This will be my first time purchasing a window unit, and I'm a little confused. Most charts indicate that a 7,000 BTU unit is appropriate for a ~250 square foot room, however from what I can tell, these do not exist (only 6,000 and 8,000 BTUs are available). Also, none of the guides I see mention ceiling height, which I figured would be important (i.e. cubic footage seems more appropriate than square footage). Finally, many say that getting a unit that is too powerful actually won't cool as effectively, but don't really explain what this means exactly - if I put a 20,000 BTU unit in a 100 square foot room, would the room really be warmer than it would be with a 5,000 BTU unit? Or do they just mean that it would be overkill?

Finally, some 6000 BTU units, like the GE AEL06LS say that they are only appropriate for 150 square foot rooms, and I don't understand why.

Anyway, my room that is 233 square feet, with ~9.5 foot ceilings. Would it be best to get a 6000 BTU unit or a 8000 BTU unit?

  • 1
    I have never dealt with sizing window units and cannot answer that question, but using an over-sized air conditioner with a built-in thermostat results in shorter cycle times with less effective cooling. What happens: The air conditioner cools down the air closest to the unit, but the air will not have a chance to circulate throughout the room and the rest of the room will still be at a warmer temperature when it shuts off. using an appropriately sized smaller AC unit, then the air will circulate more and there will be a more even temperature throughout the room before the AC unit shuts off. May 28, 2015 at 19:06
  • 8000 would be minimally oversized. Check the spec sheet for the cooling ratings. If the unit is inverter driven (which I doubt), then it will actually operate within a range, so oversizing slightly is preferable. It largely depends on the type of unit you're installing.
    – Hari
    May 7, 2018 at 18:41

3 Answers 3


The EPA has room size to BTU chart with some caveats for certain situations:


  Room size            A/C size
  ---------------     ------------
  100 up to   150 ->  5,000 BTU/hr
  150 up to   250 ->  6,000 BTU/hr
  250 up to   300 ->  7,000 BTU/hr
  300 up to   350 ->  8,000 BTU/hr
  350 up to   400 ->  9,000 BTU/hr
  400 up to   450 -> 10,000 BTU/hr
  450 up to   550 -> 12,000 BTU/hr
  550 up to   700 -> 14,000 BTU/hr
  700 up to 1,000 -> 18,000 BTU/hr
1,000 up to 1,200 -> 21,000 BTU/hr
1,200 up to 1,400 -> 23,000 BTU/hr
1,400 up to 1,500 -> 24,000 BTU/hr
1,500 up to 2,000 -> 30,000 BTU/hr
2,000 up to 2,500 -> 34,000 BTU/hr

Make any adjustments for the following circumstances:

  • If the room is heavily shaded, reduce capacity by 10 percent.
  • If the room is very sunny, increase capacity by 10 percent.
  • If more than two people regularly occupy the room, add 600 BTUs for each additional person.
  • If the unit is used in a kitchen, increase capacity by 4,000 BTUs.
  • Consider where you install the unit. If you are mounting an air conditioner near the corner of a room, look for a unit that can send
    the airflow in the right direction.

If you fall into any of those special situations, that may help you with sizing. Otherwise, since you're between sizes of available units, unless your climate is particularly mild, I'd go up a size to allow a bit extra cooling capacity for hot days.


Height doesn't matter because cold air sinks. Regardless of the ceiling height, you only need to cool the bottom 6-7 feet.


On the topic of over sized a/c units, I'd like to add this. I bought a 5 ton self contained a/c unit ( the whole unit is mounted on the outside wall with a hole for discharge above hole for return.) I bought the unit for $500 and thought what the heck I'll give it a try. WRONG!!!. The a/c could cool the small room down to 65 degrees with in 9 minutes. That sounds great but its not. The unit wasn't running enough to pull the humidity out of the air. Even though the room was 65 degrees, my skin felt sticky with the humidity still there. The humidity in Louisiana is high during the summer. I learned that its better for an a/c unit to be a little undersized than to be over sized.

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