How Do You Calculate the Size of an Air Conditioner?

How to Size a Room Air Conditioner?

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

  1. An undersized unit won’t cool adequately in extremely hot weather.
  2. 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.

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.

Length X Width = Floor Area Of Space To Be Cooled

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.

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

basic cooling capacity
Basic Cooling Capacity


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.

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.

Calculate the floor area of the space to be cooled
Calculate the floor area of the space to be cooled


How Do You Calculate the Size of an Air Conditioner PDF

How Do You Calculate the Size of an Air Conditioner PDF 2

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