How Many BTUs Does My Air Conditioner Need?

“It is hot up in the biznasty,” you may find yourself saying as the heat wave blankets your town in boiling sun rays. Air conditioning is your primary defense against this scourge and you may need to buy a new one or upgrade an old one. The first question is how many BTUs do you need? This handy chart helps you figure that out:

Via Square footage – BTUs

100 up to 150 – 5,000
150 up to 25 – 6,000
250 up to 300 – 7,000
300 up to 350 – 8,000
350 up to 400 – 9,000
400 up to 450 – 10,000
450 up to 550 – 12,000
550 up to 700 – 14,000
700 up to 1,000 – 18,000
1,000 up to 1,200 – 21,000
1,200 up to 1,400 – 23,000
1,400 up to 1,500 – 24,000
1,500 up to 2,000 – 30,000
2,000 up to 2,500- 34,000

If you don’t know your square footage, just multiply the length by the width. If you need to calculate for a triangular area, multiply the length by the width and divide by two.

If the room gets lots of shade, reduce BTUs by 10%.
If it’s very sunny, increase BTUs by 10%.
If the room has more than two people in it on a regular basis, add 600 BTUs for each.

For more info on which A/C to get, check out publishing sibling Consumer Reports air conditioner buying guides and ratings.


Edit Your Comment

  1. headhot says:

    and for every 100 watts of electrical equipment (TVs, computers, etc.) add 350 BTU.

    • Applekid ┬──┬ ノ( ã‚œ-゜ノ) says:

      Square footage isn’t so bad, but with this 100 W : 350 BTU rule I’m all up in the Al Gore energy usage category.

  2. GuyGuidoEyesSteveDaveâ„¢ says:

    I only buy HVAC units that are rated in tons. A nice 6 ton unit will cool your house down nicely.

    • AnthonyC says:

      And a mean one will spontaneously operate reverse-cycle, just to spite you.

    • TouchMyMonkey says:

      That is, central air units, the capacity of which is expressed in tons. Window units, which are the dominant type in places where they are only needed intermittently, like the Northeast, are rated in BTUs.

    • GearheadGeek says:

      You need a pretty big house for a 6-ton unit to be a good choice, though… that’s about the right amount of cooling for a 4,000 square foot house in Texas. If the unit is oversized, it’ll cool the air in the house quickly, but won’t remove enough moisture from the air for good comfort or humidity control, so in most places it’ll be cool and clammy and you’ll grow mold.

    • steve6534 says:

      Just an FYI 1 ton = 12,000 BTU.

    • freelunch says:

      reminds me of “Hands on a Hard Body”… where the east texas guy is explaining how proud he is of the A/C unit he bought from a failed Walmart store… I think he was bragging that he could get the entire house below freezing…

      great movie… The amazing thing is that I most folks I know from the area never heard of it.

      • GearheadGeek says:

        Having grown up in NE Texas, I don’t find King of the Hill amusing at all… just a dumb cartoon about people I grew up with and/or am related to. Maybe that also explains why Hands on a Hard Body wasn’t popular among East Texans?

  3. pantheonoutcast says:

    “If you don’t know your square footage….”

    Seriously? I’ve seen commenters on this site calculate the speed of tachyons.

  4. mergatroy6 says:

    Air conditioning is great until ConEd decides they can’t support the “unusually high” demand and decides to brown out Brooklyn and Queens.

  5. chaesar says:

    I’ve never used the word “biznasty.”

    • SaraFimm says:

      Neither have I. And where I live last year the whole month of July 2009 we were over 100F (17 of them over 110F with a high of 117F on the 17th).

      So tell me about this “heatwave” you’re having this summer–it sounds vaguely familiar to us in the Mohave Desert…

  6. RandomHookup says:

    The secret is to keep your biznasty properly ventilated.

  7. scoosdad says:

    An important thing to remember is not to over-buy on BTU’s. It may lower the air temperature faster, but it won’t extract humidity fast enough and you’ll end up with a cool but still sticky room.

    A feature I always make sure I have on any window unit I’ve bought in the last few years is something that shuts the unit down completely once the desired temperature is reached. Without it, the compressor shuts off but the fan keeps running constantly whether it needs to or not. This feature saves a lot of energy in the long run and keeps things quieter overall. Every unit I’ve bought with this feature also gives you the option to keep the fan running all the time if you like it that way.

    • TouchMyMonkey says:

      I went from two 12,000 BTU units to three 8,000 BTU units, and found that more smaller units work better. They’re easier on the back when they need to be installed, too.

  8. leihei says:

    “”It is hot up in the biznasty,” you may find yourself saying…”

    No, I will never find myself saying that.

  9. Guppy06 says:


    Measuring cooling power in BTUs is like measuring speed in miles. Unless, of course, the manufacturer is trying to say “This unit will only suck 5000 BTUs of heat out of the room before it stops working.” But even then it doesn’t tell you how fast it can move those BTUs.

    BTUs per hour? Per second? Per day?

    • GearheadGeek says:

      BTU/hr is the usual assumption when rating cooling in “BTU”

      • Guppy06 says:

        “Assumption?” That’s a lawsuit waiting to happen. All we need is Sharper Image to sell a ridiculously overpriced gizmo capable of “Over 9000 BTUs” (per day) of cooling.

        I can understand that putting “6000” on the box makes it seem more powerful than “1/2,” but would it kill these people to put in the extra two or three keystrokes to keep their description technically correct and useful?

        I’m tempted to say this ignorance is a shining example of why we need to go metric, but then the question becomes “Watts of cooling, or watts of electricity consumed?”

        • Ron in Missouri says:

          I have always hated Sammy Hagar’s song “I can’t drive 55” because he doesn’t specify the specific rate of speed at which he can not drive. Kilometers per hour? Meters per second? Parsecs per earth day? It’s confusing…a lawsuit waiting to happen.

    • cosmic.charlie says:

      typically it is btu/hr.

  10. AngryK9 says:

    Has anyone else noticed that all of the same people who just a few months ago were whining about it being too cold are the same people who are now whining about it being too hot?

    • blogger X says:

      It happens all the time. I, for one, did not mind shoveling all that snow. Hell, it gave me a good reason to skip the gym!

    • dolemite says:

      I hardly ever complain about it being too cold, but last winter sucked. I don’t recall ever having so much snow (VA)…maybe back in 85 or whenever we had that big snowstorn. It was like 2 feet of snow every 3 weeks…as soon as it melted, here comes another mini-blizzard. And of course I had sold my 4×4 pickup, and my little fwd hatchback with summer tires couldn’t even get out of the driveway. And of course with the ground being so saturated with water, I had water come into my basement like 4x.

    • Anonymously says:

      You can always wear more clothes, but there’s a limit to how much clothes you can take off!

    • Weekilter says:

      They’re weather wimps.

  11. AnthonyC says:

    Well, it’s certainly the *easiest* line of defense, and the most readily adjustable, but it shouldn’t be the first.
    The first should be proper insulation, ventilation, and home design appropriate for the local climate. If you do that, you’ll find you find a heck of a lot less AC than otherwise.

    • Tedicles says:

      Correct…but easier said than done, it is usually cheaper/easier to install an AC than to reconfigure the home (take it from me with 10,000 sq ft with MANY window units, one day it will be fixed, but not yet)

  12. TinaBringMeTheAx says:

    I’m confused: Why are they only concerned with square feet? Shouldn’t it be cubic feet? People with seven foot ceilings need less cooling than those with 20 foot ceilings, no?

    I mean, I know cold air sinks, but still I would think there is a difference.

    • cosmic.charlie says:

      Yes, cubic feet could also be used. But this is just a rough guide. To be precise you only need to cool as much as you are gaining from external sources (sun, conduction from outside, body heat, electronics, fridge, etc).

  13. savashley says:

    “It is hot up in the biznasty”

    That is verbatim what I said when the heat wave hit us this weekend..I was in VA Beach at the time but now back in NC, and THE HEAT IS ON

  14. H3ion says:

    Any idea why they use square footage rather than cubic footage? I would think that a room with ten foot ceilings would be harder to cool than a room with eight foot ceilings.

    • GearheadGeek says:

      I think it’s because most people’s brains begin to overheat when they try to relate to square feet, but begin to bleed when they try to grasp concepts of volume.

    • jeff_the_snake says:

      possibly because that extra empty space is not generating or retaining much heat. you’re right, cubic feet would be a better measurement but since heat rises the square footage works just fine for simplicity’s sake.

  15. dolemite says:

    I’ve got an old central air unit. 2 of my best purchases was a programmable thermostat (around $50), and a small window unit for the bedroom( around $100). The house AC rarely turns on at night because I set the temp to 79, but our bedroom stays a cool 70. Now, when most people are spending $150-$200 on electricity in the summer, ours is usually around $80.

  16. kataisa says:

    Does anybody know how or where to recycle old air conditioners?

    • Kevin says:

      Leave it by the street and somebody with a rusty pickup truck will show up and take it away.

      • freelunch says:

        just don’t be one of those jerks that leaves the empty casing on the curb…. it’s not nice to trick people… *pouts away*

  17. Kevin says:

    Something I like to consider when purchasing an air conditioner is cubic footage rather than square footage. I used to live in an apartment in a building from 1870. It had 10′ ceilings in some places. If you bought an ac to cool a 10×10 room, the chart assumes an 8′ ceiling, or 800 cubic feet. If you have high ceilings, in my case the room was 10x10x10, an AC sized from a standard chart would have been too small.

    Another thing to consider, which may cost slightly more but will be well worth it in comfort, is instead of buying one large AC for a big area, buy several small ACs. Our apartment was about 1,200 square feet, although by the time we closed the doors to the stair wells and closets, we were only had to deal with about 900 square feet. We ran 6,000 BTU units in the kitchen and bedroom and an 8,000 BTU unit in the living room. Despite, the strange floor plan, the entire apartment was kept comfortable with this configuration.

  18. Shonky McShonk says:

    hw bt jst gttng clng fn. ssss. w ddn’t hv c n th trnchs f rq. whn bm hnds ths ngrn slm trrrsts r cntr s f y hv c thn. sft lb scm.

  19. seth1066 says:

    A/C Trivia / Bar Bet

    How many BTU’s does the average automobile put out? Pretty small area, not even 50 Sq. Ft.

    Answer: That 50 sq ft is surrounded by a glass… In excess of 40,000 BTUs.

    • GearheadGeek says:

      On top of being surrounded by glass (and having a very hot engine just on the other side of the firewall) is the fact that your car’s AC needs to haul down the internal temperature from ridiculous highs VERY quickly. It’s assumed that the house AC will run and prevent the house from heating up too much, and that it’ll be acceptable to cool the house slowly over the course of hours if it DOES get hot in there. The demand profile is very different between the 2 types of systems.

  20. Ixnayer says:

    Most companies will just look at your old system and sell you the same size, but if you have a “builders grade” system, it may be undersized. You don’t want to undersize or oversize your system. When getting estimates, go with the company that actually takes the time to calculate your house’s needs.