Generally, you need 9,000 BTUs for a 400 square-foot room. This is the average figure for standard rooms in standard climates. The actual figure will vary depending on several factors and considerations, including the number of occupants and the quality of insulation.
Consider two rooms, for instance. Both are 400 square feet in size. However, one is the master bedroom that serves two adults, whereas the other is a guest room that may even remain unoccupied throughout the summer season.
Or, imagine two living areas – both 400 square feet large – but one located in a 50-year old building and the other in a newly-built home. The older home is likely to have more inadequate insulation, with gaps all over.
These gaps allow col air to escape and warm air to enter the room compared to the newly-built home with new weather-stripping. Therefore, you need a more powerful AC for the older room.
The rest of this guide discusses how to find the perfect AC size depending on the condition of the room, the number of occupants, and several other factors.
Three Ways to Determine the Right Size AC for a 400 Square Foot Room
Before we get to the details, though, we must mention that there are three broad approaches to help you determine the number of BTUs needed for your 400 square foot room.
General AC BTU Sizing Recommendations
The first approach is to use general AC sizing recommendations. These figures are general guidelines and serve as a great starting point when shopping for an air conditioner. They can also prove a valuable point of reference when you’re in a hurry. They’re as follows;
- For rooms below 150 square feet, you need a 5,000 BTU AC
- From 150 to 249 Square feet, you need a 6,000 BTU AC
- From 250 to 299 square feet, you need an 8,000 BTU AC
- From 300 to 349 square feet, you need a 10,000 BTU AC
- From 350 to 399 square feet, you need a 12,000 BTU AC
- From 400 to 449 square feet, you need a 14,000 BTU AC
- From 450 to 499 square feet, you need a 15,000 BTU AC
- From 500 to 599 square feet, you need an 18,000 BTU AC
From 600 square feet onwards, add 2,000 BTU for every additional 100 square feet. So, for instance, a 1,000 square-foot room requires a 26,000 BTU air conditioner.
What we must stress about these figures is that they’re best used as general guidelines. Think about them as the first point of reference when shopping for an air conditioner. For instance, we now know that, roughly, you need a 14,000 BTU AC for a 400 square-foot room.
Online AC BTU Calculators
If you have time, you can then determine a more exact BTU requirement for your room based on the exact conditions in your room. The easiest way to do this is using online AC sizing calculators.
Beware that similar calculators exist for heating. Sometimes the same calculator may even calculate both. So before you begin entering figures and making calculations, make sure to set it to the correct mode for cooling AC calculations.
For most AC size calculators, you need a few things, namely;
- The size of the room (400 sq. ft. for this case)
- The ceiling height (8.0 feet for a standard home)
- The type of room (bedroom, kitchen, dining, etc.)
- Insulation conditions (good, average, or bad)
Most calculators ask for many more pieces of information for greater accuracy. You may even need to name your geographic location.
For the best outcome, we recommend finding an air conditioning BTU calculator for your region or one that allows you to pick your state (and possibly city) from a drop-down menu.
These calculators are more efficient because they automatically factor in the climatic conditions of your geographic location. Moreover, it saves you from the trouble of attempting to describe your climate.
Do the math, manually
The final and arguably best way to determine the AC size needed for your 400 square foot room is to do the math manually. The good news is – it’s not that difficult.
- Measure the floor size: This should be easy given that most rooms are either rectangular or square. Get your tape measure and measure the length (longer edge) and width (shorter edge) of the floor.
- Calculate the area: For squares and rectangles, the area is calculated as length x width. If it’s a square room, it will be 20 feet by 20 feet. Four hundred square-foot rectangular rooms are usually either 25 feet x 16 feet or 40 feet by 10 feet, though other measurement combinations are possible.
- Convert the area to feet: Some people prefer to take measurements in meters rather than feet. After finding the area of the room in square meters, convert the figure to square feet. One (1) square meter = 10.764 square feet.
- Determine the BTU requirement: The standard recommendation is 20 BTU per square foot. So, multiply the room area (in square feet) by 20 to determine the ideal BTU requirement for the room. For 400 square feet, it works out to 8,000 BTUs.
- Adjust for unique room and climate factors: Finally, you need to adjust the ideal BTU value (in d above) to determine the specific BTU requirements for the room. We discuss the most crucial considerations in the next part.
Top Factors to Consider When Calculating Air Conditioner BTU Sizing
Ideally, you may need to consider over a dozen factors, including the room’s purpose and even its position within the house, such as whether it’s on a lower or upper floor. However, the three most crucial considerations when determining the right size air conditioner for a 400 square-foot, and any room size for that matter, are as follows;
The desired temperature change
We buy air conditioners because it’s too hot inside the house. Therefore, the purpose of the air conditioner is to bring down indoor temperatures. The big question is, how much do you need to bring down temperatures in your home to make it cool and comfortable?
It’s easy to determine the desired change. Find the difference between the unaltered outdoor temperature and the desired temperature (most people are comfortable at 70˚F to 80˚F). Generally, it takes 0.24 BTU to raise the temperature of one pound of air by 1˚F.
Therefore, it follows that you may need a smaller AC if you live in a generally cooler area with average summer temperatures around 85˚F than in hotter climates where average summer temperatures are 95˚F or higher.
The average home has a ceiling height of 8.0 feet. Indeed, all the general recommendations above assume a ceiling height of 8.0 feet. If your ceiling is lower or higher, you need to adjust your BTU requirements to reflect the difference.
Experts recommend adjusting the figure by 10% up or down accordingly for every foot. For a higher ceiling, adjust the figure upwards by 10% for every foot. For lower ceilings, adjust it downward by 10% for every foot. For instance, if the room has a 10-foot ceiling, you need to adjust the standard 20 BTU/sq. ft. recommendation upward by 20% to 24 BTU/square foot.
Making adjustments to correct insulation variations is a little challenging. However, it’s just as important as the first two points above. Why? Because heat tends to move from warmer to cooler areas until there’s no longer a temperature difference.
During the summer, the hot, sweaty air from outside will seep inside the house via any available avenues, including cracks in the walls and gaps under the door, until the air inside is as hot as that outside.
Ideally, you want to bring in a professional to help you determine the R-Value (resistance value) of the home, i.e., the home’s resistance to conductive heat flow, before making a decision. However, it would help if you had more cooling BTUs for a poorly insulated home.
Other important considerations when calculating the cooling BTU requirement for a 400 square-foot room are as follows;
- The number of occupants: The body dissipates heat, contributing to higher temperature levels in the room. The higher number of people, the higher the likely temperature rise. It’s recommended to add 600 BTU for every additional occupant (i.e., from the second occupant onward).
- The presence of ceiling fans: If you intend to use ceiling fans as part of your overall air cooling strategy alongside the air conditioner, a smaller AC may do. Why? Because fans create a “cooling” feeling as they rotate. The breeze makes us feel cooler. So, you may not need as much AC cooling as in a room without a fan.
- The efficiency of the AC: Every air conditioner has an efficiency rating in EER, CEER, or SEER. These efficiency ratings tell you how much of the cooling BTUs go to good use. A 90% efficient 10,000 BTU air conditioner, for instance, only puts 9,000 BTUs to good use. The other 1,000 BTUs are lost. Keep this in mind too.
- The number of doors and windows: This factor is often addressed under insulation. But, just so you don’t forget, more doors and windows mean more entry points for warm air from outside and exit points for the cool air from inside the house. Therefore, you may need a larger AC for a room with more windows and doors.
So, What Size AC Do You Need for 400 Square Feet?
Ideally, you need 8,000 BTUs. However, the actual figure can range from about 7,000 BTUs to over 15,000 BTUs depending on your geographic location, the number of occupants, the ceiling height, the room’s insulation factor, and several other considerations.