Calculating the Right BTU For Your Space (Proper Sizing)

For those who are not electricians, the different measurements for power can be wildly confusing. But if you are looking into an air conditioner unit for your home, you have likely run across BTU as a measurement. BTU is a British Thermal Unit and measures the amount of air pushed through the unit each hour. Meaning smaller BTUs are meant for small rooms, while larger ones are for large open spaces.

Deciding how much BTU you need from an air conditioner depends on the square footage of the room you are wanting to cool down.

As a general rule of thumb, you can determine a sufficient BTU by multiplying the square footage of the room by 20. For example, a 1,000-square-foot room will need about 20,000 BTU to work efficiently!

BTU is important so that you do not get an AC unit that is too large and wastes energy, or is too small and does not cool the room well. Luckily it is easy to figure this measurement out! However, it is best to get a good overall understanding of AC unit efficiency. That way you can select the proper unit that works best for you!

What Does BTU Stand For? 

BTU is an acronym for British Thermal Unit. Essentially, this is just a unit of measurement that describes how much energy the AC unit uses each hour to remove heat from a room. If your air conditioner uses 10,000 BTU, it means that it is transferring 10,000 units of hot air from your home every hour. 

It is a very convenient tool to make sure you are not getting an AC that is too large or too small for your desired space. The measurement is easy to understand and very user-friendly. By being able to find the correct amount of energy you need, you will know exactly what AC unit to buy. That way you do not spend hours wondering which unit is the right one for you!

Getting the wrong-sized air conditioner is not great for two reasons. One, if the unit is too large you can easily overpay on your electricity bill for cool air that you do not need. Contrarily, if the unit is too small it can make your room sticky and hot. This means it will also break faster, costing you more money down the road.

How to Calculate BTU 

Many energy conversions are confusing, particularly for those who are not well-versed in energy calculations! You do not need to know the energy capacity of your house, just the square footage of the room you are aiming to install the AC in. There are a few other factors to consider, but it is easy to understand on your own.

To find the BTU you need, you first need to measure your room by the length and width of the space. Then you will multiply those two numbers together to get the square footage. Now to find the BTU you will simply multiply the square footage by 20. Here is a quick example:

Length: 40 feet, Width: 30 feet.

40 x 30 = 1,200 square feet

1,200 feet x 20 = 24,000 BTU

If you are wanting to cool an area that has a more complicated layout than 4 walls, just try your best to get a good estimation of the square footage. BTU does not need to be exact. In fact, you may want an AC unit that is just above the BTU you need. It is always better to have a bit more cooling power than not enough.

There are a few other factors to consider. If your room is heavily shaded or never sees direct sunlight, you can reduce the BTU by 10%. And if it sees a lot of sunlight, you should increase this by 10%. 

And if the room is a kitchen, you will want to add 4,000 BTU to accommodate the heat from cooking. Finally, if it is a regularly used room for your home you can add an extra 600 BTU per person. These alterations are recommended, but not required, to ensure the AC unit can be as efficient as possible!

What Size Air Conditioner is Right? 

Even if you do not want to calculate the exact amount of BTU you need for your home, there are some easy and general suggestions to follow. Again, these are all based on square footage. So you should have a rough idea of this measurement of space in your home to determine the size of air conditioner you need. Here is a chart of average room sizes and their equivalent BTU needs!

Room TypeAverage Square FeetBTU Needed
Bedroom/Office130 sq ft2,600 BTU
Living Room340 sq ft6,800 BTU
Kitchen170 sq ft3,400 BTU
Dining Room200 sq ft4,000 BTU
Basement800 sq ft16,000 BTU

Under 500 Square Feet

A 500-square-foot space is about 20 ft by 25 ft. This is equivalent to a bedroom or office space. It does not take a lot of energy to cool down an area of this size. You will likely need an air conditioner unit between 5,000 and 10,000 BTU. 

These units tend to be the smallest on the market. Of course, you can find all kinds of options available to you. But it is great to look for a compact air conditioner for this size room. As you do not want a large but weak unit taking up unnecessary space!

500 to 1,000 Square Feet 

To put it in perspective, a 500 to 1,000-square-foot space is the size of a small apartment. Roughly 30 ft by 30 ft. For this sized area, you will need approximately 16,000 BTU to properly regulate the air. However, this can vary if the space has multiple rooms or a lot of sunlight.

If this room is not well insulated, you might want to up your BTU capacity on the air conditioner unit. Poorly insulated areas can leak a lot of the cool air. So it may be good to consider using the cooling capacity of 2,000 BTU. This just ensures that the room can always be well maintained, especially if you live in an old home or apartment building.

1,000 to 1,500 Square Feet

A room within this range can be roughly 40 ft long and 30 ft wide. A room this size will generally need an air conditioning capacity of 22,000 BTU. When rooms get this big it is better to get exact calculations for BTU, but this is a good estimate to start with.

If this room is well insulated and does not see too much redirect sunlight, you could consider lowering the BTU capacity. Natural moderators of temperature like insulation of shade and reduce the need for extra air conditioner power. For rooms like this, you can reduce the BTU by about 3,000 BTU.

1,500 to 2,000 Square Feet 

Rooms that are between 1,500 and 2,000 square feet are the point where you will be needing one of the larger air conditioning systems. These tend to space measuring 40 ft by 45 ft.  You can approximate needing 30,000 BTU to cool the whole space. 

However, if the space has multiple large windows with lots of sunlight, you will need to increase this estimation to take into account the extra heat. If this is one large room, a single unit should suffice. But if you are trying to cool home this size the cool air will not properly reach the rooms farthest from the unit.

Over 2,000 Square Feet 

Rooms over 2,000 square feet can be roughly 50 ft by 45 ft in width and length. Since these rooms are large they usually need a lot of cooling capabilities to keep the whole space comfortable. Generally, you will need 34,000 BTU and above to maintain the cool air.

These tend to be the largest air conditioner unit. They are often capable of cooling many rooms at once with the doors open. If this space contains a kitchen or a highly trafficked area in the home, you may want to increase the BTU by a few thousand to accommodate the extra heat.

Is a Wall Through System the Same as a Window System? 

A wall-through AC system is not exactly the same as a window system, although they do work similarly! Both systems are used to cool a single room, unlike central cooling units. They are both similar in size and efficiency as well. The biggest difference is their placement and ventilation systems.

A window air conditioner unit shows the difference in its name! Window air conditioner units are meant to sit on a window sill so they are easy to install and do not require tricky installments. These units are very easy to install on your own and work great for budget homes.

Wall-through air conditioners are also pretty self-explanatory, as they are installed by going through walls. These systems require the wall to be cut and fit to the size of the vent for the air conditioner, where the air is then vented through this space. This system is more difficult to install on your own as it requires cutting through the wall precisely. That way, you do not lose cool air or energy between cracks in the wall.

If you are going for energy efficiency, the wall unit is slightly better than the window unit. That is, if the wall through the unit is properly installed and insulated. This is because the window unit is not as precise or secure as the well unit, even though it is easier to install. 


It is easy to feel overwhelmed when you are looking to beat the heat with a new air conditioner, but have no idea what to look for in a unit! It is convenient and easy to simply measure your space and convert its BTU needs with a simple calculation. Even if it is not exact, this number gives you a good range to look for while shopping and hopefully takes a bit of the stress away.

Here is an extra tip for those who made it through the whole article! If you want to improve the efficiency of your air conditioner even further than the BTU, keep the system in an AUTO setting. This will ensure that your AC unit is efficient at using the fan only when it is needed. All while still keeping your room consistently cool. It is easier than constantly adjusting the setting and can save you money on your energy bill in the future!