s too small, it won’t be able to handle the needs of your home. While buying a large unit may seem like a great way to ensure you have enough cooling power, you are essentially throwing money down the drain and wasting energy in the process. 

How big is your home?

If you’re wondering what size central air conditioner you need for your home, you won’t need to call a professional or make an appointment. You will need to do a little math, however, and take some measurements if you’re not sure of the square footage of your home. You will also need to learn a few terms like tonnage and BTU, which we will cover below.

BTUs and Tons

BTU is a term you’ll need to familiarize yourself with, and it stands for British Thermal Unit. BTU measures thermal energy, and lets you know how much energy is needed to cool or heat one pound of water by a single degree.

It’s something you will need to understand when shopping for a new central air unit, just like tonnage. A ton in the HVAC world doesn’t actually refer to weight, although central air conditioner units themselves can be quite heavy. Instead, when you hear “ton” it refers to the cooling capacity of an air conditioner. The important thing to remember is that one ton is the equivalent of 12,000 BTUs per hour of cooling power.

Calculating Square Footage

If you already know the square footage of your home, proceed to the next step. If not, you’ll need to measure the square footage of each room you’ll need to cool. That includes kitchens, bedrooms, bathrooms, and any other “living” areas of your home. If you have a finished basement or attic apartment, don’t forget about those or any area that has ductwork.

Finding the square footage of any room is easy. Just get a tape measure and measure the length and width, and then multiply those figures together. For example, a room that’s 8’ x 10’ would be 80 square feet. Once you have the total for all the rooms, add those numbers together, and you’ll have the square footage for the floor space in your home.

Converting Square Feet into BTUs

Here’s where things can get tricky. You’ll need to take the square footage of your home and multiply by a BTU measurement, which will vary depending on where you live. To find that, you can use an “average” to get a rough range, and most professionals put that number at around 25 to 30 BTU per square foot. By those standards, a home that’s 1,200 square feet at 25 BTUs would be rated for 30,000 BTUs.

This gives you a good range to shop by but doesn’t cover tonnage. To find that number, you simply divide your BTU number by 12,000. That same 1,200 sq. ft. home would need a 2.5-ton central air unit using those calculations, but several other factors come into play as well.

If you live in Texas, your cooling needs will be entirely different than homeowners that reside in Maine. That’s where a climate map comes in handy. There are plenty of BTU zone maps online for the United States, but focusing you’re your county is the best way to narrow things down. Online maps can also be outdated and are subject to change, so contacting your local utility company and asking a few questions could save you a lot of time.

ac btu zone map

Other factors to consider 

Now that you have a number to go by, it’s important to remember it’s just a range, and there are several factors that are difficult for homeowners to account for on their own. 

Is your current ductwork in good shape or wrapped with an outdated material? That’s important, but so is the type of roof you have. Homes built on a concrete slab will have different needs than houses with basements or unfinished crawlspaces as well. Are you thinking about chopping down a large shade tree that’s a little too close to your home? Well, that could affect your cooling as well.

Should I call in a professional?

The only way to get a number that’s truly accurate does involve bringing an HVAC technician to your home. The advantage of hiring a pro is the fact they can accurately gauge your home's needs through a Manual J calculation based on your location, and dozens of other factors the average homeowner will never consider.

That includes things like humidity, your ceiling height, shade trees, and even how many people live in the home.  While that may seem daunting and can be expensive, you can often find deals or discounts through services like Home Advisor and Thumbtack. If you’re considering placing a service call for a professional assessment, our tool can help you find the right professional for the job.


Share Your HVAC Quote/Cost