AC BTU Calculator — Free Online Air Conditioner Sizing Tool

How to Use the AC BTU Calculator

1. Enter the room size in square feet: Measure the room by multiplying its length by its width in feet and enter the total square footage. For a 20-foot by 25-foot room, enter 500. For irregularly shaped rooms, measure each section separately and enter the combined total. If you need help calculating the area, use our square footage calculator first. Include only the area you want to cool, not adjacent rooms with separate HVAC zones.
2. Set the ceiling height: Enter the ceiling height in feet. The standard is 8 feet, which is the baseline for BTU calculations. If your room has 9-foot or 10-foot ceilings, enter the actual height and the calculator will proportionally increase the BTU requirement. For vaulted ceilings that vary in height, use the average height. Rooms with standard 8-foot ceilings receive no ceiling adjustment.
3. Enter the number of occupants: Type the number of people who regularly use the room. Each person generates approximately 600 BTU of body heat per hour. The calculation assumes two people as the baseline, so additional occupants above two increase the BTU requirement. For bedrooms, two is typically appropriate. For family rooms or home offices with multiple occupants, enter the actual number.
4. Select environmental factors: Choose the sun exposure level from the dropdown: Heavy Sun for rooms with large south or west-facing windows that receive direct sunlight, Moderate Sun for average exposure, or Mostly Shaded for north-facing rooms or those shaded by trees or other buildings. Select the insulation quality: Poor for older homes with single-pane windows and minimal wall insulation, Average for standard construction, or Good for well-insulated modern construction with double-pane windows. Toggle the Kitchen switch on if the room includes a cooking area, which adds 4,000 BTU for oven and appliance heat.

The results show the base BTU requirement, the adjusted BTU after all factors, the tonnage equivalent, a specific equipment recommendation, and a list of each adjustment that was applied. Use the recommended unit size when shopping for window units, mini-splits, or central AC systems.

AC BTU Calculation Formula

Variables Explained

• Room Square Footage: The floor area of the room to be cooled, measured in square feet. This is the primary factor in determining cooling capacity. Larger rooms require more BTU to maintain comfortable temperatures. The base formula uses 20 BTU per square foot for cooling.
• Ceiling Height Factor: Rooms taller than 8 feet contain more air volume that needs to be cooled. The calculator multiplies the base BTU by the ratio of actual ceiling height to the standard 8 feet. A 10-foot ceiling results in a 1.25 multiplier (25% increase), while a 12-foot ceiling results in a 1.5 multiplier (50% increase).
• Sun Exposure: Heavy sun exposure (large, unshaded south or west-facing windows) increases the heat gain by 10%, while shaded conditions reduce it by 10%. Moderate exposure uses no adjustment. This factor accounts for solar heat gain through windows and walls.
• Occupant Heat Load: Each person beyond two adds 600 BTU to the total. The human body at rest generates about 400 BTU per hour, and during light activity this rises to 600 BTU or more. This adjustment prevents undersizing AC for rooms with regular gatherings.
• Kitchen Adjustment: Cooking areas add 4,000 BTU due to heat generated by ovens, stoves, and other cooking appliances. This is a significant addition that can increase the total BTU by 20% to 40% for smaller rooms. Kitchens that are part of an open floor plan should include this adjustment for the combined space.
• Insulation Quality: Poor insulation increases BTU needs by 20% because conditioned air escapes and outside heat enters more readily. Good insulation reduces needs by 15% because the room retains cool air better. Average insulation uses no adjustment.

Step-by-Step Example

Calculate AC BTU for a 600 sq ft living room with 10-foot ceilings, 4 occupants, heavy sun exposure, average insulation, and no kitchen:

1. Base BTU: 600 x 20 = 12,000 BTU
2. Ceiling adjustment: 12,000 x (10/8) = 15,000 BTU
3. Sun exposure (heavy, +10%): 15,000 x 1.1 = 16,500 BTU
4. Extra occupants: (4 - 2) x 600 = +1,200 BTU, total = 17,700 BTU
5. Rounded: 18,000 BTU = 1.5 tons
6. Recommendation: Mini-split or small central AC (1.5 ton)

This living room needs an 18,000 BTU (1.5-ton) air conditioning unit. A standard window unit tops out around 14,000 BTU, so a mini-split system or small central AC would be the appropriate choice for this space.

Practical Examples

Example 1: Angela's Studio Apartment

Angela lives in a 400-square-foot studio apartment in Phoenix, Arizona. The apartment has 8-foot ceilings, large west-facing windows (heavy sun exposure), average insulation, and includes an open kitchen. She lives alone. She needs to size a window AC unit:

• Base BTU: 400 x 20 = 8,000 BTU
• Ceiling adjustment: none (8-foot standard)
• Heavy sun exposure: 8,000 x 1.1 = 8,800 BTU
• Occupants: 1 person (below baseline of 2, no addition)
• Kitchen included: 8,800 + 4,000 = 12,800 BTU
• Rounded: 13,000 BTU, approximately 1.1 tons

Angela needs approximately 13,000 BTU of cooling capacity. A 12,000 BTU window unit would be close but slightly undersized given the Phoenix heat and west-facing windows. She selects a 14,000 BTU window unit, which provides a small safety margin for the extreme Arizona summers. The unit costs about $0.20 per hour to run, using our electricity calculator at 1,400 watts and $0.14/kWh.

Example 2: Brian's Home Office Addition

Brian converted his garage into a 300-square-foot home office with 9-foot ceilings. The room has poor insulation (the garage was not insulated to residential standards), moderate sun exposure, and two occupants (Brian and his business partner). No kitchen area:

• Base BTU: 300 x 20 = 6,000 BTU
• Ceiling adjustment: 6,000 x (9/8) = 6,750 BTU
• Sun exposure: moderate (no adjustment)
• Poor insulation: 6,750 x 1.2 = 8,100 BTU
• Occupants: 2 (baseline, no addition)
• Rounded: 8,000 BTU, approximately 0.7 tons

Brian needs about 8,000 BTU for his home office. A standard 8,000 BTU window unit or a mini-split system would handle this load effectively. However, Brian should also consider upgrading the insulation in his converted garage, which would drop the requirement to about 5,700 BTU (with good insulation) and save 30% on ongoing cooling costs. The insulation upgrade pays for itself within two to three cooling seasons.

Example 3: Michelle's Two-Story Home

Michelle is sizing a new central AC system for her 2,200-square-foot two-story home in Atlanta. The home has 9-foot ceilings on both floors, moderate sun exposure, good insulation (new construction), 4 regular occupants, and a kitchen on the first floor:

• Base BTU: 2,200 x 20 = 44,000 BTU
• Ceiling adjustment: 44,000 x (9/8) = 49,500 BTU
• Sun exposure: moderate (no adjustment)
• Good insulation: 49,500 x 0.85 = 42,075 BTU
• Extra occupants: (4 - 2) x 600 = +1,200 BTU, total = 43,275 BTU
• Kitchen: 43,275 + 4,000 = 47,275 BTU
• Rounded: 47,000 BTU = 3.9 tons

Michelle needs approximately 4 tons of central air conditioning. She can choose between a single 4-ton unit for the whole house or a zoned system with a 2.5-ton unit for the first floor (which has the kitchen and higher heat load) and a 1.5-ton unit for the second floor. The zoned approach offers better efficiency and comfort control, as the upstairs receives the natural heat rise from below.

AC BTU Sizing Reference Table

Base BTU assumes 8-foot ceilings, moderate sun, average insulation, 2 occupants, and no kitchen. Adjust upward for unfavorable conditions.

Tips and Complete Guide

Choosing Between AC Unit Types

The right type of air conditioning depends on your cooling needs, budget, and installation constraints. Window units are the most affordable option ($150 to $600), suitable for single rooms up to about 14,000 BTU, and require no permanent installation. Portable AC units offer flexibility but are generally less efficient and noisier than window units. Ductless mini-split systems ($2,000 to $5,000 installed) provide quiet, efficient cooling for one to four rooms without ductwork and are ideal for additions, converted spaces, and homes without existing duct systems. Central air conditioning ($3,500 to $7,500 installed) is the most efficient option for whole-house cooling when ductwork already exists. Consider energy efficiency ratings (SEER2 for central, EER for window units) along with BTU when making your decision.

Energy Efficiency and Operating Costs

The SEER2 (Seasonal Energy Efficiency Ratio 2) rating measures how efficiently a central AC converts electricity into cooling. Higher SEER2 means lower operating costs. As of 2026, the federal minimum SEER2 for residential central AC is 13.4 in northern states and 14.3 in southern states. ENERGY STAR certified units achieve SEER2 of 15.2 or higher. To estimate monthly operating cost, divide the BTU by the SEER2 rating, multiply by hours of operation, and multiply by your electricity rate. For a 36,000 BTU unit with SEER2 15 running 8 hours daily at $0.16/kWh: (36,000 / 15,000) x 8 x $0.16 = $3.07 per day, or about $92 per month. Use our electricity calculator for detailed cost projections.

Regional Climate Considerations

Your geographic location affects AC sizing beyond what the basic formula captures. In hot, humid climates like the southeastern United States, prioritize dehumidification capability alongside BTU rating. Slightly undersizing in humid climates can actually improve comfort because the unit runs longer cycles, removing more moisture from the air. In hot, dry climates like the southwestern United States, BTU capacity is the primary concern since humidity is not an issue. In northern climates with short cooling seasons, a slightly smaller unit may be cost-effective since it handles the typical summer day while only struggling during rare heat waves. A professional ACCA Manual J load calculation considers your exact climate zone, home orientation, window specifications, and insulation values for the most precise sizing.

Common Mistakes to Avoid

• Sizing AC based solely on square footage: The basic 20 BTU per square foot formula is a starting point, not a final answer. Ceiling height, sun exposure, insulation, occupants, and kitchen heat can change the required BTU by 30% or more. Always apply all relevant adjustments for accurate sizing.
• Buying an oversized unit for faster cooling: An oversized AC cools the temperature quickly but fails to dehumidify properly, leading to a cold and clammy environment. It also short-cycles, increasing wear on the compressor and reducing equipment lifespan. Properly sized units maintain both temperature and humidity at comfortable levels.
• Ignoring the difference between BTU cooling and BTU heating: Our BTU calculator focuses on AC cooling. A furnace or heat pump BTU calculator uses different multipliers (typically 30-40 BTU per square foot for heating, depending on climate). Do not use cooling BTU to size heating equipment or vice versa. Our BTU calculator handles both heating and cooling calculations.
• Neglecting ductwork condition: For central AC, even a perfectly sized unit performs poorly if the ductwork is leaky, undersized, or poorly insulated. Duct losses of 20% to 30% are common in older homes with ducts running through unconditioned attics. Have ductwork inspected and sealed before upgrading the AC unit.
• Not considering future changes: If you plan to add occupants, convert a space to a kitchen, or remove window shading (like cutting down trees), size your AC for the anticipated future conditions. It is more economical to install the right size once than to replace an undersized unit later.

Frequently Asked Questions