Wall air conditionre (C) D Friedman Room Air Conditioner BTU Size FAQs
Questions & answers on choosing the size for a room air conditioner

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Air conditioner BTU size questions & answers:

This article provides frequently asked questions and answers about how to choose the right size air conditioner for a room, apartment or building - BTU size requirement FAQs.

This article series provides an air conditioner BTU chart shows how to choose a room air conditioner for window or through-wall mounting. We provide room air conditioner or window air conditioner BTU sizing & choice charts.

We also show how to actually calculate how much BTU cooling capacity you need based on building area or square feet, and we warn about dehumidification problems if you buy an air conditioner that is too big for the space you are cooling.

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BTU Cooling Capacity Questions, Answers, FAQs

Room air conditioners (C) D Friedman

Question: different cooling loads in Trinidad/Tobago's tropical climate - how to size a unit

(Apr 19, 2011) winston said:

I live in a tropical country which is Trinidad and Tobago. Could the size of the unit for cooling the same size room in Trinidaq be different.


Choosing the Air Conditioner Size for a Warm Humid Climate like Trinidad/Tobago:

In a hot humid climate I can understand the temptation to buy a bigger, more powerful air conditionr.

But watch out:
You still want to choose the size of your cooling unit based on room size. As we mention above at "Watch out: Don't Buy an Oversized or "Too Big" Air Conditioner", if you buy a unit that is too big in BTU capacity, you'll find that it cools down the room, but it won't dehumidify adequately and you won't be comfortable. - Editor

Question: Cooling loads in Dubai

(Apr 22, 2011) rashed said:

thank you for the very usfull information

my question is if im living in a country (UAE DUBAI)which is very very hot in summer around 45 c
(which is 110 - 120 f) could you please show me a methode which i can use to callculate the BTU needed accurately for cooling a room (example 20 square feet)

its always sunny and bright in summer
we use cemint bulldings and not wood
the roof is 11 feets
no isulation is usd (only a layer of tar on roof for water proof purposes)

Question: using one compressor for two separated cooling areas

(May 29, 2011) Rick said:

I have an area of 600sq ft. it is seperated as follows: family room (300sq ft), dinning room (200sq ft) and a parlor (100sq ft) which seperates the family room and dinning room.
there is an archway on either side of parlor (aproximately 60 x 78"h). ceiling height is 102".
Q: can one 1500btu compressor run two AC'S at opposite ends?
Q: if one compressor placed at one end, will family room be too cool and dinning room not get enough?


In some product designs (for example Sanyo split systems) one compressor unit may be designed to power two or even up to four indoor air handlers or blower units. The location of the compressor won't be the problem - that unit has the job of delivering liquid refrigerant to the indoor air handler or blower unit.

It's the location of the air handlers or blower units that will determine the adequacy of your room cooling.

Question: cooling requirements in Arizona

(June 11, 2011) Allen said:

I live in AZ. Summers up to 120, humidity only in July/Aug, and want to ac a 720 sq. ft, garage. It has R-18 insulated doors (3), insulated ceiling, and one metal side door, and very small vent window at 8ft. Ceilings are 10ft. Only cold month (40 degree nights) is Dec.

Should I use a window or portable? Which is most effecant and cost effective? And what BTU would get me down to 75-80 degrees in the worst months?

flag like


Allen, from the chart above you probably need 12,000 - 15,000 BTUH. A neat installation would be to frame an opening through the wall and install a through-wall or window-type unit. Only if you want to be able to use the portable A/C unit in another space would I go that route.

Question: cooling loads in teh Caribbean - higher mean heat, increase the BTUh / sqft

(June 25, 2011) Keith Edwards said:

In the caribbean the mean heat is a little higher so the amount of BTU per square foot shoul be increased but by how much

Question: faulty A/C design or layout - engineers got their calculations very wrong

(July 2, 2011) David Owings said:

IAbout 6 weeks ago I had a large server room 100 sq ft X 300 sq ft x 9ft with 180 Large IT server Units commission.

The design spec should have the room temp set at 23 c and alarm set at 25 c, but the room is now reaching temps of 35 c. I have asked the designers and IT engineers how could they have got their calculations so wrong after you all approved the design spec and calculations provided. I also starting to have overloading circuit problems

They are now blaming each other, saying that I will need to reduce the number of units or make the room larger or put in even more AC units.
I have asked designers and IT engineers to provided me with the designed validation and verification sign off and their DFMEA’s

Please can you advise me on possible reasons how they could have messed up so badly and how go about correcting this major problem.
Regards David


David: regarding design errors in air conditioning system layout and specifications, your question is beyond my expertise, and lacks detail that would make an answer possible except for some general speculation. Having recently done some heat loss calculations using a rather detailed analysis provided by Slant Fin I found that

1. the software did not allow input that accurately described the actual R-values of the particular buildings floor, walls, ceilings;

2. we had to guesstimate on solar gain and losses at windows;

3. local design experts at our plumbing supply house felt that the software had over-designed our system by 50%;

Your own design team may not have had adequate or accurate information about the performance of the building. Or there could be an installation or even a maintenance issue. A dirty blocked filter, crimped flex duct section, or duct connection that has come loose, can be all it takes to subvert the system.

Before blaming the design I'd suggest a very expert and thorough complete inspection of the sytsem.

Question: how do I select a refrigeration compressor?

(July 6, 2011) Jignesh Jain said:

Please Give Me Details Of How To select Refrigeration Compressor?


Jignesh Jain: If you are selecting a refrigeration compressor to replace a burned-up unit, you will want the same horsepower unit as the one you are replacing; take a look at the data tags on your equipment or at the installation and service booklet for your unit to see the capacity of the unit you are replacing.

Question: trying to size a condenser

(July 7, 2011) Russ Sutton said:

I am trying to size a condensor unit for a tree cooler and want to know if the size sounds correct to you. The building is 41'x21'x9.6' ceiling. The wall R value is 5.08 (1220 sf of concrete block) and the ceiling is 861 sf with an R value of 20. Floor is 6" concrete on grade. There are no windows and only two entrance doors. The building is quite tight and their is no air exchange/intake except when doors are opened to obtain tree seedlings.

Average outside temp during operation is 65F and inside tree cooler the temp is to be at 34F. The RH inside the cooler should be kept at 95%. According to my calculations (and I am not a ME) I came up with a 1 ton condensor and then sized it to 1.5 tons for opening of doors, etc.

Does this sound adequate? The current 20 year old condensor is about 3.5 tons (RLA = 22.3 amps) and has had problems for years so is being replaced. My opinion is that it was too big and by being oversized it shortened its life. Any advice you can provide would be helpful. Thanks.

Question: can I put in two units to meet total coolingload - two window units?

(July 20, 2011) wilcoss bazile said:

i am in a four rooms appartment and i wonder if since i knwow the total btu i need to cool my entire area which is about 14000 btu can't i just put two 7000 btu windows a/c to sums up the total or its better if i get a single unit with that total?


Yes Mr. Bizile, you could install two 7K BTUH Air conditioners to obtain the total 14,000 you estimate is needed; an advantage is that when you do not need the full cooling capacity of both units it may be more economical to run just one of them; a second advantage is you may be able to get better cool air distribution; the disadvantage is that it may cost more to buy and install two smaller window A/C units than the single larger one.

Question: cooling a two story home

(July 23, 2011) Mike said:

I have an older (1920's) brick home 2 story (very little insulation). I am trying to figure the size AC I should put in for the downstairs, the AC will be in the dinning room/kitchen area but also needs to cool the seperate living room. Rooms are aprox 350 square feet each, with 9" ceilings, dining room attaches to living room by single door that is directly in line with window AC will be installed in.

The dining room gets a fair amount of light (it's a brite room but not excessive)

My plan is to get a 22000 btu unit but I am worried about that being too much. The rooms are currently cooled by a 12000 in the d/k area and a 6000 in the living room. On really hot/humid days 95+ those units will get the house down to maybe 76 degrees.

I really do not think a 18,500 will do the job. Really just want one window unit downstairs instead of two. What do you think? There is also a open stair case in the living room leading to the upstairs


Mike if you want to try just installing one window unit, perhaps a combination of leaving doors open and a few small circulating fans strategically placed will work - or if not you'll know you need a second unit because of the floor layout.

Question: window unit for a backyard shop

(July 27, 2011) ronnie said:

I am looking for a window unit for a "shop" area in my backyard. I am not trying to keep this area sooled but would like to be able to work in the shop and have the temperature at least enjoyable. It is a 24 x 24 with open rafters exposed, wrapped with vinyl siding when built, 2 overhead doors and 1 window and 1 entry door. I would like to be able to turn it on just before work starts and keep it bearable once inside. Any suggestions would be great.


Ronnie you are describing a 576 sqft area that sounds uninsulated, with little solar gain through windows, but significant indoor air exchange with outside whenever you open the garage door.

If you mostly keep the garage door shut and just use the entry door and considering the lack of insulation, you probably want a 12,000 BTUH window unit. I'd rather have the unit slightly on the small side than way too big, both to avoid freezing yourself out and more to avoid inadequate dehumidification that comes from a too-big unit.

Question: what is a tree cooler?

(July 27, 2011) ronnie said:

I am looking for a window unit for a "shop" area in my backyard. I am not trying to keep this area sooled but would like to be able to work in the shop and have the temperature at least enjoyable. It is a 24 x 24 with open rafters exposed, wrapped with vinyl siding when built, 2 overhead doors and 1 window and 1 entry door. I would like to be able to turn it on just before work starts and keep it bearable once inside. Any suggestions would be great.


Sorry Russ, there has been so much help and Q&A activity at InspectAPedia that some queries may have been pushed out of view too rapidly and missed notice. I don't know what a tree cooler is, so I'm not sure we can be of help other than with more general guidance about the concept. If it would help I can give you a link to a technical paper on refrigeration design that is quite thorough.

Question: cooling a room with a high ceiling and one side open to the outdoors

(Aug 30, 2011) Anonymous said:

Is it possible to cool a room that is 12' x 17' with a 10' high ceiling that has one side open to the elements, like a garage.

There will be a 12' x 10' opening all day. People will be eating and socializing in the room so I want to try to keep it cool. I thought a portable unit would be better than misters, particularly as I am in Tampa Fl--high humidity. Can you give any suggestions?



The most effective way you'd cool a room that has one end open to outdoors would be to install an air curtain such as you can find at businesses that cool the building interior while keeping the doors open. i am afraid you'd find it rather costly for a private room.

Short of an air curtain approach, you can install cooling in the room using room or portable A/C units but they will be operating at a near total loss - likely to be very high in energy cost and very inefficient.

Question: do we include basement in area needing to be cooled?

(Aug 30, 2011) Daniela said:

Could you please tell me is we need to include the basement when calculate the total square feet to be cooled?

Our house has one floor open to the basement of 1000 sf per level and a big cathedral entry room which is 9x21 and 16 feet height. What would be your suggestion for the air conditioning capacity that we need to install? Thank you



Normally you would not include the basement in your cooling square feet estimation unless you plan on occupying and thus wanting to cool that space.

But as you'll read in our comments to Anon below about cooling a room open at one end, if your cooled space is open to the basement and you don't plan to install any air curtain nor doorway, and if the spaces are basically joined through a large opening, you will indeed need to consider the basement in your square footage calculation.

Question: problem with over-sized cooling unit

(Dec 1, 2011) Duane K. Sheren said:

A local dealer just installed a 3 ton AC/heat unit for our park model house. The trailer is 12 x 36 feet and the Florida room is 11 x 22 feet.

By my calculation this is 674 square feet. I should have done the research earlier but it seem this unit is way too big for our house. What do you think?


Duane based under 700 sqft, in an insulated conventional structure, you'd figure 13,500 t0 15,000 btuh or about 1 to 1.5 tons. But your installer may figure that your building has minimal insulation and that there is no insulation and a lot of heat gain in the Florida room.

You don't mention what part of the country you're in but that would be a related factor.

If your system is oversized you'll find that it will cool nicely but you won't be comfortable because it won't dehumidify adequately.

Why not give the installer a call and ask their reasoning for a unit this large. Keep us posted - it will assist other readers.

Question: how can I heat and cool an area that's drafty and has one electrical outlet?

(Dec 4, 2011) donna said:

hi, i have a outside storage room (12x17) that i have re-roofed and insulated. it has 2 windows, 35x48 inches, and your standard door opening. it is probably a littly drafty around door and windows.

As i had this room repaired , i decided-- hey, i would like to use it year round for an art studio and/or exercise room/ bridge card room. This room has one regular electrical outlet.

PROBLEM: how can i heat and cool this area most of the months of the year. i live in alabama . please help me if you can. any input you could give me would be greatly appreciated. i have searched the net but am lost! however, i did find your article enlightening thank you again donna

Question: cooling a restaurant, cooling a warehouse

(Mar 22, 2012) jacques said:

I am doing a restauramt of 1200sqf with 50 seat, and commercial kitchen with a exaust fan. The ceiling is of 21foot with all insulation in place in the roof and wall and wall of the partition of the kitchen. How much ac do I need??
Thank you for your help

(Apr 16, 2012) Chandrasekhar said:

Insulated ware house measuring 15 m Length x 8 m width x 6 m Height
Entrance – large door with air curtain

Climate –6 summer months, when daytime temperature go up to 50ºC (122 ºF), night time temperature in summer up to 35 ºC (85 ºF)
Is it advisable to go for a single unit (4 ton / 5 ton A/C) or multiple units (2 or 2.5 ton A/C x 2 units)?

Which combination will be power efficient?


Chandrasekhar, I am not sure, but in general I like the multiple unit approach for operating efficiency - when the cooling load is lower you can run a smaller, more efficient unit.

Ask your HVAC engineer to do the calculations for you, comparing SEER, local energy rates, and the payback period for the added cost of a pair of systems as opposed to one large one.

Question: high humidity and a window A/C

(May 16, 2012) Tom said:

We live near the beach in CA and it does not get over 85 but it gets very humid but we have an old house poorly insulated and late afternoon sun blasts the flat roof house through big windows and it's very hard to get the hot air out.

So the air conditioner is more to get out the hot air and dehumidify. I'm thinking 12,000 BTU's window ac should handle the front of the house, dining room and living room connected by large opening. The square footage is 346. Would 12,000 be overkill?



Tom, indeed it's difficult to vent a flat roof -

but an air conditioner does not "get out" the hot air - in your climate more likely the A/C unit recycles indoor air through the cooling unit until air temp. at the thermostat has reached the set temperature.

If your A/C is too BIG, indeed it won't dehumidify adequately. You'll just have cool damp indoor air.

But if it's way undersized it may not be up to the dehumidifying job either. I'd measure the sqft. and use the smaller end of recommended BTUH capacity for your space, solar gain, and heat loss or gain properties - that is if you have a lot of glass facing the sun you need more capacity.

Question: how can I find out how many tons is my cooling unit's capacity?

(May 19, 2012) Angel said:

how I can find how many tons is my unit?


(May 24, 2012) artemis2854 said:

For Angel--one ton of refrigeration is 12,000 BTUh. You can find this on the faceplate of the unit. On a window unit, it's usually on the side of the unit that's inside the house. Central units are usually on the rear. Hope this helps.

Thanks Artemis.

Use Artemis's tip and take a look at the data tag on your cooling unit.

Question: do we really need to calculate volume or just area to get cooling needs?

(July 3, 2012) abdul qadir said:

u have explained in thist article for a.c. Calculate the s.f. But no need to take area complete volume.l*b*h.


We don't agree Abdul. Calculating the cubic volume to be cooled is important in sizing an air conditioner as is special factors that affect building heat gain such as ceiling shape, cathedral ceilings, dark colored roofs, number of windows, sunlight direction, etc.

Just consider that cooling a room with a 15 foot cathedral ceiling may require more BTUs than a roof of the same square foot floor area that has just an 8-foot high flat ceiling.

(Mar 14, 2013) Mike said:

Since the air to be cooled and dehumidified is a cubic volume of air, why are you measuring the square area of a space to determine the cooling needs? Don't AC units work in 3 dimensions instead of 2? Assuming all rooms have the same height ceiling would seem to be a bad assumption. ceiling heights vary a foot or more in different rooms of my house.

Reply to Mike:


I agree with you that cubic volume makes sense, but that's not how typical BTU guides work. I think it's out of a wish on the part of manufacturers and industry experts to simplify thinking for consumers.

In fact they've got a point of sorts. Cubic volume would still be inaccurate unless it considered additional factors affecting heat gain or loss including not just the presence of windows and doors and types of glazing, but even the shape of the cubic space

. For example a 1000 cubic foot space whose ceiling is flat is not going to have the same cooling properties and needs as one whose cathedral ceiling has a significant slope - all other things being equal.

So we're back to being asked to accept that these guides are a general rule of thumb, not something offering engineering precision. In general we see BTU requirement charts for air conditioners will just consider square feet, with an adjustment for general climate: the BTU chart for buildings in Jamaica should be different from buildings of the same size in Northern Minnesota. - Moderator

Question: split A/C units in Puerto Rico: experts suggest 65-70 BTU/sqft

(Aug 12, 2012) Brad said:

I had A/C units (splits) installed in my house in Puerto Rico. Upstairs bedrooms. Cement structure with flat roof. Louver windows. Boths B/Rs are approximately 270-280 Sq. Ft. They recommended minimum 18,000 BTU in each, and said the 10'3"x27' room should be over 18,000 BTUs....That is 65-70 BTU/sq ft. Why so much!?!


Perhaps local experience with climate and heat gain for your building type?


(Jan 16, 2013) Wally Miller said:

I am heating a 1200 square feet house with a window A/C unit. How many BTUs of heating do I need?

Question: My A/C blows fuses and doesn't cool anything

(1 hours ago) Anonymous said:

I have an office that gets the hot afternoon sun, about 400 sq ft. The air conditioner i have downs blows off if a few lights are on, and doesnt cool anything. I put in 4 double paned windows that helped.but not much. The building is very old. What can i do?


Anon, you need to have a service tech diagnose the problem - it sounds as if the blowing fuse is a clue that may point to a seized compressor - which would also explain lost cooling.

Question: Through-wall A/C units for mobile homes

(Mar 23, 2015) Elyse said:
Can you put a thru the wall a/c unit in a mobile home?



It is physically possible to do so provided there is actually framing inside the wall that is capable of supporting the weight of the air conditioner - the cost will be above installing a window unit that fits since you'll need to cut a wall opening, frame that opening to preserve the structural strength of the wall, and install flashing and sealants so that there are no leaks into the building.

Question: what size of a window unit do i need for a 1200 sq ft room at 60 degrees

(June 4, 2015) Fred said:
what size of a window unit do i need for a 1200 sq ft room at 60 degrees


12,000 - 22,500 BTUs from the table at BTU CHART for AIR CONDITIONERS / HEAT PUMPS but I'm not sure what you mean by 60 degrees.

Question: how to cool a 700 sq.ft. apartment

My apt. Is 700 square feet. I want a window mounted air conditioner that cools the whole apt. What size should I get?



Question: how to position the A/C window fresh air unit vent during the hottest time

(July 26, 2015) janie said:
I would like to figure out what position the vent in my ac window unit should be in during hottest part of day.



During the hottest time of the day you can increase the amount of cooling from a window AC unit by closing off the outside air vent.

Question: filter size for portable A/C units

(Oct 7, 2015) Anonymous said:
What size air filter for a 4000 BTU air conditioner central unit


Anon, filters for air conditioners are matched to the air handler or for portable or window units, they fit either a built-in slot, or the filter is a thin material sized to fit in the clip-in filter opening behind the air conditioner air inlet grille.

Take a look at your A/C unit where the filter is installed and you'll see the size needed. Some portable A/C filters are sold intended to be cut with scissors to fit your unit

Question: Sources for window mount solar air conditioners

(Nov 22, 2015) doug barna said:
please give me imformation on all your window mount solor air conditioning


A number of solar-powered window A/C units are currently on the market, Doug. The units typically run on 48VAC, and units are available with an Energy Efficiency Ratio or EER of nearly 19, with a cooling capacity of 1600 BTUh (850 watts).

Some of these were adapted from units originally made for Recreational Vehicles (RVs) and long distance truckers' semis. Manufacturers include a number of Chinese suppiers, as you'll see below. We have no performance / reliability data on these units.

Take a look at ARTICLE INDEX to SOLAR ENERGY and also see




Where we link to both heating and cooling design guides including solar systems.

Question: Finding old AC parts

(Nov 23, 2015) Morrie said:
I am looking for a circetoard for a airconditoner the identefication NO OLD art is MIRIP14 THERMOCON


Morrie, I'd start a search with the manufacturer's brand and model . Thermocon air conditioners are porduced by Kia whose parts are stocked world-wide

Question: design for mounting the compressor

(Dec 7, 2015) Tambe Vaibhav said:
Can you suggest the design for mounting the compressor of AC?


Sorry, I cannot imagine guessing at a compressor design with no information whatsoever about the air conditioner compressor or other system components. And Sorry but we don't do homework for students - it would not be helping you out.

Question: why are Air Conditioner BTU charts based just on square feet or meters rather than volume?

(Feb 27, 2016) Anonymous said:
I do not think that BTU VS AREA is correct approach. the correct should be BTU VS VOLUME NOT AREA, as cooling/ heating is not for floor but is for space which refers to volume not area

2016/03/14 Anonymous said:
How much BTUs for a room in southern U.S 12x14/x4? Ceilings are not standard. They are taller than 12 feet. So I put 14 to be safe. AC will mostly be used at night. Roof is not insulated. Windows may be leaky.



Originally I agreed with you and certainly for unusual homes with very high ceilings volume needs to be considered. But as most homes in North America have ceiling heights that are rather standard, most HVAC people take the shortcut of just looking at area.

at BTU CHART for AIR CONDITIONERS / HEAT PUMPS you'll also find discussion about calculating BTUh requirements for high ceilings, high heat-gain buildings, and hot climates.

Anon, thank you for the question. Please help us out by telling me what clarifications, changes, or additions to the BTU sizing chart given above would make that more clear and easier for you to read.


2016/03/14 lucy said:
I need a window ac for my 210 sq. ft. room. I have a flat, non insulated flat roof in this area and it is very hot during the summer. The height of the room is only 7 ft. The room is a bedroom with a small bathroom. What size ac do you recommend? Do I have to find something with very low amps also?


Lucy, I'd choose a unit in the 7,500 - 9,000 BTUH range, as indicated in the air conditioner sizing tables at BTU CHART for AIR CONDITIONERS / HEAT PUMPS. If you go too big the A/C will cool but not dehumidify.


Article Series Contents


Continue reading at BTU CHART for AIR CONDITIONERS / HEAT PUMPS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see PORTABLE AIR CONDITIONER BTU CHART for portable room air conditioner BTU sizing advice.


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