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You need about 18,000 BTUs for 900 square feet. This is the figure most experts will give you for rooms between 700 square feet and 1,000 square feet before considering the room’s unique features.

The figure is based on the general recommendation of 20 BTUs per square foot. Thus, multiplying 20 by 900 gives you 18,000 BTUs.

If you’re in a hurry, use it. It works in most cases. However, if you have the time, you can manually calculate the right BTU value for your 900 square foot room. It’s a simple, straightforward process as long as you have a few details about the room and your home.

We’ll discuss how to do the calculations shortly. But first, it helps to understand how AC capacity is measured.

## Understanding British Thermal Units (BTUs)

Air conditioner cooling capacity is measured in British Thermal Units (BTUs). One BTU is defined as the amount of heat required to raise one pound of water temperature by one degree Fahrenheit. One BTU is also approximately equal to 1055 joules or 252-gram calories.

You may also come across air conditioner capacities given in tons. Just remember that one ton is equivalent to 12,000 BTUs. Therefore, 0.5 tons = 6,000 BTUs and 3 tons = 36,000 BTUs.

Typically, you need more cooling BTUs for a larger room because of the larger volume of air. Conversely, smaller rooms have a lower air volume, therefore, can run on smaller air conditioners.

## Factors That Determine How Many BTUs You Need

First and foremost, the amount of cooling BTUs required for a given application depends on the size of the application, i.e., the size of the room. However, a few other considerations apply;

### Type of application/room

Two 900 square-foot rooms may require completely different AC cooling capacities because of their purpose in the home. Take an example of the dining area and the basement. The dining area is typically an open area with doors and windows that enable strong airflow.

Conversely, basements tend to be closed-up, with few windows and doors and minimal airflow. Therefore, the basement is likely to be hotter than the dining room.

### Your geographic location

People living in the north rarely need air conditioning. And even those who do require very little of it. Why? Because the north is naturally cool. Even the hottest summers in Alaska and North Dakota rarely exceed 70˚F.

Meanwhile, the southern states are warm throughout the year and become scorching hot in the summer. So you need more air conditioning if you live in these southern states.

### How many people use the room?

Humans transmit heat too, and the hotter the air around us, the more heat we transmit. Now, imagine if you have ten people in the room! It instantly makes the area a lot hotter. As a result, you need more air conditioning power for rooms that typically house more people.

For example, a 900 square-foot living room that serves a family of seven people requires more air conditioning than a 900 square-foot living room that only serves two people.

## Three Ways to Determine What Size AC You Need for 900 Square Feet

You can determine the correct size AC size for your application in three ways, i.e., using general expert recommendations, using online BTU calculators, and manual calculation.

### General BTU Sizing Recommendations

HVAC experts have developed generalized sizing guidelines to help shoppers pick the closest possible air conditioner size for different room sizes. The recommendations are as follows;

- For areas up to 250 square feet, you need a 6,000 BTU AC
- From 251 to 350 square feet, you need an 8,000 BTU AC
- From 351 to 450 square feet, you need a 10,000 BTU AC
- From 451 to 550 square feet, you need a 12,000 BTU AC
- From 551 to 700 square feet, you need a 14,000 BTU AC
- From 701 to 1,000 square feet, you need an 18,000 BTU AC
- From 1,001 to 1,200 square feet, you need a 21,000 BTU AC
- From 1,201 to 1,400 square feet, you need a 23,000 BTU AC

From 1,401 square feet onwards, add 1,500 BTUs for every additional 100 square feet. So, for instance, you need about 27,500 BTUs for a 1,700 square foot room/home.

A key advantage of using these general BTU sizing recommendations is that you don’t have to do any calculations. You can quickly identify the best AC size for your application as long as you know the area of the room.

Unfortunately, generalized recommendations can be a bit misleading because they don’t factor in your geographic location or the unique conditions of your room.

### Using BTU Sizing Calculators

If you want a more accurate figure, consider using online calculators. An online calculator will give you an answer in as little as two minutes if you have the required information ready. In many cases, you’re required to provide information about the following;

**Area of the room**

For standard square and rectangular rooms, calculating areas is as simple as multiplying the length and width of the floor. However, if the room comprises different shapes, begin by dividing the floor into standard shapes, i.e., rectangles, squares, and circles. Then calculate the area of each part separately and add all the areas.

**Room type**

For which room are you buying the AC? Is it the bedroom, kitchen, garage? You need to input the room type because different rooms in the home have different air conditioning requirements. For example, bedrooms typically require less air conditioning than kitchens, and kitchens may need less air conditioning than basements. Rooms on the uppermost floors are also generally hotter thus need more air conditioning.

**Number of occupants**

How many people use the room on a typical day? Is it one person, two people, or perhaps five or ten people? You need to indicate the number of occupants because humans are a significant heat source in the home. We transmit a lot of heat, especially when exposed to a hot environment. The higher the number of occupants, the greater the heat output.

Once you input the above data, the calculator does the math in the background and displays the results in a matter of seconds.

The main benefit of using online calculators is that they’re very accurate and often free. Also, you get the results right away without doing the math yourself. However, a key downside is that you need to key in data, some of which may require that you measure your room.

### Calculate the AC Size Manually

The third and arguably the best way to determine what size air conditioner you need for a 900 square-foot area is to do the math yourself. The process is a little more involving than the first two. However, you’re guaranteed a more accurate figure that considers all the unique conditions of your location and home.

**Determine the area of the room**

Suppose you’re confident that it’s 900 square feet, then great. However, if you’re not, then this is the time to measure. Measure at length and width and multiply the two numbers to determine the area. Remember to take your measurements in feet to calculate the area in square feet.

**Multiply the figure by 20**

The general recommendation is to add 20 BTUs of cool air for every square foot. Therefore, you need 20 x 900 = 18,000 BTUs for a 900 square-foot area. If you calculated the area in square meters, convert the figure to square feet before multiplying by 20. One square meter equals 10.764 square feet.

**Adjust for ceiling height**

The 20 BTUs/square foot recommendation is for standard-height homes with 8-foot ceilings. If the house has a lower or higher ceiling, you need to adjust the metric accordingly to account for the lower or higher air volume. For example, reduce the number by 10% per foot for lower ceilings and increase it by 10% per foot for higher ceilings.

**Add 600 BTUs for every extra occupant**

The 20 BTUs/square foot recommendation also assumes single occupancy. If the room houses two or more people at any given time, add 600 BTUs to the final BRU requirement for every extra person.

For example, if the room regularly houses six people, you need to add 600 BTUs for the five extra people. In total, you need an extra 3,000 BTUs. So, if you initially needed 18,000 BTUs, you’ll now require 21,000 BTUs.

**Is the area exposed to sunshine?**

Rooms directly exposed to sunshine can be a little hotter than the rest of the home during the summer. At the other end, rooms in heavily shaded areas tend to remain cool even in the scorching heat. To this end, adjust your final BTU requirement upwards by 10% for sunlight-exposed areas and downwards by 10% for those in heavily shaded areas.

**Add 4,000 BTUs for kitchens**

The kitchen tends to get pretty hot during the summer because of all the cooking. Therefore, you’re advised to add 4,000 BTUs to the final BTU requirement to account for the excess heat.

## Summary

The manual calculation approach guarantees an incredibly accurate measure of the amount of cooling BTUs required for any application. You can even factor in the insulation factor of the home/room a determined by an HVAC professional.