How to Size a Room Air Conditioner

How to Size a Room Air Conditioner (Worksheet 1)

To buy an energy-efficient room air conditioner, you must first determine what capacity, or size, you need. This is important for two reasons:

  • An undersized unit won't cool adequately in extremely hot weather.
  • An oversized air conditioner can also adversely affect your comfort. The unit may switch on and off too often, without running long enough to dehumidify the room properly or cool the space uniformly. And it will consume more energy than necessary.

Use the simple calculation method described below to estimate the cooling capacity you'll need to maintain a temperature of 24°C with a humidity level of between 30 and 50 percent. You can find a more detailed calculation method in another NRCan publication entitled Air Conditioning Your Home (the "Useful Information Sources" page explains how you can order this booklet or view it on-line). If you're cooling a large space, consult a qualified contractor who will consider such factors as the amount of space to be cooled, the number of occupants, the insulation levels and the size, orientation and R-value of windows and doors.

Step 1.
Calculate the floor area of the space to be cooled.

To calculate floor area, multiply the length of each room or open space by its width. Don't estimate – take a few minutes to measure accurately. Units are often bought to cool one or two rooms. Enter your numbers into the blanks provided. (The calculation for a typical open-concept living/dining room will guide you through the process.)

x =
6.1 m x 3.7 m = 22.6 m2
(20 ft. x 12 ft. = 240 sq. ft.)

Remember to combine the total floor area of all the space you'd like to cool with a single air conditioner.

Step 2.
Calculate the basic cooling capacity you'll need to meet the typical Canadian cooling load.

Use this table to find the basic cooling capacity in Btu/h you'll need based on total floor area to be cooled.

9–14 100–150 5000
14–23 150–250 6000
23–28 250–300 6500
28–33 300–350 7250
33–38 350–400 8000
38–41 400–450 8750
41–46 450–500 9650
46–51 500–550 10 500
51–65 550–700 12 500
65–93 700–1000 15 000
93–111 1000–1200 17 700
111–149 1200–1600 19 000–24 000
149–167 1600–1800 24 000–27 000
167–260 1800–2800 27 000–33 000

*Based on rooms with two occupants and average windows, insulation and sun exposure.

22.6 m2 (240 sq. ft.) NEEDS 6000 Btu/h

Note: If cooling capacity is more than 12 000 Btu/h, consider installing two smaller units. Otherwise, a single unit might require a larger amperage circuit (20–30 amperes) or a dedicated 240-volt circuit; consult an electrician for more information.

Step 3.
Answer the questions in Worksheet 1 (below). If a question doesn't apply to your situation, leave the space blank. Then add or subtract from the basic cooling capacity to arrive at the approximate size of room air conditioner you should buy.

Worksheet 1

Additional Sizing Considerations

Add to or Subtract
From the Basic
Cooling Capacity

  +6000 Btu/h
If the space faces due north or northeast or is heavily shaded, subtract 10 percent of the basic cooling capacity. + 600
x 0.10 =
If the space is extremely sunny (e.g. windows face west and southwest), add 10 percent of the basic cooling capacity.  
x 0.10 =
If the space is poorly insulated, add 15 percent of the basic cooling capacity.  
x 0.15 =

If the space includes a working kitchen, increase cooling capacity by 4000 Btu/h. + 4000
+ 4000 =
If more than two people regularly occupy the space, add 600 Btu/h for each additional person.   ___ (number of additional people) x 600 =
If the air conditioner will run only at night, subtract 30 percent of the basic cooling capacity.  
x 0.30 =

Approximate size of room air conditioner to buy

10 600 Btu/h

Source: Natural Resources Canada (NRCan) - Office of Energy Efficiency