Selecting the proper size air conditioner for your room or space is one of the most important decisions when purchasing a new unit. An air conditioner that is too small will not be able to cool your space efficiently, while one that is too large can be wasteful, noisy, and expensive to operate. Here is a comprehensive guide on how to calculate cooling capacity and choose the right BTU rating when buying an AC.
How to Determine the Correct Air Conditioner Size
Choosing an air conditioner with the proper cooling capacity involves carefully calculating the size of the space you need to cool. Here are the key factors to consider:
Measure Room Dimensions
First, meticulously measure the length, width and height of the room in feet that you want to cool. Be sure to measure from wall to wall for an accurate floor space. Also account for any open adjacent spaces like a kitchen or hallway that will need cooling.
Make note of the room’s total square footage by multiplying length by width. Also calculate the room’s cubic feet by multiplying length x width x height. Having both measurements is useful.
Window Unit vs Central AC
Window AC units are sized by square footage, while central air conditioners use a room’s cubic feet. Know which type of AC you are installing as it changes the calculation.
Consider Window Area
Windows admit a lot of heat from sunlight. Add up the total square feet of all windows in the space. If there is a large window area compared to floor space, you may want to increase capacity by 10%.
Factor in Room Use
Rooms used frequently or occupied by many people need more cooling power. Allow for extra capacity if the room will be crowded or used heavily. Similarly, rooms containing heat-generating appliances require larger ACs.
Understand BTUs
BTU or British Thermal Unit is the standard measurement of cooling capacity. The higher the BTU rating, the more powerful the AC unit. Calculate the minimum BTUs needed based on your room dimensions.
Allow for Future Expansion
It’s smart to purchase an AC unit with 10-20% more capacity than your calculations show to handle possible future use. A slightly oversized unit can run more efficiently than one working at full capacity.
By carefully following these steps and doing the measurements properly, you can zero in on the optimal BTU rating for adequate comfort and efficiency.
Air Conditioner Sizing Calculations
Here are the specific formulas and calculations to determine the appropriate BTU power for both window air conditioners and whole-home central air systems.
Window AC BTU Calculation
For square footage:
- 100 to 150 square feet = 5,000 BTUs
- 150 to 250 square feet = 6,000 BTUs
- 250 to 300 square feet = 7,000 BTUs
- 300 to 350 square feet = 8,000 BTUs
- 350 to 400 square feet = 9,000 BTUs
- 400 to 450 square feet = 10,000 BTUs
- 450 to 550 square feet = 12,000 BTUs
- 550 to 700 square feet = 14,000 BTUs
- 700 to 1,000 square feet = 18,000 BTUs
- 1,000 to 1,200 square feet = 21,000 BTUs
For cubic feet:
Take the room’s cubic feet and multiply by 20 to get the minimum BTU needed. For heavily used rooms, multiply by 25 instead.
Example:
Room size: 10 x 12 x 8 feet high = 960 cubic feet
960 cubic feet x 25 = 24,000 minimum BTUs
Central AC BTU Calculation
Multiply the cubic feet of your entire home by 25 to 35 BTUs per cubic foot. Use 25 for milder climates and 35 for very hot locations.
Example:
House with 5 rooms at 10 x 10 x 8 feet = 4,000 cubic feet per room
Total cubic feet = 5 rooms x 4,000 cubic feet = 20,000 cubic feet
20,000 cubic feet x 35 BTU per cubic foot = 700,000 minimum BTUs for central air conditioner
As shown in the examples, accurately measuring room dimensions and performing some simple math makes choosing AC cooling capacity much easier.
Factors that Lower BTU Requirements
Certain factors allow you to reduce the BTU requirements after initial capacity calculations:
- Shaded home – Heavy shade around your home from trees or overhangs reduces heat gain through windows, lowering BTU needs by 10%.
- Higher SEER rating – Central ACs with SEER 14 or above are more efficient so capacity can be reduced by about 5%.
- Newer windows – Well-insulated, modern windows admit less heat than old single pane windows, allowing a 5-10% reduction.
- Northern climate – Homes in cooler northern regions need smaller ACs than southern hot climates. Remove 10% capacity.
- Whole home fan – Using a whole house fan to move air reduces AC run time, allowing a 5-10% downsize.
- Higher thermostat setting – Setting the thermostat higher (78+ degrees) lowers BTU needs substantially, allowing for a unit one size smaller.
Carefully factoring in these elements can prevent getting an oversized AC and lead to better comfort and efficiency.
Signs You Need a Larger Air Conditioner
Here are signs that your existing AC may be underpowered and you need to move up to the next size:
- AC runs all the time but never reaches set temperature
- Cooling is uneven with hot and cold spots
- AC frosts over frequently from working too hard
- High cooling bills from AC constantly running
- Upstairs is always hotter than downstairs
- You often lower thermostat by several degrees to compensate
- House never “feels cold” even at full blast
If you experience these issues, carefully re-measure your rooms and windows and recalculate the needed capacity. Installing a right-sized or slightly oversized AC can provide better comfort.
Is Bigger Always Better for Air Conditioners?
It’s a common myth that it’s always wise to purchase the largest possible air conditioner. In reality, units that are oversized for your cooling needs can cause a number of problems:
- Higher cost – Larger ACs have higher purchase prices and installation costs. Electricity bills also rise since it runs less efficiently.
- Short cycling – An oversized unit will cool your home quickly but then shut off until the temperature rises again. This frequent short cycling reduces system lifespan.
- Lack of humidity control – Oversized ACs turn off before they’ve had a chance to remove humidity from the air.
- Noisy operation – When an AC is too large, it switches on and off rapidly, causing disruptive noise and vibrations.
- Cooling inconsistency – Rooms get too cold before humidity is lowered, then become clammy as unit continuously cycles on and off.
For optimal comfort and efficiency, it’s always wise to have an AC properly matched to the size of your space.
Choosing Central AC Unit Tonnage
Air conditioner size for central air systems is rated in tons. Each ton represents 12,000 BTUs of cooling capacity. Here is a tonnage guide:
- 1.5 tons = Good for 550 to 900 square feet
- 2 tons = Good for 900 to 1,200 square feet
- 2.5 tons = Good for 1,200 to 1,500 square feet
- 3 tons = Good for 1,500 to 2,000 square feet
- 3.5 tons = Good for 2,000 to 2,500 square feet
- 4 tons = Good for 2,500 to 3,000 square feet
- 5 tons = Good for 3,000 to 4,000 square feet
Again, correctly measuring room sizes and calculating total cubic feet is key to determining the right system tonnage. Consider the variables previously discussed as well.
Special Case: Additions and Renovations
If you are installing AC in an addition or renovating part of your house, be sure to do separate calculations just for the new space. This ensures proper cooling for the new construction without oversizing your existing central air system.
Many homeowners choose to add a smaller secondary AC system custom tailored for additions rather than upsizing their current unit. This can provide better comfort and efficiency.
Installing Multiple AC Units
For very large homes, it’s often preferable to install multiple smaller AC units rather than one huge central system. Benefits of going with several smaller ACs include:
- Zoned comfort – Different units can be set to customize cooling for occupied rooms like bedrooms vs. unused spaces.
- Reduced noise – Smaller ACs make less noise than one giant compressor.
- Enhanced efficiency – Individual ACs don’t have to work as hard to cool smaller zones. Units can be shut off in unused rooms.
- Air filtration – Separate ACs allow for better air filters to remove indoor pollutants.
- Lower cost – Depending on home layout, smaller units may have lower purchase and install costs than one giant AC.
For very large or multi-story homes, consulting an HVAC pro on multiple air conditioning units can provide advantages over a single large system.
Air Conditioner Size Comparison Tables
Here are helpful comparison tables to guide you in choosing the right air conditioner size based on square footage or BTUs:
Window Air Conditioner Sizing
| Square Footage | BTU Rating |
|-|-|
| 100 – 150 sq ft | 5,000 BTUs |
| 150 – 250 sq ft | 6,000 BTUs |
| 250 – 300 sq ft | 7,000 BTUs |
| 300 – 350 sq ft | 8,000 BTUs |
| 350 – 400 sq ft | 9,000 BTUs |
| 400 – 450 sq ft | 10,000 BTUs |
| 450 – 550 sq ft | 12,000 BTUs |
| 550 – 700 sq ft | 14,000 BTUs |
| 700 – 1,000 sq ft | 18,000 BTUs |
| 1,000 – 1,200 sq ft | 21,000 BTUs |
Central AC System Sizing
| Square Footage | System BTUs | Tons |
|-|-|-|
| 1,000 – 1,500 sq ft | 24,000 – 36,000 | 2 – 3 tons |
| 1,500 – 2,000 sq ft | 36,000 – 48,000 | 3 – 4 tons |
| 2,000 – 2,500 sq ft | 48,000 – 60,000 | 4 – 5 tons |
| 2,500 – 3,500 sq ft | 60,000 – 72,000 | 5 – 6 tons |
| 3,500 – 4,500 sq ft | 72,000 – 96,000 | 6 – 8 tons |
| 4,500 – 6,000 sq ft | 96,000 – 120,000 | 8 – 10 tons |
These tables provide a quick and easy BTU or tonnage recommendation estimate for both window units and central air based on the square footage you are cooling.
Air Conditioner Sizing Tips and Recommendations
- Accurately measure room dimensions for total square footage and cubic feet. This is vital for proper AC sizing.
- When in doubt, go larger. An AC that is slightly oversized is better than one that is undersized.
- Size the unit based on needed capacity at the hottest part of summer. It’s okay if AC is large for spring and fall use.
- For improved efficiency, look for the highest SEER rating you can afford when choosing the AC model.
- Install the AC unit in a shaded location if possible. This will increase its lifespan.
- Have an HVAC professional do a detailed heating/cooling load calculation for the most precise sizing.
- Choose Energy Star certified AC models for the best efficiency and proper sizing.
Carefully following these air conditioner sizing recommendations will ensure your unit provides ideal cooling capacity, comfort, efficiency, and performance.
Frequently Asked Questions About Air Conditioner Sizing
How do I measure room size to size an AC?
Use a measuring tape to meticulously measure room length, width and height in feet. Multiply L x W for square footage. Multiply L x W x H for cubic feet. Be sure to account for any open adjacent spaces.
What size AC unit do I need for a 500 sq ft room?
For a 500 square foot room, you will generally need a 6,000 BTU window air conditioner or 18,000 BTU central AC (1.5 tons). Make sure to calculate cubic feet as well.
My current AC is undersized. What are signs I need more capacity?
Signs of an underpowered AC are: unit running constantly but unable to cool, uneven temperatures, AC frosting over, high electricity bills, hot upstairs, having to lower thermostat substantially.
Should I buy a larger AC unit than my calculations show?
It’s wise to purchase an AC unit with 10-20% higher capacity than calculated as a buffer. A slightly oversized unit will run more efficiently than one working at full capacity.
What are disadvantages of an oversized air conditioner?
Oversized ACs cost more upfront, have higher electrical bills, short cycle on and off leading to wear and tear, lack humidity control, and cause noisy operation.
For a new home addition, should I get a separate AC?
For room additions, it’s often preferable to install a smaller secondary AC custom-sized for just that space rather than oversizing your existing central air unit.
What are benefits of installing multiple small AC units vs. one large one?
Benefits of multiple smaller ACs include: enhanced zoned comfort control, lower noise, increased efficiency, better air filtration, and possibly lower overall cost.
Conclusion
Choosing the optimal air conditioner size for your room or home is crucial to comfort and efficiency. Performing careful square footage and cubic feet calculations based on methodical room measurements will determine the correct BTU or tonnage rating. Consider all factors including window area, room use, climate, SEER rating, and future needs when sizing your AC. Allow for a small buffer with extra capacity if desired. Follow these guidelines to find the ideal AC unit for your cooling needs. With a properly sized, energy-efficient air conditioner, you can stay comfortable on even the hottest summer days.