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Generally, you need an 18,000 BTU air conditioner for an 800 square-foot room. It’s a general figure assuming standard climatic conditions and sound insulation. It also assumes that you’re shopping for a conventional area, such as the living room or bedroom.

However, if you have time, you can use online calculators to determine the best AC size for your application based on your unique needs, including your geographic location and the number of people who use the area.

Even better, you can do the math on your own without online calculators. It may sound daunting, but it’s a straightforward process once you’re committed to it.

## Begin By Understanding BTUs

Before we get to the calculations, it helps to understand how air conditioners are sized and the most popular AC sizes.

The cooling capacity of an air conditioner is measured in British Thermal Units (BTUs). One British Thermal Unit (BTU) is defined as the amount of heat (or energy) required to raise the temperature of one pound of water by one degree Fahrenheit.

In cooling, the BTU rating defines an appliance’s capacity to remove heat from an area to lower the temperature of that area – measured per hour. It answers the question – how much heat can the AC remove per hour?

As such, a 5,000 BTU air conditioner can remove 5,000 British Thermal Units worth of heat per hour, while a 10,000 BTU unit can remove 10,000 British Thermal Units per hour.

Common AC sizes include 8,000 BTUs for small spaces, 12,000 BTU units for medium-sized applications, and 24,000 BTUs for slightly larger applications.

It’s also worth mentioning that air conditioners can also be sized in tons rather than BTUs. For instance, you may come across one-ton ACs, two-ton ACs, etc. Just remember that one ton = 12,000 BTU. Therefore a two-ton AC = 24,000 BTU and a 0.5 ton unit = 6,000 BTU.

## The Importance of Finding the Right AC Size

Typically, you need a larger air conditioner for a larger room. Alternatively, you can combine multiple air conditioners to serve a single room. The choice is yours. Some people even buy a single, whole-home air conditioner to serve the entire home.

The point is – it’s critical to find the right air conditioner size for your application. Buying an AC that’s too small or too big for the applications comes with serious consequences as follows;

### Risks of an Undersize AC

An under-size air conditioner is one that’s too small for the room. A good example is buying an AC designed for a small bedroom and using it in a 3,000 square-foot basement. The potential risks include;

**Overworking the AC:**When the AC is undersized, it needs to work harder and longer to reach and maintain the thermostat setting. Running harder and longer may cause issues such as faster wear and tear. It may also necessitate more frequent and costly repairs.**Inadequate air conditioning:**An even bigger problem is that you may not achieve the desired comfort levels in the first place. This is especially true if the AC is too small for the room. It may never reach or maintain the temperature on the thermostat.**Noise issues:**An overworked air conditioner is often a noisy air conditioner. Even if we ignore the likely damages to the fan that may cause noise, the AC must work harder, meaning the fan will produce more noise.

### Risks of an Over-size AC

An oversize AC is one that’s too big for the application. For example, if you buy a whole-home air conditioner rated 48,000 BTU and placed it in the bedroom. The risks include;

**Over dehumidification**: One thing many consumers tend to forget is that air conditioners are also dehumidifiers. They remove just as much moisture from your home as they do hot air. Removing too much of the warm air (faster than you can replace it) may leave your indoor air dry, exposing you to allergies and dry skin, among other health issues.**Too many start-stop cycles:**All modern air conditioners self-regulate by automatically going on and off to maintain the desired indoor temperature. If the AC is too big, it will reach the target temperature faster, resulting in shorter on-off cycles. Frequent on-off cycles can damage the motor and compromise the functioning of the compressor.**Higher cooling costs:**First off, bigger ACs consume more power. But their higher cooling costs here don’t usually arise from the massive size of the AC alone – the frequent on-off cycles are just as big a culprit. Air conditioners draw the most power (maximum power during any run cycle) when starting. More start episodes can, therefore, significantly increase your power bill.

## How to Determine the Best AC Size for your 800 Square Feet Application?

You can determine the best air conditioner size for your 800 square-foot application in three broad ways;

### Follow General AC Sizing Guidelines

This is the simplest but also the least accurate method. Basically, you just need to follow the standard guidelines set out by HVAC professionals. Some of the most common recommendations are as follows;

- For up to 250 square feet, get a 6,000 BTU AC
- For 251 to 300 square feet, get a 7,000 BTU AC
- For 301 to 400 square feet, get a 9,000 BTU AC
- For 401 to 550 square feet, get a 12,000 BTU AC
- For 551 to 700 square feet, get a 14,000 BTU AC
- For 701 to 1,000 square feet, get an 18,000 BTU AC
- For 1001 to 1,200 square feet, get a 21,000 BTU AC

From 1,201 square feet onwards, add 3,000 BTU for every additional 200 square feet. So, for instance, you need about 30,000 BTU of cooling power for a 1,800 square foot area.

### Use Online Calculators

If you’re determined to get a more accurate figure and have the time, online AC sizing calculators will give you a ball-park value in less than three minutes.

You’ll need to provide the size of your application (in this case 800 square feet), your area code, the type of room (kitchen, sitting room, dining area, basement, etc.), and perhaps the type of AC you intend to buy. Some calculators may even ask for the efficiency of the AC.

We just want to stress two things;

**Your geographic location**: Always work with an AC sizing calculator that considers your geographic location. The app/software may ask that you click/touch your location on a given map or enter your zip code. Whichever method they use, this information is critical as different parts of the country have different climates, with entirely different cooling needs.**Type of room/application:**The best AC sizing calculators also factor in the variations in the cooling needs of the different rooms in the home. For instance, the basement is typically hotter, thus, it may need a more powerful air conditioner. The same applies to the kitchen.

### Manual Calculation

The final and best approach to determining the best AC size for your 800 square-foot room is to do the math manually. Proceed as follows;

**Measure the size of the room (in feet)**

Are you sure it’s 800 square feet? The only way to find out is through manual measurement. Get your tape measure and measure the length (longer edge) and width (shorter edge). For circular rooms, measure the radius or diameter.

**Calculate the area (in square feet)**

For rectangular and square rooms, multiply the length by the width to determine the area in square feet. Meanwhile, to determine the area of triangular shapes, use the formula – area = ½ Length x Width. For circular sections, use the formula, where π = 3.14.

**Add the sections and convert to square feet**

If the room comprises multiple sections, add the areas of all the sections to determine the total area. Then check again to ensure you took your measurements in feet and calculated the area in square feet. If you took the measurements in meters and calculates the area in square meters, convert the figure for the total area to square feet. 1.0 meters-squared = 10.76 feet-squared.

**Multiply the figure by 20**

Under normal circumstances, you need about 20 BTUs of cooling power for every square foot. Assuming that you’ve determined that the room is indeed 800 square feet, multiply the figure by 20 BTU/sq. ft. to determine how many BTUs you need under standard conditions. It comes to 16,000 BTU.

**Adjust for important factors**

This is the most important part of the manual calculation process. Otherwise, there’s no difference from using the available sizing tables found online.

- Add or deduct 10% from the final figure per foot if your ceiling is higher or lower than 8.0 feet, respectively.
- Add or deduct 10% if the room is exposed to sunlight or located in a heavily shaded location, respectively.
- Add 4,000 BTU to the final figure if you’re shopping for an air conditioner to use in the kitchen.
- Add 600 BTU for every extra person if the room typically houses two or more occupants at any given time.
- Adjust for insulation. You’ll need first to determine the area’s R-Value (resistance value). An HVAC professional is needed here.

## So, How Many BTUs Exactly Do You Need for 800 Square Feet?

For general purpose air conditioning, you need about 18,000 BTUs for an 800 square-foot area. However, once you adjust for your geographic location (climate) and factors such as shading, ceiling height, the purpose of the room, and insulation, you may find that you need a slightly smaller or much bigger AC.

An 800 square-foot basement in Arizona that serves as an entertainment center for four occupants, for instance, may need a 24,000 BTU air conditioner or higher.