Kara Zorn on July 25, 2014 58 Comments 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. Learn More: Portable Air Conditioners vs. Window Air Conditioners 5 Tips for Maintaining Your Window Air Conditioner Why a Heat & Cool Window A/C Unit Makes a Good Investment Cooling Capacity Calculator: For Tips for Finding the Right BTU for You
Hasan says September 12, 2015 at 12:04 pm 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? Reply
Hodor says May 24, 2016 at 10:46 am Square feet. This is very infuriating because I have to convert into metric. Reply
David Walls says April 20, 2018 at 8:52 am It’s infuriating because of your unwillingness to accept the situation. Reply
Andrea D says May 31, 2018 at 2:06 am LOL, David Walls, I feel the same way when I seek help and find the answers in metric, I always have to convert using an online calculator. Glad it works both ways, sorry but I never thought of it the other way around, so I’m not being rude – just gave me a chuckle. Have a great day! Reply
Sacul says July 13, 2019 at 10:29 am You said if ceiling is more than 8 which is traditional we must increase btu so my question is by how much? Any formula? Ty for the knowledge mam. Reply
Anthimos says July 4, 2018 at 6:49 pm Go to any other country…it’s metric. Period. The U.S. Congress passed a bill back in 1974 that by 1980 we would adopt the metric system. I guess the American People (most of them are either unwilling to adapt, too lazy, or too stupid)…..the choice is yours. Reply
Charles says October 17, 2018 at 10:26 pm why should we adopt metric? Is your answer because other countries have metric. Sounds like a stupid reason to me, let’s do what everybody else does. Seems like all countries have a way of doing things and people can not just convert, in surgery most all things are metric and being from the US its not so horrible everybody has to stop take off their shoes and calculate or better yet call Anthimos, because he/She is apparently not stupid. I think USA is a good country, yes we have issues like all countries, Corruption, bad Politicians, those who are money hungry. But overall I do not have a issue with USA. Our country doesn’t starve our people to the degree you see in so many others, Medicine is great, we have many good things and suffer from the bad things most countries do, Rich power hungry people who rule. Hey, why not keep our focus on metrics or something stupid and such a burden to think you would need to take a few minutes and convert. Why has the earth grown and nurtured so many big fat entitled punks. I get so sick of listening to so much BS and crap that is so meaningless. If you want to bitch then bitch about child abuse, starvation or things that actually need to be fixed not talked about. Put that into your Calculator conversion and when the answer pops up, take the calculator slightly turn it north and shove it into your cry baby butt. All the way up, don’t hold back. Reply
USAUSAUSA says April 22, 2019 at 8:14 am Says the person too unwilling/fat/lazy to type one number into google at the end of his calculating. You’ll survive through the rage I think. Reply
Sandro says June 11, 2019 at 3:49 pm There are two kinds of countries on this planet. Those that use the metric system… and those that have put a man on the moon. Reply
Dave Barnett says March 5, 2020 at 10:28 pm Same Country with the best actors and film sets LOL GO FX.
Ted says May 27, 2016 at 9:38 am 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 Reply
irrelevant says July 12, 2016 at 3:43 pm 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. Reply
Steven says July 28, 2018 at 8:15 am When calculating sq. footage of a room one only needs to measure the lenght and width of the room. Then multiply them and that’s the sqaure footage. One only needs the height if one wants to determine the total volume or cubic footage. Say your room is 10 ft. long and 20 ft. wide then 10 x 20 ft= 200 square ft. Now multiply the height 200 sq ft. x 10 ft. and 2000 cubic ft. is the volume or the amount of space in the room. BTU will usually apply to rooms with 8 ft. high ceilings so keep in mind when shopping the amount of sapce needed to cool as well as the sq. footage. Hope this has been helpful. Reply
Danny says June 5, 2016 at 10:25 pm Awesome thanks for all the helpful information. The only thing is like to see is humidity and how much it effects the situation. Reply
jennifer summers says October 12, 2016 at 9:08 am 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– Reply
Tanisha says October 21, 2016 at 4:43 pm Hi have a one bedroom size 14 by 16 and need help in fining the right air condition for my space Reply
michael says October 27, 2017 at 4:45 am What part of the country do you live in where it gets 100 degrees in the summer or where it gets 125 degrees like phoenix arizona.That should be figured in with those equations that are slightly underpowered.. Reply
Corey says October 30, 2016 at 9:09 am 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? Reply
Zequek Estrada says January 11, 2017 at 10:27 am 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. Reply
Scott says January 13, 2017 at 12:37 pm 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. Reply
michael says October 27, 2017 at 4:50 am You could buy a mini split a/c system.. That can put the cold part of an a/c system almost any where. But they are more expensive. Reply
Murray says February 13, 2017 at 9:04 pm 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. Reply
PAG says July 8, 2017 at 5:18 pm “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. Reply
Mike says July 9, 2020 at 1:54 pm Maybe we should get the rest of the world to change to our system. I kind of like it. Reply
haron says February 20, 2017 at 3:39 am 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? Reply
Jenna Hunter says March 13, 2017 at 7:39 pm 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! Reply
John says March 20, 2017 at 2:17 pm 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. Reply
Mike K says March 27, 2017 at 1:46 pm I’ve got a 350sf room with 15ft ceilings. How many BTU unit do I need? Reply
Mike K says March 27, 2017 at 1:48 pm Second post – my windows open left to right. How do I install a window unit? Reply
Michael O'Brien says April 12, 2017 at 9:03 am 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? Reply
Annika Larson says April 27, 2017 at 4:47 pm 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. Reply
Laura says April 28, 2017 at 12:33 pm 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. Reply
Basheer says May 1, 2017 at 1:29 pm How many tons of A/C required for 280sqmeter ware house height is 20feet Reply
dee adler says May 5, 2017 at 3:57 am 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! Reply
James A Friedsam says May 13, 2017 at 5:10 am 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! Reply
Joseph says June 7, 2017 at 8:32 am 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? Reply
Pamela Norris says June 11, 2017 at 9:32 pm 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 Reply
Ravishanka says June 26, 2017 at 7:30 am 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… Reply
Afton Jackson says July 3, 2017 at 10:30 am 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. Reply
LORI says July 8, 2017 at 4:55 am 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 Reply
Paul says July 18, 2017 at 8:19 am 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? Reply
Danny Martt says August 7, 2017 at 11:59 pm Just measure the length of the room by the width of the room to get your square feet. The formula is usually done with 8 ft ceilings you add I think about 1000 BTUs for the extra two foot of heights. I will take your measurements and tell you what size you need to get Reply
Danny Martt says August 8, 2017 at 12:05 am You have basically 200 square foot room. It said you because you have over 8 ft ceilings a 6000 BTU air conditioner should be perfect Reply
Peter Battin says September 4, 2017 at 1:38 am The type of unit you need is called a “casement” air conditioner. Not many models available but they do exist Reply
Linda says October 24, 2017 at 9:56 am I live in United States and Most of us are happy we have feet and inches not meters. Just because the world order would like for everyone to do that doesn’t mean that we have to. Reply
Kiril says November 19, 2017 at 3:00 pm My apartment is 700 sq feet with two bedrooms is it enough one big ac or I should take two or three smaller? Reply
Pete says January 5, 2018 at 10:38 am I have a metal sleeve that is 25″ wide and 15″ high that the AC slides into in the wall. Where do I find replacement AC units? The area is about 750 sq ft. Thanks Reply
Jon says March 31, 2018 at 4:34 am In your example you forgot to divide Area C in half. It should be 22.5 square feet. So your total square footage is actually 267.5 feet. Thanks for the chart! Reply
Brien Kroeger says April 2, 2018 at 7:00 pm I am seeking to cool 4 rooms. One is a great room @ 20 x 25 x 19′ ft at the peak with a ceiling fan mounted there. There are two other rooms @ 12 x 13 x 9 and one other @ 7 x 12 x 9. There are no doors to shut these rooms off to air circulation. I like the “through the wall” mounted units commonly seen in restaurant dining rooms and in motel rooms. What size unit would you recommend for this configuration? I don’t require the heating option only the cooling phase. Reply
jimmy says May 20, 2018 at 5:54 pm one thing is not considered in this artical and its very important the height of the room Reply
Robert DeGroat says July 28, 2018 at 10:48 pm I have a 480 sq ft area, with cathedral ceiling, the center being 22 ft high and a nine foot tall center divider in the room, creating in effect two rooms. Will one window unit, mounted in the top of a window cool all, and what size would be best. Also I live on the Gulf coast. Reply
Marifatullah Farooqi says January 14, 2019 at 2:02 pm Thank you from your useful information . Thank you so much. Reply
Big daddy in florida says January 15, 2019 at 7:38 pm If you don’t know what size unit to install just pay someone else that knows what they are doing to do it for you.There are many other factors involved in choosing the right size a/c unit for the space that you need to cool other than just Sq. Footage or area.Insulation R value in walls and ceilings.Windows,doors and non insulated glass.Cubic ft of air if ceilings higher than 8 ft or vaulted.Where the return(s) are located.How many rooms.Thats why in a new house you have to have Energy Calculations performed by an expert to determine what size unit for your particular house or situation.They add up all these factors for you to determine the right size unit and where to install it before you can even get an a/c permit.Average is 500 Sq ft for 1 ton (12000) Btu.The average 10x12x8 ft. Ceiling room with insulated walls and ceilings should cool easily with a 5000 btu window unit.The same room in an attic without insulation and 2 windows would be around 10000 btu.The unit would probably never be able to keep up in the summer and end up burning up anyway because the room was never set up to run a/c in it in the first place.Insulation is the 2nd biggest factor in a/c sizing that most people ignore and find out the hard way in electric bills and burned up a/c units.Oversizing is just as bad or worse as undersizing the unit, it won’t run long enough to pull out the humidity and just pump in cool air without removing all of the humidity in the air and the a/c unit won’t cycle long enough to do it resulting in a cool but humid space.Window units (shakers)are made for 1 unit per room and you need 1 for each different size room you want to cool.1big unit won’t cool an entire 2 bed 2 bath apt. That’s where central air or multi zone ductless mini splits come in.Ductless Mini splits are the best answer to window units and are fairly easy to install and more efficient than window units and cost less than central air. Reply
[…] the window and screw it in to make sure it is secure before you start using it too. However, the HVAC system gets installed in the backyard or on the side of the building, so you never have to mess with it […] Reply
