Air Conditioner BTU Calculator (With AC BTU For Room Size Chart)

“I have a 400 sq ft room with 9 ft ceiling height. Sweating at 84°F temperature, I get a lot of sun in South California. I need an air conditioner; what size AC I need? I’m interested in AC BTU for this room size.”

This is a question that landed in our inbox. We figured the best way to help people calculate the BTUs of their future AC unit is to create an easy-to-use Air Conditioner BTU Calculator. And that’s exactly what we did.

Namely, you will find these 2 key resources in this article:

1. Air Conditioner Room Size Calculator. Here you insert the room square footage, ceiling height, sun exposure, climate, and inclusion of kitchen, and the AC unit size calculator will automatically tell you how many BTU AC unit you should go for.
2. Air Conditioner BTU Chart. We have also calculated air conditioner BTUs for 100 sq ft, 150 sq ft, 200 sq ft, and up to 3,000 sq ft rooms/homes, based on the AC BTU sizing rule of thumb. We summarized all the air conditioner room sizes and BTUs in a neat chart (below the calculator).

First of all, let’s cover all 5 key factors that determine the air conditioner BTU size you will need:

1. Room size (square footage). This is the most important factor; larger rooms or homes will need more BTUs. The HVAC rule of thumb for cooling is that you need 20 BTU per sq ft of living space (8 ft ceiling height).
2. Ceiling height. If you have high ceilings, you have more air that needs cooling. That’s why we add +12.5% BTU for every additional foot of ceiling height over the standard 8 ft ceiling. Example: If you have a 10 ft ceiling, you need 25 BTU per sq ft (+25.0%) of cooling output.
3. Sun exposure. If you have several windows, a south-oriented room that gets a lot of sun, you would add +10% to air conditioner BTUs. If your room is heavily shaded, you would reduce the AC BTUs by -10%, in accordance with the Energy Star guidelines.
4. Climate. If you live in a sunny state (California, Florida, Texas), you would add +10% to the air conditioner BTUs. If you live in a colder state like New York, Illinois, Minnesota, and so on, you would reduce the AC BTU capacity by -10%.
5. Inclusion of kitchen. If the rooms you want to use the AC for includes the kitchen, you should add +4,000 BTU to the overall air conditioner size. That’s because the kitchen has many heat-generating appliances like an oven, dishwasher, cooking space, and you want to account for that as well.

Before we check the air conditioner BTU calculator and AC BTU chart, let’s solve the above example to show how to use these 5 factors adequately:

Air Conditioner BTU For 400 Sq Ft Room (Example)

Alright, we have a 400 sq ft room, 9 ft ceiling height (+12.5%), and high sun exposure (+10%), in California (+10%). It’s not evident from the comment in our inbox, but let’s presume this room doesn’t include a kitchen.

Now, for every air conditioner BTU size estimation, we start with the basic HVAC rule of thumb, which states the following:

20 BTU per sq ft of living space (8 ft ceiling height).

If this 400 sq ft room would be an average room (no correction factors), we would need an 8,000 BTU room air conditioner (since 20 BTU per sq ft × 400 sq ft = 8,000 BTU). But this is not an average room, and we have to account for the following factors:

• Increase AC BTU by 12.5% since we have a 9 ft ceiling height.
• Increase AC BTU by 10% since we have high sun exposure.
• Increase AC BTU by 10% since we have a hot climate (California).
• No kitchen = No additional BTUs needed.

We can see that we have to increase our 20 BTU per sq ft standard size by a total of 32.5%. That means we will need 26.5 BTU per sq ft for this particular room. Now that we have the adequate BTU per sq ft estimate, we can calculate BTU for this room size like this:

Air Conditioner BTU (400 Sq Ft) = 400 Sq Ft × 26.5 BTU Per Sq Ft = 10,600 BTU

We can see that we would need about 10,600 BTU to adequately cool this 400 sq ft room. In practice, we would install an 11,000 BTU or 12,000 BTU portable or window air conditioner. It makes sense to get a slightly bigger AC unit (but not by much; heavy oversizing leads to short cycling) just to be sure we won’t be sweating since the summers are, as you can quite easily feel yourself, becoming hotter.

Now, if you calculate air conditioner BTU manually like this, you can see that you have to run through quite a few calculations and account for AC BTU sizing correction factors.

To smooth out this process, we have created the before-mentioned air conditioner room size calculator. You just insert the square footage, ceiling height, and other correction factors, and the calculator will automatically estimate the air conditioner BTUs needed (you should also check the air conditioner BTU chart below the calculator that makes this room AC sizing even easier):

Air Conditioner BTU Calculator

Let’s look at one example to illustrate how this online AC BTU calculator works:

Let’s say we want to cool a 500 sq ft flat and want to calculate the air conditioner BTUs we need. This is a New York flat with average sun exposure that includes a kitchen and a standard 8 ft ceiling height.  You just slide the 1st room size slider to ‘500’, the 2nd ceiling height slider to ‘8’, pick ‘Average’ sun exposure, ‘Cold’ climate, and tick off ‘Yes’ for kitchen inclusion. Here is the result:

This particular 500 sq ft flat would need a 13,000 BTU air conditioner.

Here is the full air conditioner BTU chart, based on the 20 BTU per sq ft HVAC rule of thumb (without corrections for ceiling height, sun exposure, climate, and kitchen inclusion):

Air Conditioner BTU Chart

With the air conditioner room size calculator and this chart, you are now well-equipped to figure out the air conditioner BTU capacity you need. If you would need a bit of advice, you can use the comment section below, give us some numbers, and we can help you out with air conditioner sizing.