How Many BTU per Square Foot (AC BTU Calculator)

Size is a critical consideration when buying an air conditioner for your home. If you get it wrong, the unit will not run effectively or efficiently. The capacity of an air conditioner is measured in BTU (British Thermal Units). If you know the size of your room, you can calculate how many BTU per square foot you need to cool the space.

In this article, we will teach you a simple method that you can use to estimate the size of the ac you need. We will also show you how to get a more accurate result using our air conditioner BTU calculator.

The Estimation Method: How Many BTU per Square Foot

This method relies on the rule of thumb: 20 BTU per sq ft.

What Size Central AC do I need?

Central air conditioners come in several sizes. You can calculate how many square feet a central ac can cool using the estimation method.

Example: How many square feet does a 3-ton air conditioner cool?

Step 1: First, you need to convert tons to BTU. That is simple if you know how many BTUs there are in a ton.

One ton equals 12,000 BTU. Therefore, we will multiply three by 12,000 BTU.

3 x 12, 000 BTU = 36, 000 BTU

Step 2: Calculate how many square feet it can cool using the sq ft to BTU formula. That means dividing 36,000 BTU by 20 BTU per square foot (rule of thumb).

36, 000 ÷ 20 = 1,800 square feet.

In other words, a 3-ton air conditioner can cool a 1,800 square feet room.

Central Air Conditioner BTU to Room Size Chart

The following table shows central air conditioners of varying capacities and the corresponding room sizes they can cool.

Air Conditioner BTUAC Tonnage SizeRoom Size (sq ft)
12,000 BTU1 ton600 sq ft
18,000 BTU1.5 ton900 sq ft
24,000 BTU2 ton1,200 sq ft
30,000 BTU2.5 ton1,500 sq ft
36,000 BTU3 ton1,800 sq ft
42,000 BTU3.5 ton2,100 sq ft
48,000 BTU4 ton2,400 sq ft
60,000 BTU5 ton3,000 sq ft

You can consult the air conditioner BTU chart above to find the size of ac you need for your home. Let’s look at an example.

Example: What size ac unit for 1,500 square feet?

The corresponding central ac size on the BTU to sq ft chart is 30,000 BTU.

In short, you need 30,000 BTU to cool a 15,000 square feet house.

If you can’t find the exact size of your room on the chart, choose the closest option.

Example: How many BTU for 500 sq ft?

The closest size to 500 on the chart is 600 sq. ft., which corresponds to 12,000 BTU.

So, you need 12,000 BTU to cool a 500 sq. ft. room.

How Many BTUs Do I need to cool my Room?

The most common room air conditioners are portable and window air conditioners. If you know the area of your home, you can figure out what size you need using the estimation method.

Example: How many BTU do I need for a 12x12 room?

Step 1: Calculate the area in sq. ft.

12 x 12 = 144 sq. ft.

Step 2: Calculate how many BTUs you need by multiplying the area by 20 BTU (rule of thumb)

144 x 20 BTU =2,880 BTU

In simple terms, you need 2,880 BTUs to cool a 12 x 12 room.

If you want to get the size in tons, use our BTU to Tonnage Calculator. Type the size of the ac in the box, and the calculator will automatically convert BTU to tons.

Room Air Conditioner BTU to Tons Chart

The following table shows different air conditioner sizes (BTU) and their corresponding capacities in tons.

Air Conditioner BTUAC Tonnage SizeAir Conditioner Options for This Size
5,000 btu 0.42 tonWindow AC, Portable AC
6,000 btu 0.50 tonWindow AC, Portable AC
6,500 btu0.54 tonNA
7,000 btu0.58 tonPortable AC
7,500 btu 0.63 tonNA
7,700 btu 0.64 tonWindow AC
8,000 btu0.67 tonWindow AC, Portable AC
8,500 btu0.71 tonPortable AC
9,000 btu 0.75 tonWindow AC
9,500 btu 0.79 tonWindow AC
10,000 btu 0.83 tonWindow AC, Portable AC
11,000 btu 0.92 tonWindow AC, Portable AC
12,000 btu 1 tonWindow AC, Portable AC
13,000 btu1.08 tonPortable AC
13,500 btu1.13 tonPortable AC
14,000 btu 1.17 tonWindow AC, Portable AC
14,500 btu 1.21 tonWindow AC
15,000 btu 1.25 tonWindow AC, Portable AC
18,000 btu 1.5 tonWindow AC
22,000 btu 1.83 tonWindow AC
24,000 btu 2 tonWindow AC

It is important to note that there are only a few tonnage sizes for air conditioners. They include 1.5 ton, 2 ton, 2.5 ton, 3 ton, 3.5 ton, 4 ton, and 5 ton.

However, we have included a variety of tonnage sizes in the chart for the convenience of BTU to Ton conversion. Otherwise, you should not expect to find them on the market.

The Accurate Method: Using our Air Conditioner Room Size Calculator

The method above will only give you an estimate since it does not consider various factors. These include the insulation, ceiling height, and the number of windows and doors.

One way of getting a more accurate answer is by calling in a professional contractor. They will use the recommended manual J process to calculate your home’s cooling load. This process considers many factors.

However, a contractor will charge you for the service. Paying the extra cost may not be necessary, especially if you want to install a room air conditioner.

Fortunately, our BTU calculator simplifies the process for you. All you need to do is fill in some basic information on the boxes. The calculator will do the rest.

How to Use the Calculator

Our calculator takes several factors into account. They include the following.

Climate Region

Different climate regions will not experience the same weather patterns. In particular, they will have varying levels of temperature and humidity. If your area experiences high temperatures and humidity, you will need a sizable air conditioner.

The calculator has seven zones. Choose the region your area falls in from the drop-down menu on the climate region box.

Home Size

The size of the house was the only factor we considered in the estimation method since it plays a critical role. The bigger the room, the harder your ac will have to work to keep it cool. Therefore, you will need an ac with a higher capacity.

On the calculator, type the area of your room in square feet in the home size box.

Insulation Grade

The insulation also helps in regulating the temperature in the house. If yours is in good condition, your ac will not have to work as hard. However, a room without proper insulation will need a sizable ac.

The calculator has three options on the drop-down menu depending on the age of the house: Good (After 2000), Average (1975-2000), and bad (before 1975). Choose one depending on the house’s year of construction.

Sun Exposure

If your house is exposed to the sun for many hours during the day, the indoor temperature will rise. The more sun exposure it gets, the more the ac will have to work. You have to take this into account when choosing the size of your ac.

There are three options on the drop-down menu. Good, average, and bad. Choose one.

Number of Doors and Windows

Hot air can enter or leave the house through doors and windows, thus making the ac work harder. If they are too many, you will need a sizable ac to handle the extra workload.

Count the number of doors and windows in your house, and fill the numbers in their respective boxes.

Let’s demonstrate how to use the calculator using an example.

How many BTU do I need for a 20x20 room?

We will work with the following conditions

  • Climate Region: Zone 5
  • Insulation Grade: Good
  • Sun Exposure: Good
  • Number of Windows: 6
  • Number of Doors: 2

The area equals 20 x 20 = 400 square feet.

After filling all the boxes, the calculator displays the recommended ac capacity as 7,400 - 9,000 BTU. That can either be a portable or window air conditioner.

Cooling BTUs per Square Footage for 7 Climate Zones

The following chart shows the cooling BTUs per square foot needed for different climatic regions.

Climate ZoneCooling BTUs Per Square Footage
Climate Zone 1 & 2 (hot)22-30 BTUs / Sq. Ft.
Climate Zone 3 (warm)20-24 BTUs / Sq. Ft.
Climate Zone 4 (moderate)18-22 BTUs / Sq. Ft.
Climate Zone 5 (cool)16-20 BTUs / Sq. Ft.
Climate Zone 6 (cold)14-18 BTUs / Sq. Ft.
Climate Zone 7 (very cold)12-16 BTUs / Sq. Ft.


Buying the right size ac unit will ensure your house feels comfortable all year long. However, finding the right size for your cooling needs can be challenging if you’re only relying on instincts.

Luckily, you can now calculate the size of the ac unit you need using a simple estimation method. For a more accurate result, you can use our AC BTU calculator.

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