What Is an AC Size (BTU) Calculator?
An AC Size (BTU) Calculator is a free tool used to determine the correct cooling capacity needed for a room air conditioner. It uses your room's dimensions and environmental factors like insulation and sun exposure to provide a precise recommendation in BTUs (British Thermal Units) per hour, helping you choose an AC that is both effective and energy-efficient without overspending. This ensures your space stays comfortable on hot days.
Last updated: November 8, 2025
How to Use the BTU Calculator
Our free BTU calculator gives you an accurate AC size recommendation in four easy steps:
- Enter Room Area: Input your room's square footage (length x width).
- Provide Ceiling Height: Enter the height of your room's ceiling in feet.
- Select Insulation Quality: Choose whether your room's insulation is good, average, or poor.
- Indicate Sun Exposure: Specify if the room is mostly sunny or shady during the day.
- Click "Calculate": Get your instant, accurate BTU/hr capacity recommendation.
Understanding Your BTU Calculation
Your air conditioner's performance depends entirely on choosing the right size for your space. Our BTU calculator provides a precise cooling capacity recommendation to ensure your room stays comfortable and your energy bills stay low.
- Estimated BTUs/hr: This is the key number. It represents the amount of heat the AC unit needs to remove from your room every hour to be effective. Match this number as closely as possible to the BTU rating on an air conditioner you're considering.
- Tonnage Equivalent: For central air systems, BTUs are often converted to 'tons'. This is provided for reference, where 1 ton of cooling equals 12,000 BTU/hr. This calculator is primarily for single-room units.
According to the Department of Energy, an improperly sized air conditioner can increase your electricity usage by up to 15% and provide less-than-optimal cooling performance.
Key Factors That Affect BTU Requirements
More than just square footage goes into finding the right AC size. These factors are built into our calculator:
- Room Size: Larger rooms require more BTUs to cool the greater volume of air.
- Sunlight: Rooms that receive a lot of direct afternoon sun require about 10-15% more cooling power.
- Insulation: A poorly insulated room with drafty windows will need a more powerful AC unit to compensate for the constant heat gain.
- Occupancy: While minor for most residential rooms, a space regularly occupied by many people will need slightly more cooling power (add ~600 BTU per person).
Example BTU Calculation
Here is a common scenario for sizing an AC unit for a master bedroom.
| Input | Example Value |
|---|---|
| Room Area | 300 sq ft |
| Ceiling Height | 8 ft |
| Insulation | Average |
| Estimated Cooling Load | ~8,000 BTU/hr |