What Size Air Conditioner Do I Need for a 20×20 Room?


20×20′ is a medium size room. The room can get very hot in the summer. So, what size air conditioner is suitable for a 20×20′ room?

As a general rule, a 20×20 feet or 400 square feet room should use a 12,000 BTU or 1 Ton air conditioner. If your room is heavily shaded, you can save cost by using a 9,000 BTU or 0.75 Ton air conditioner. Although a 12,000 BTU air conditioner is still sufficient if your room is very sunny, I will recommend you go for an 18,000 BTU air conditioner so that it won’t have to run continuously.

There are many factors to be considered when deciding what size air conditioner to use for a room. Using a bigger size air conditioner doesn’t mean that it is better. In fact, an oversized air conditioner can’t remove sufficient moisture in the air and thus, unable to give you the level of comfort that you supposed to get.

Factors to Consider When Sizing an Air Conditioner

Most HVAC contractors use a rule of thumb to calculate the cooling load of a room and then, recommend to you what size air conditioner you should get. However, we must take into account other factors that might affect the cooling load requirement to prevent us from getting the wrong size air conditioner.

If your room is very sunny, you need a bigger air conditioner

Do sunlight glaze into your room through the window in the afternoon? If so, your room is very sunny and gains a lot of heat. So, you’ll need a bigger air conditioner to counter the heat and provide sufficient cooling.

Solar heat gain is the biggest factors when it comes to calculating the cooling load for a room, especially when the room has large area of windows. Some sunny rooms need air conditioner twice as much size as shaded rooms.

For a 20×20′ room, the base capacity needed as per Energy Star is between 9,000 BTU and 10,000 BTU. If the room is very sunny, they recommend increasing it by 10%. However, the available cooling capacity for most air conditioners is 9,000 BTU, 12,000 BTU, 18,000 BTU, and more.

Thus, if your room is very sunny, you may need to upgrade from 10,000 BTU to 11,000 BTU which is one size bigger from 0.75 Ton to 1 Ton air conditioner. But, if you put thick curtains or tint the windows, you may remain using a 0.75 Ton air conditioner.

More people, more heat and more cooling is needed

I guess you’ll hold more than 2 people in a 400 square feet room. The base capacity that we got earlier is based on two persons only. Thus, for each additional person, we need to add 600 BTU.

Let’s say the 400 square feet room is the living room and 5 persons regularly sit there. So, there are 3 people in addition to the 2 people. Hence, we need to add 1,800 BTU on top of the base capacity needed. Therefore, if the living room is very sunny, we’ll need a total capacity of 12,800 BTU which is still a 1 Ton air conditioner.

However, if you put thick curtains or tint the windows, you only need a base capacity of 10,000 BTU which you can opt for a 0.75 Ton air conditioner. But now, with 5 persons in the room, the capacity needed is 11,800 BTU and the air conditioner goes back to 1 Ton.

Kitchen has a lot of heat, so you need a bigger air conditioner

If your 400 square feet room included the kitchen, you need to add 4,000 BTU due to the heat gain from kitchen appliances. Thus, you easily need to upgrade the air conditioner by one size if the kitchen is included.

However, you may not need to upgrade your air conditioner all the time. Sometimes, we use the kitchen heavily in the evening where the sunlight is not so strong anymore. Thus, the room is not getting as much heat as before in the afternoon and you may remain it at 1 Ton capacity.

Save cost by going for a smaller air conditioner if your room is heavily shaded

As opposed to a very sunny room, a heavily shaded room can ignore a jump in air conditioner size. Energy Star also recommended that you can reduce the capacity needed by 10%. Thus, you end up with a base capacity needed between 8,100 BTU and 9,000 BTU.

If your room doesn’t have direct sunlight or you plan to put curtains or tint the windows, you can opt for a 9,000 BTU or 0.75 Ton air conditioner. But, as mentioned earlier, if you have a lot of people in the room, you may need to use a 1 Ton air conditioner.

Other factors to consider before finalizing the air conditioner size

Even though you only have let’s say 5 people in the room, you should consider if you’ll hold more people for a birthday party or family gathering. I will allow some safety factors whenever I’m sizing an air conditioner because I wouldn’t want to have insufficient cooling when I throw a party at my house.

Furthermore, not everyone likes to close their window curtains whenever they are using the air conditioner. Sometimes, we’ll want to have natural light shining in our room. I don’t like it when I need to close the curtains every time I use the air conditioner or else it’s not enough cold. So, I’ll add in more BTU and get one size bigger air conditioner.

Besides, I’ll consider the decrease in the performance of the air conditioner over time and add in more BTU. No matter how great the air conditioner is, every mechanical equipment degrades over time. Your air conditioner may get dirty internally that is beyond washable and reducing performance after a few years.

Therefore, I suggest you don’t get the minimum size air conditioner but allow some buffer for the future. Generally, I like to have my air conditioner running at about 70% of its maximum capacity so that it won’t be loading at full capacity all the time and prolong the lifespan of the air conditioner. Not to mention, if abnormal heat strikes, I know my air conditioner can handle it.

What Happen If You Use a Wrong Size Air Conditioner?

Air conditioners are expensive. So, the first thing is you wasted money. Duh…but here is the deal, an oversized air conditioner will cool a room quickly, so quickly that not enough moisture is being extracted out from the air. Thus, an oversized air conditioner unable to dehumidify the air to a comfortable level.

You’ll feel very clammy and cold at the same time with an oversized air conditioner. Furthermore, the oversized air conditioner will switch on and off more frequently which can shorten its lifespan. Also, an oversized air conditioner costs you more on the electricity bill.

On the flip side, an undersized air conditioner causes you only one problem which is not enough cold. But, giving sufficient cooling is the purpose of an air conditioner. Hence, many people, even HVAC contractors, rather oversizing the air conditioner than undersizing it.

I have a tight budget, what other options do I have?

If you think that you need a 12,000 BTU or 1 Ton air conditioner but you only have the budget for a 9,000 BTU or 0.75 Ton air conditioner, try some of the following options.

Cheaply partition your space to cut out only the space that truly need air conditioning

Many living rooms are connected with the dry kitchen. But, the dry kitchen usually doesn’t need to be air-conditioned. Thus, you can separate the living room from the kitchen so that a small air conditioner can cool the living room sufficiently.

Try to DIY misting the condenser or shade the condenser to improve the cooling

You can set up a cheap misting system to mist the condenser so that the condenser can reject more heat and improve the cooling. Besides, you can choose a shady location that has plenty of wind blow to put the condenser so that the wind carries away more heat from the condenser and improve the cooling.

Yu Chang Zhen

Yu was working in the air conditioning industry for the past 7 years, covering from design to installation and maintenance. Yu wish to share all the things he learned with the people around the world.

Recent Posts