When it comes to selecting your window air conditioner, choosing the right size is the most important decision you will make. Selecting a unit too small will be too underpowered to cool your room, and you will never reach the cool temperatures you are looking for.

On the other hand, picking an oversized unit will cool the room quickly, but will inefficiently cycle on and off costing you extra money. Window air conditioners also dehumidify the air on top of cooling it. An overpowered unit will run less often leaving more humidity in the air and your room feeling muggy.

Room air conditioners receive a BTU rating, starting from around 5,000 BTUs all the way up to the most powerful around 36,000 BTUs. Once we calculate the square footage of the space we want to cool, we can decide on how powerful of a unit we need to go after. In this article, we will look at measuring and calculating our space below and then compare our calculation to the BTU conversion chart (Shown Below) to determine our BTU rating.

## How to Calculate Square Footage of Your Room

The first step in choosing a room air conditioner is measuring your space to calculate square feet. To get the right number you will need to do some measuring and a little geometry. So, let us cover a couple simple geometric formulas to get us started.

The formula used to calculate the square footage of a typical square or rectangular is going to be the formula used for most rooms. This formula is simple:

*L x B = Square Footage*

### How to Measure a Square Room:

If we have a room that measures 10 feet by 12 feet our formula would be:

*10ft x 12ft = 120 square feet*

### How to Measure a Triangle-Shaped Room:

Our next formula is to calculate the area of a triangle. This will be less commonly used, but might come in handy.

*L x B x ½ = Square Footage*

So for the triangular space pictured above our formula is:

*10 x 12 x ½ = 60 square feet*

### How to Measure an Odd-Shaped Room:

Below we have an odd shaped room that will require a few extra calculations to get the final square footage; we will walk through those steps below:

The first step is measure each wall and map that out as we’ve done below.

Next, we’ll break this room into smaller pieces and calculate the square footage of each of those pieces.

We’ll calculate area “A” first. We know both walls are 15 feet each, so calculating this section is easy.

*15ft X 15ft = 225 square feet*

Calculating “B” will require us to use the same formula. Both walls in this space measure 4.5 feet each. So:

*4.5ft x 4.5ft = 20.25 square feet*

The formula for C is similar but requires us to half our calculation of L x B. It is also important to note that the two “legs” (non –diagonal walls) be used to calculate this formula. So in this instance:

*6 ft x 7.5 ft = 45 square feet*

**Note:** We’re using decimal numbers instead of fractions to make the calculations easier to manage. To convert from inches to decimals measure your space and keep the footage as a whole number. Take your number of inches and divide them by 12. So if you have a wall 10 feet 8 inches it would calculate to: 10 Feet + 8/12 inches (.67) = 10.67 feet

### Putting it Together

Now that we have calculated the area of three pieces of our space, we’re ready to add them up to get our overall square footage of the room.

**Total Square Footage**= Area “A” + Area “B” + Area “C” = 290 Total Square Feet

### Selecting the BTU Rating of Our Room Air Conditioner

Comparing our calculation to the chart below, we see that 290 sq ft comes in at the high end of a 7,000 BTU unit. You will not find a great selection of window air conditioners at the 7,000 BTU Range, but we are very close to the 8,000 BTU range as well, and moving to that BTU range is justifiable in this case.

## BTU Chart Based on Room Size |
|||
---|---|---|---|

Room Size |
BTU’s Needed |
Room Size |
BTU’s Needed |

150 sq. ft. | 5,000 BTU’s | 700 sq. ft. | 14,000 BTU’s |

250 sq. ft. | 6,000 BTU’s | 1,000 sq. ft. | 18,000 BTU’s |

300 sq. ft. | 7,000 BTU’s | 1,200 sq. ft. | 21,000 BTU’s |

350 sq. ft. | 8,000 BTU’s | 1,400 sq. ft. | 23,000 BTU’s |

400 sq. ft. | 9,000 BTU’s | 1,600 sq. ft. | 25,000 BTU’s |

450 sq. ft. | 10,000 BTU’s | 1,900 sq. ft. | 28,000 BTU’s |

550 sq. ft. | 12,000 BTU’s | 2,700 sq. ft. | 36,000 BTU’s |

To determine the most accurate BTU estimate, you should also consider these factors:

**Ceiling Height**

The above estimates assume you have traditional 8-foot ceilings. If your ceilings measure higher than 8 feet, you’ll want to increase your BTU level.**Sunlight**

If your room or space is sunny during the day, increase your BTUs by 10 percent.**Shade**

If your room is shaded for most of the day, decrease your BTUs by 10 percent.**Number of Occupants**

If more than 2 people will occupy your room or space regularly, you should add an additional 600 BTUs of cooling power for each person.**Kitchen:**

If you are installing your window AC in a kitchen, increase the BTU level by 4,000.

One of the most common mistakes in selecting a room air conditioning unit is the purchase of an underpowered unit that will not properly cool the room. With the proper measurements and calculations, you will be able to determine the square footage of your room and make the wise choice for your next room AC.

Hasan says

I have a question. Why we are not considering the room temperature and the temperature outside the room? Besides why don’t we consider the heat generated in the room by some electronic or electrical component?

Hodor says

Square feet. This is very infuriating because I have to convert into metric.

Ted says

not sure i figured sq ft for room a/c correct room is 10 ft high 10 ft long 20 ft wide still can not figure size of window ac i should get

irrelevant says

6000/7000 btu/hr and you should be fine. unless you have a lot of people in the room at once or some sort of equipment that generates a lot of heat, then you may want to up it a little.

Danny says

Awesome thanks for all the helpful information. The only thing is like to see is humidity and how much it effects the situation.

jennifer summers says

My apartment is 700 square feet. According to a chart I looked at, it would require 14,000 BTU’s. I will be using it primarily in my bedroom, so how many BTU’s would be required? It would be for a portable air conditioner.

Thanks much–

Tanisha says

Hi have a one bedroom size 14 by 16 and need help in fining the right air condition for my space

Corey says

I have a living room, kitchen, dining room and tv room that totals 600sq but the rooms are parted off and have wide openings to walk into them. will this be a problem with air flowing threw to cool each room well if I install one ac unit?

Zequek Estrada says

I appreciated the formulas included for how to measure a room. There was one thing that stopped me from getting an air conditioner this past summer. It was that I learned that bigger isn’t always been when it comes to this.

Scott says

I had never considered to calculate the square footage of the area to decide what kind of AC unit I would need. I can see why this would help you choose a unit that would be just right for your situation. I am looking for a way to cool off my kitchen during the summer. I’ll have to take the measurements this weekend and then consult a professional about it.

Murray says

Sorry we live a world of metric measurements Australia gave away feet and inches in the 60’s lets hope Mr Trump can get your country up todate.

PAG says

“when in Rome”, Murray. As far as I can tell, this is a United States based website; therefore, it uses US based measurement. If you wants metric – find an Aussie website.

I don’t go to Aussie websites to look up plants and then grumble about the height and width being in meters.

haron says

I want to install 8pcs of air conditioners in a ware house measuring 60by 40meters with a height of 16 meters how will I lnstall them?

Jenna Hunter says

I never knew that the first step in choosing a room air conditioner is measuring your space to calculate square feet. Our house is pretty big and we had it extended the kitchen about 12 feet a year ago. I will be certain to talk to an expert to make sure we get the right unit for our house!

John says

My wife and I are looking at getting a new air conditioner because our old one isn’t working very well. The article mentions that the size of the space being cooled directly corresponds with the BTU’s per hour a unit produces. Based off the chart provided, I don’t think our old unit was large enough to handle the space we wanted it to cool, so we’ll need to look for a unit with a higher BTU rating.

Mike K says

I’ve got a 350sf room with 15ft ceilings. How many BTU unit do I need?

Mike K says

Second post – my windows open left to right. How do I install a window unit?

Michael O'Brien says

There is no mention about ductless, split system units. Just wondering why as I was under the impression that they were superior to “window” units and I’m actually in the market for one right now. Could you shine some light on that subject please?

Annika Larson says

Our AC unit is quite old and hasn’t been very efficient lately. We are looking to have a new one installed, and we want to make sure we choose the right size. Thanks for the information on calculating room size to determine the capacity needed.

Laura says

Mike, you are probably going to have to get a portable AC unit. I had to buy one for my last apartment which had crank-type Windows. They are much more expensive and a PITA to install as you will have to rig your window with plywood or plexiglass (costs a fortune to get it cut to fit, too!) and the ACs aren’t that good in my (unfortunate) experience. I spent a hot, uncomfortable and expensive summer trying to use that AC unit. You are welcome to my barely used unit! LOL. Good luck with staying coo this summer.

Basheer says

How many tons of A/C required for 280sqmeter ware house

height is 20feet

dee adler says

i’m confused! or perhaps just had a bad AC. my room is 10×21=210. i bought an lg 8,000 btu. it did very little unless u sat right next to it and worse it was sO noisy. horrible decibel level.

i’m thinking about a 12,000 btu but i’m worried about the potential for increased decibel!

James A Friedsam says

Thats messed up on so many levels. Its like you are bashing and insulting this lovley author because she is American. You are so rude!

Joseph says

Hi! I found this guide to be very helpful. I am planning to buy a mini/split for an odd shaped space. One question I have is how many btu to add if the ceiling is above 8 feet? My ceiling is 9ft. 10% also?

Pamela Norris says

I have an attic bedroom, it gets about 100 degrees during the day.. My window is very small, only measuring 18.5 inches wide.. Should I buy a regular window air conditioniner or a portable one.. Ceilings are slanted and low.. Would 7000 BTU work?

Miserable in Indiana

Ravishanka says

I should A/C our lecture room which have 100 people studying in one time…there has 1500 sqare feet in my lecture room..please help me to choose the suitable btu value for that work…

Afton Jackson says

I didn’t realize how important it is to check an air conditioner’s BTU rating to ensure it has the power to cool the square footage in your home. My sister is having a custom home built for her family soon. This article should be able to help her choose an air conditioner that can keep her family comfortable in the summer.

LORI says

Would a ceiling fan help my square footage calculations at all? I have little square footage and high cielings. My it conditioner of 13 years will not cool the kitchen living room square footage as high as my ceiling fan. Thank you so much for your help. I cleanened mine of 12/13 46

Paul says

Coming at this from a different angle. I have a 36ft square room with devices that generate around 8000 BTU. The question is: will i need an A/C Unit or is the room sufficient enough to deal with the heat?