How To Figure Out How Many BTUs You Need In Your A/C

Hot times are coming and you may find yourself buying a new air conditioner, but how do you figure out how many BTUs you need? Silencedotcom over in the Consumerist Forums posted this BTU guide:

Length of Room x Width of Room [in feet] = Area
(Area x 30) + 2000 = BTUs you need

UPDATE: This Energy Star chart is what you need:
btusneeded.jpg• If the room is shaded, reduce the BTUs by 10%.
• If the room is very sunny, increase the BTUs by 10%.
• If you plan on placing the air conditioning unit in your kitchen, add 4,000 BTUs.
• If more than two people will regularly be in the room (e.g., an office), add 600 BTUs per person.

We’re no A/C experts but that formula roughly jibes with the number of BTUs Lowes.com recommended for our apartment size. — BEN POPKEN

Properly Sized Room Air Conditioners [Energy Star]
Little air conditioning lesson for everyone [Consumerist Forums]

Comments

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  1. faust1200 says:

    I love optical illusion pictures like this. After 40 minutes I could see the a/c unit mounted in the wall.

  2. bsheairs says:

    There’s an air conditioner in the picture? I’m a bit distracted at the moment…

  3. vr4z06gt says:

    does that make sense? According to your algorithm my bedroom should require almost 13000btu’s. For an 18×18 bedroom that doesn’t make any sense.

    18x18x30+2000=11720 + 10%(sunny)=12892btu!

  4. Is this using Meters or Feet?

  5. silencedotcom says:

    @vr4z06gt: Actually, that’s about right. I’ve always thought the adding 10% was a bit excessive. I’ve put A/C’s in before and never added the 10% to really sunny rooms and I’ve been ok.

    @thnkwhatyouthnk: Feet, sorry to say.

  6. It tells me that I need 22000 btus to cool my bedroom. Seems a bit high to me.

  7. ncboxer says:

    Here is the Energy Star guide- http://www.energystar.gov/index.cfm?c=roomac.pr_properly_s

    For an 18 x 18, which equals 324 sq feet, it recommends a base of 8000. If you add 10% of sunny and round up, you’re at 9000.

  8. Thanks for taking pictures of your house Ben!

  9. silencedotcom says:

    Alright, the Enegry Star website is a lot more accurate than my ballparking, since it only had 1 figure for all rooms.

    I’ll adjust next time I post it, thanks!

  10. Paul D says:

    We’re no A/C experts but that formula roughly jives with the number of BTUs Lowes.com recommended for our apartment size.

    Jibes.
    Jibes!
    JIBES!

    J-I-B-E-S!

    *wink*

  11. bambino says:

    I prefer jives. Reminds me of the soul brother on Airplane!

  12. edbro says:

    Another factor is the floor level. I have a 2 story house and the bedrooms, which are on the second floor, get much hotter than the ground floor. Heat rises so I have to counteract not only the normal heat of the room but also the rising heat from the ground floor.

  13. Promethean says:

    What are these the estimates for? To go from sweaty to frostbite is 10 minutes? These quotes, both the original formula & energy star, seem a bit excessive. My bedroom is about 300 sq., moderately shady. I have a 4,000 BTU in there, and it gets chilly after 20 minutes. I can only imagine what 8,000 – 10% would do. My living room, about 600 sq. ft, is sunny and has a 5,000 BTU. This too works quite fine to cool the room, despite not being the 15,400 BTU behemoth Energy Star recommends.

  14. morph3us says:

    let’s play the game everyloves…”name that girl”

    any takers?

  15. roothorick says:

    We have a massive (somewhere in the ballpark of 80,000BTU) air conditioner that’s tasked with covering our entire condo via an intricate layout of stock house stand fans. It can JUST BARELY keep up on a moderate day. So, that chart sounds right about right.

  16. Paul D says:

    @bambino

    Jus’ hang loose, blood. She gonna catch ya up on da’ rebound on da’ med side.

    What it is, big mama? My mama din’ raise no dummies. I dug her rap!

    Cut me some slack, Jack! Chump don’ want no help, chump don’t GET dick help…

  17. Scott in DC says:

    be careful – if you get an airconditioner that’s oversized it won’t run long enough to remove humidity from the air. Wet air feels hotter than dry air… so removing the humidity is good!

    On the hottest day of the summer you want that air conditioner to run continuously – if it’s cycling on and off on that super-hot day, it’s oversized.

  18. anatak says:

    not possible in all areas, but we’re going no-A/C this year. Dehumidifier and ceiling fans only.

    not that there’s much of a choice – this house has casement windows and no duct work.

  19. elf6c says:

    Sweet, sweet cheesecake photos. . .

    Wait, the article is about air conditioning?

  20. jeffislouie says:

    Make sure not to buy too much AC as well.
    If you do, it won’t cool properly and will end up icing over. The humidity in the room will rise and you’ll have spent too much for an ac designed for a bigger room.
    It’s almost better to go a little under, in my experience.
    I cooled an 800 sq foot apartment with a single 12000 btu a/c with no problems.
    Another thing to keep in mind is that it is very difficult to ‘bend’ the air. So putting an AC in the living room facing away from your bedroom will leave your bedroom hot.
    Depending on the config of your apartment, placing a larger unit in the main room and a smaller unit in the bedroom makes for some chilly evenings – even when it is really hot outside.
    I have a 10000 btu unit in my living room and an 8000 btu cooling the dining room and bedroom (the unit blows into my bedroom) and so far, it’s all good.
    My apt. is around 750 sq feet.
    At night, all I run is the small unit and it freezes my bedroom.
    :-)

  21. jeffislouie says:

    @anatak:

    not possible in all areas, but we’re going no-A/C this year. Dehumidifier and ceiling fans only.

    not that there’s much of a choice – this house has casement windows and no duct work. “

    You actually do have some choices, btw…
    They make casement air conditioners that will fit the windows and they also make portable ac’s that you can move into rooms. As long as you have a window in the room for the AC to vent out hot air, they work pretty well….
    Casement units and portable units are more expensive than regular ones though…

  22. AcidReign says:

    …..@anatak: Since air-conditioning has only been around for 75 years or so, I suppose it’s POSSIBLE to live in Alabama without it. 95 degrees F and 100% humidity feel great! Not.

    …..To go without, and be even remotely comfortable, you need high ceilings and big open windows. Ideally, the the building should be shaded from afternoon sun by lots of trees, too.

    …..We went without AC for about a week, two years ago. (Waiting on the installation of a new outdoor compressor unit.) With 9 foot ceilings and an attic fan, we were pretty miserable. Fortunately, the first part of that week, Hurricane Dennis blew by, and we had three days of clouds and rain, to keep the heat down.

  23. Jiminy Christmas says:

    A worthy reminder: If you find yourself shopping for an air conditioner give some serious thought to where you will plug it in. A decent-sized window unit (say, 12,000 BTU or more) will draw at least 10 amps. A typical residential circuit is 15A. If you have anything else substantial on the same circuit you will probably overload it and trip the breaker. A couple of lamps and an alarm clock and you would be okay. A refrigerator would be out of the question.

    Also, if you are considering a large unit verify what voltage it runs on. A typical household receptacle is in the 110v -120v range. Something like a 20,000BTU air conditioner will likely need 220v. Older air conditioners as low as 10,000BTU can also require 220v. You don’t want to lug something like that home only to discover that you don’t have a 220v receptacle and having an electrician install a new circuit for you will run $500.

  24. FLConsumer says:

    @anatak: They DO make casement window ACs. If you own the place, look into mini-split ACs. By-far the ultimate air cons, even better than central system. We’re just a little behind the technology here in the USA, which is porbably why most of the mini-split systems are made in Japan.

    That chart’s a bit off…at least it doesn’t match up with the heat load calculations I did here. Granted, proper heat load calcs are designed to handle the absolute worst day of the year and actual conditions are far better than that most of the time.

  25. anatak says:

    @AcidReign: you apparently missed the “not possible in all areas” part of my post. I wouldn’t try this in the south either with out having a house that was properly designed for no A/C. And yes, that is very possible – even in the heat and humidity of the south. And 95*? is that it? I’ve seen this pulled off in 105* heat + humidity. But thats OK, I knew that response was coming.

    @FLConsumer:
    @jeffislouie:
    My casements don’t slide, they tip out. I don’t want them hanging wide open all the time, and I’m not too keen on taking them apart. Like I said, there not much of a choice. There are choices like portables (something else taking up floor space), or others, but they are just not that favorable. But thank you, thank you, thank you all for pushing A/C on me. Thats just what our power grid needs, more A/Cs. Personally, I’d rather use less power.

  26. jeffislouie says:

    @anatak:
    Sorry to hear your casements don’t slide.
    That makes it difficult.

    “But thank you, thank you, thank you all for pushing A/C on me. Thats just what our power grid needs, more A/Cs. Personally, I’d rather use less power.”

    Some people don’t mind the heat and humidity of summer. Kudos to you if you can stand it.
    Personally, I sweat like a pig in the summer heat and don’t like smelling like a bag of two day old sliced onions, so it’s AC all the way for me!
    Best of luck this summer.
    I hope it’s mild.

  27. lore says:

    Or, you can live in certain areas like the San Francisco Bay Area (near the Bay) where you never really need A/C. Nice, dry, ocean-cooled air. :-)

  28. anatak says:

    @jeffislouie:
    Thanks, Jeff. I’m sure we’ll survive. The last owners lived here with no A/C and no dehumidifiers for 40+ years.

    We have the something closer to the original “A/C” – proper home planning. A breezeway, placing the house to capture the prevailing winds, larger overhangs, established trees… These things make a big difference, but they are the last thing on the list for builders today. Gotta design your own, or go for an older home to find this usually.

  29. JNighthawk says:

    Everyone who is saying things like “it seems very high for my room”: it’s a measurement of SQUARE feet, not CUBIC feet. Don’t take into account height.

  30. JNighthawk says:

    And I take that back. I only saw the chart, I didn’t see the formula before-hand, so I assumed (18x18x30) was width by length by height.

  31. bowersst says:

    I’ve noticed that it is often much cheaper to buy two 6000 BTU air conditioners than it is to buy one 12000 BTU unit. Would the tables scale the same if 2 A/C’s are used?

  32. Chicago7 says:

    If nobody wears any clothes in the house, does that mean you can lower the BTUs by 10%???

  33. FLConsumer says:

    @bowersst: You got it. I have two 6,000 BTU ACs here as emergency cooling. It’s half of what I need for this place according to the ASHRAE’s Manual J heat load calcs, but they do an amazing job.

    As a benefit, if you have them both in the same room, you can set one 1-2F below the other one and you’ve ended up creating a 2-stage system, which normally costs $$$ when done with a central AC. The 2-stage setup will mean better humidity removal and a very stable temperature.