air conditioners size

Most air conditioners are largely oversized, and many people think this is a good thing. But this is nonsense - as it is nonsense to size an AC by an AC sizing chart, without considering other elements.

Oversized air conditioners

An oversized air conditioner is less effective and wastes large amounts of energy. If the air conditioner unit is oversized, it will cool the room quickly but it will also remove some of the necessary humidity. Only a properly sized unit removes humidity effectively as it cools.

Oversized air conditioners will cycle on and off more frequently, making indoor temperatures fluctuate more... A smaller unit will be more eco-friendly, quieter and will provide higher comfort.

Besides, a smaller non-oversized unit is also more appropriate to the standard sizes of most existing duct systems, and will last longer.

Too small units

Make sure you buy an air conditioner with the right size. A too small unit is also a wrong decision: an undersized air conditioner will over-dry the air and will not cool it well.

Before buying

Before estimating the size of an air conditioner, consider improving your home's insulation (attic and ceiling, walls...). Also fix air leaks… and improve shading devices and landscape shading.

A good shading system - through reflective roof coatings, awnings, overhangs or trees and landscaping -, high levels of insulation and a proper sealing will lower your cooling requirements and reduce the size of the air conditioner unit or system...

Sizing Charts and square footage

The size of an air conditioner depends partially on the square footage of the house. A good air conditioner sizing chart - like the one we list below, based on values suggested by Energy Star - can help you in such task.

Obviously, the exact AC size depends also on elements like the insulation levels, the ductwork, the height of the walls, the area of the windows, etc. AC sizing charts are simple tools to get a first estimate...

Note: Most room AC have cooling capacities between 5,500 Btu and 14,000 Btu per hour. 12,000 Btu per hour is often said to be the equivalent to a «ton». The size of an air conditioner is commonly measured by the number of British Thermal Units (BTU) of heat they can remove (per hour). The ton (12,000 Btu per hour) is another common rating term, but BTU is more universal. AC sizing charts match the square footage of the house or room with the needed capacity in BTU (per hour).

AC Sizing chart

Area To Be Cooled (square feet)	Area to Be Cooled (m2)	Capacity Needed (BTUs per hour)
100 to 150	9.3 to 14	5.000
150 to 250	14 to 23	6.000
250 to 300	23 to 28	7.000
300 to 350	28 to 33	8.000
350 to 400	33 to 37	9.000
400 to 450	37 to 42	10.000
450 to 550	42 to 51	12.000
550 to 700	51 to 65	14.000
700 to 1,000	65 to 93	18.000
1,000 to 1,200	93 to 111	21.000
1,200 to 1,400	111 to 130	23.000
1,400 to 1,500	130 to 139	24.000
1,500 to 2,000	139 to 186	30.000
2,000 to 2,500	186 to 232	34.000

Adjustments to the AC sizing chart

To improve the estimate given by the AC sizing chart, you should consider some adjustments to it:

- reduce the needed capacity by 10% if the room is very shaded, or increase it by 10% in the opposite case;
- increase the capacity by 600 BTUs per person, if the room is occupied regularly by more than two people;
- increase the capacity by 4,000 BTUs in the case of kitchens.

If ceilings are higher than 8 feet, or if the room is located directly under the attic, then your cooling needs might also be one to two Btu classes higher than that suggested by the AC sizing chart.