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How Big of an AC System Do I Need?

Have you ever wondered how we choose the optimimal air conditioner size for your home? Let’s take a look at the formula which determines how we select the right size AC system for optimal performance.

If you’re in the market for an air conditioner, one of the most important decisions you’ll make is how big of a unit you will need. There is a mathematical formula that you can use to make this determination. The formula is not difficult to work and, in addition to consulting with an air-conditioning technician, it can help you get a ballpark estimate of how much air conditioner you really need to cool your home. Other things factor into this decision, as well, that you need to take into account.

The Formula

About.com gives a good way that you can determine how much air conditioner you’re going to need. The first thing you’re going to need to do is to determine the area of your room. This is a simple formula. Simply multiply the length times the width of your room and you have the area.

After you have the area of your home, multiply that by 25. The 25 corresponds to British thermal units, usually abbreviated BTU. The air conditioner will be measured in BTUs. Just for the sake of example, if you had a room that was 20’ x 20’ that would be 400 ft.². You would need a 10,000 BTU air conditioning unit to cool off that space. Of course, that’s a very large room.

What About Going Small?

Air-conditioning units are going to cost more if they have a larger capacity, of course. Because of that, you may be tempted to go small on your air-conditioning unit and try to buy something less expensive. There is a huge pitfall involved in doing this. The problem is that you’ll end up with an air-conditioning unit that is running almost constantly. That means that you’ll pay huge electricity bills, which can quickly offset any savings you believe you got by cutting down on the size of the equipment you purchased.

Go Big?

You don’t want to go too large, either. Where air-conditioning is concerned, there really is such a thing as just right. If you have too large of an air conditioner, it’s going to pull the moisture out of the air very quickly. This makes the interior environment very unpleasant.

There’s another thing that can happen with too large of an air conditioner. The air conditioner may not actually be able to pull all of the moisture out of the air before it manages to cool the room. This means that you have the unpleasant experience of a clammy feeling room, which is something you definitely don’t want.

Where air-conditioning is concerned, there is such a thing as the right size unit for the right size room. Your air-conditioning technician can help you make this determination and ensure that you buy something appropriate.


One final factory should you take into account is how well insulated your home is. The better your insulation the better the results you’ll get from your air-conditioning. Problems from inadequate insulation are similar to the problems that you have if your air-conditioning unit is too small.

No matter how efficient, modern and advanced an air-conditioning unit is, it cannot cool a room if all of the cool air is leaking out the doors and the windows. Think of air-conditioning season in much the same terms as you think of heating season. Remember that the converse usually applies, however. For example, during the heating season you want to open your drapes up to let sun in so you get passive solar heating. During the cooling season, keep those drapes closed, keep the windows closed and keep the doors close so that you keep the cold air in the hot sun out. With the right size unit, you can have great results from AC.