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Generally, you need a 10,000 BTU air conditioner for a 500 square foot area. It’s an easy calculation if you apply the standard BTU sizing formula, i.e., 20 BTU for every square foot.

However, it’s also a generalized figure with many assumptions. For instance, you may need a bigger AC for the same 500 sq. ft. room if you live in a warmer climate and a smaller unit if you live in a colder climate.

The rest of this guide discusses the various factors and considerations to help you pick the most suitable BTU size.

## Understanding British Thermal Units (BTUs)

The cooling capacity of an air conditioner is measured in British Thermal Units (BTU). The term was first coined in 1800. One (1) BTU is the energy needed to raise (by heating) or lower (by cooling) one pound of water by one degree Fahrenheit at sea level.

When used in the context of an air conditioner, the BTU rating refers to the amount of heat the air conditioner can remove from the air per hour, i.e., BTU/hour. The metric is often given as just BTU (rather than a ratio, i.e., BTU/hour). However, it’s important to know at the back of your mind that it’s BTU/hour.

You may come across some air conditioners that use tonnage as opposed to BTU for measurement. All you need to remember is that a one-ton air conditioner = 12,000 BTU. Therefore, a two-ton AC = 24,000 BTU and so forth.

## General BTU Requirements for Various Room Sizes

Although you can manually calculate the BTU requirement for ay room, as we’ll find out shortly, HVAC experts have also developed air conditioning BTU charts for general sizing as follows;

- For 150 sq. ft. you need 5,000 BTUs
- For 250 sq. ft. you need 6,000 BTU
- For 300 sq. ft. you need 7,000 BTU
- For 350 sq. ft. you need 8,000 BTU
- For 400 sq. ft. you need 9,000 BTU
- For 450 sq. ft. you need 10,000 BTU
- For 550 sq. ft. you need 12,000 BTU
- For 700 sq. ft. you need 14,000 BTU
- For 1,000 sq. ft. you need 18,000 BTU

Beyond 1,000 square feet, add 3,000 BTU for every additional 200 square feet. Thus, for a 1,500 square-foot room, you need about 25,500 BTU.

If you’re short on time, consider using the above chart. You can see that it recommends between 10,000 BTU and 12,000 BTU for a 500 square-foot area. To be precise, it recommends an 11,000 BTU air conditioner, which is close enough to what we recommended at the beginning.

## Calculating your Exact BTU Requirement

Unfortunately, generalizations don’t always work. For example, an 11,000 BTU air conditioner would work in ideal circumstances, i.e., in an area with average climate, sound insulation, and located in an area that’s not exposed to too much sunlight.

But, very few homes are located in the ideal area, right. The room you seek to condition may be located in the extreme cold of Alaska, where even the hottest summers are about 40 ˚F, or in the middle of Texas, where the average summer temperature is about 70˚F. An 11,000 BTU air conditioner wouldn’t have the same cooling effect in these two areas.

Therefore, a better way to determine the best air conditioner size is to calculate your requirements manually. Fortunately, it’s a simple process.

### Determine the size of the exact size of the room

It’s possible that the room in question isn’t even 500 square feet. To be entirely sure, get your tape measure and calculate the area in square feet.

**Measure the length and width**: Since most rooms are either rectangular or square, determining the area should be easy. Measure the length (longer side) and then the width (shorter side). All measurements are taken at the floor level. You don’t need the height measurement for this step.**Multiply L x W to determine area**: You can easily determine the area of a square or rectangular space by multiplying the length and width. Most 500 square-foot rooms are either 25 feet x 20 feet for rectangular rooms or approximately 22.36 feet x 23.36 feet for square rooms.**Convert meters-squared to feet-squared**: Some people find it easier to take measurements in meters rather than square feet. If you decide to go that route, make sure to take all the measurements in meters. Then multiply the length x width to find the area of the space in square meters. Then, convert the figure to square feet. One square meter equals 10.764 square feet.

### Calculate the area of unique spaces

If the room is an unusual shape or comprises squares and rectangles plus a few unusually shaped areas, then proceed as follows;

**Divide it into standard-shape areas**: Nearly all floor designs can be subdivided into squares, rectangles, triangles, and circles. A few may also comprise oval shapes.**Calculate the area of each space separately:**We already know that for squares and rectangles, you multiply the length by the width to determine the area. For triangles, the formula is; Area = ½ Length x Width. For circles, it’s: Area = πR-squared where π = 3.14 and R = the radius of the circle.**Find the sum of all the areas:**This step is as easy as adding the individual areas for all the differently-shaped sections of the room.**Multiply the area by 20 BTU**: To determine the BTU requirement for the area calculated above, multiple the figure 20 BTU. If it’s precisely 500 square feet, the final figure will be 10,000 BTU.

### Adjust for a few critical factors

This is where BTU-sizing variations emerge for similar-size rooms. It’s also where you’ll address the unique conditions of your application. Experts recommend that you adjust your BTU requirements based on four key factors.

**Ceiling height **

The air in the home or any room occupies every square inch of the room from floor to ceiling. Therefore, the ceiling height determines the amount of heat the room contains at any given time. A higher ceiling means a greater volume of hot, sweaty air in your home and vice versa.

Since the 20 BTUs/square foot formula assumes a standard 8-foot room, you must adjust it for lower or higher ceilings. Experts recommend adjusting the BTU requirement upwards or downwards by 10 for higher and lower ceilings, respectively. Therefore, for a 9-foot ceiling, you need a 110% x 10,000 BTU = 11,000 BTU AC whereas for a 7-foot ceiling, you need a 90% x 10,000 = 9,000 BTU AC.

**Sunlight and shade**

Rooms located in areas directly exposed to sunlight experience more heat. The opposite is true. Areas with plenty of shading typically experience less heat.

As a result, experts recommend adjusting the BTU requirement depending on the location of the room. A room located in an area exposed to direct sunlight (no trees nearby) increased the BTU requirement by 10%. If it’s located in a generally shady area (plenty of trees nearby), lower the BTU requirement by 10%.

**The number of occupants **

How many people will use the area at any given time by your estimates? One, two, five? You need to factor in the number of occupants because air conditioners are purposely acquired to cool people, not the home’s physical structure or your furniture. Moreover, humans release a lot of heat, especially during hotter weather.

The 20 BTU/square foot assumes a one-person occupancy. For every additional occupant, add 600 BTU to the final BTU requirement. Therefore, if it’s a bedroom that houses three people every day, you need to add 1,200 to your AC BTU requirements for the extra two occupants.

**Climate **

If you live in a warmer climate, the difference in the ambient air temperature the desired room temperature will be much higher. For instance, temperatures in Phoenix, Arizona, can rise to 105˚F in the middle of summer. Meanwhile, even the hottest summers in San Francisco, California, rarely exceed 61˚F.

You can therefore see that you may not even need an air conditioner in the first place in San Francisco, whereas you need a very powerful AC in Phoenix. Some experts say you may need an AC twice as strong for the hotter climates as the milder climates.

**Number of doors and windows**

Finally, the number of doors and windows also directly impacts the air conditions inside your home. A room with three doors and four windows naturally allows more hot air to enter and more cool air to exit, leaving the area generally hotter than a room with, say, just one door and one window.

Here too, the recommendation is to adjust your BTU requirement by 10% for every additional opening beyond one door and one window.

## So, How Many BTUs do You Need for 500 Sq. Ft.?

On average, you need 10,000 BTUs for a 500 square foot room. However, the actual figure can vary from zero to as much as 20,000 BTUs.

Assuming that it’s a 9-foot high room, with two doors and two windows that regularly houses three occupants and is directly exposed to sunlight, you’d need a 16,379.9 BTU AC (approximately 16,500 BTUs), and that’s assuming you live in a moderate climate, such as Florida, not Phoenix, AZ.