Beat The Summer Heat – What Size Air Conditioner Do I Need?

by Donald Grummett

A larger air conditioner will not always provide more cooling. It should be sized for the room area. If properly sized it should operate long enough to remove moisture from the air. It is the moisture removed from the room which actually produces the feeling of cooling. Unfortunately many people in their rush to purchase a window air conditioner buy too large a model.

Too large a unit can actually provide less cooling than a smaller, properly sized unit. This is because with an oversized air conditioner its run time is too short. Consequently it does not remove enough moisture from the room to produce a feeling of comfort. If you’re A/C seems to cycle off and then back on every few minutes and does a poor job of cooling, it is probably oversized for the room.

Air conditioners are rated in BTU’s. British Thermal Units are a measurement of how much heat is being transferred form the room being cooled to where the heat is dumped out of doors. This is why the cooing coil inside the room is cool to the touch, while the part that hangs outside is extremely hot.

A window air conditioner used in the average household room can be as low as 4000 BTU. While 5000, 6000, and 8000 are the most common sizes purchased.

How do I determine what size A/C I need? Following is a basic sizing chart for calculating the window air conditioner needed for a room. Calculate the size of the room by multiplying the width of the room by the length of the room. Use your room size to determine what BTU’s are required to cool that particular room size.

room in sq ft—–room in sq m—–A/C size in BTU’s

100 – 250…………9 – 23…………5000 – 6000

250 – 400………..23 – 37…………6000 – 8500

400 – 550………..37 – 51…………8000 – 11000

550 – 875………..51 – 81………..11000 – 15000

Based on a room occupied by two adults, having an average insulation, number of windows, and sun exposure.

Which is the best room for placement of an air conditioner?

Wherever it gives the most comfort is where it should be installed. If you only have one air conditioner then we usually suggest the bedroom. This is because most people find that if they can sleep comfortably during a hot summer night then their next days work will be a lot easier. Getting through a hot day at work becomes a lot easier when you know you will sleep comfortably that night.

Some people do not like a cool sleeping environment so they will tend to place it in a living room or family room. Or some people will place it in the family room because the whole family can sleep there. Get out the sleeping bags and turn it into an adventure for the young ones. Beats waking up in the morning to find a dog, cat, and four kids in the bed with you.

Also window air conditioner can be noisy, which may disturb sleep. Others find the drone of the fan actually promotes sleep because all the neighbourhood noises are drowned out. Some people say the air conditioner works as well as a sleeping pill

But whichever room you eventually pick don’t expect the impossible from you’re A/C. It is not meant to cool more than one room or one open area. Air conditioners will not cool other rooms? This is because the blower in a window air conditioner is not very powerful. It is sized to cool one open area, not a number of interconnected rooms. Therefore once again a couple of smaller units may be better for your home than one large unit.

Lastly, remember to properly secure the air conditioner in the window. Once installed a piece of wood to stop the window from being lifted up is a good idea. If you have wooden sills the simple addition of a wood screw at the top of the raised window (the one you had to raise to install the A/C) will only take a minute. Otherwise an intruder could gain access to the house by lifting the window and letting the A/C fall out. This could result in the loss of both your valuables and the air conditioner.


Copyright 2005 by Donald Grummett. All right reserved. Service manager of MG Appliance Repair Services in Ottawa, Canada. In the trade over 30 years as a technician, business owner, and technical trainer. For more information about appliances including FAQ, Stain guide, Recycling, and Newsletter visit