Air Conditioner Size Calculator

How to Use This Calculator

1. Measure your room's length and width in feet for accurate square footage
2. Enter the ceiling height (standard is 8 feet; older homes may have 9-10 feet)
3. Specify the typical number of people who occupy the room regularly
4. Select sun exposure level based on which direction your windows face
5. Choose insulation quality based on your building's age, construction, and condition
6. Select room type (kitchens generate significant heat from appliances)
7. Click "Calculate BTU Needed" to see personalized recommendations

For open floor plans, calculate the total connected square footage rather than individual rooms.

What is BTU?

BTU stands for British Thermal Unit, the standard measure of thermal energy used throughout the HVAC industry. In air conditioning, BTU/hr (BTUs per hour) indicates the cooling capacity of an AC unit - specifically, how much heat it can remove from a space in one hour. Technically, one BTU is the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit.

A higher BTU rating means more cooling power, but bigger is not always better. Choosing the correct BTU capacity is crucial for comfort and efficiency. An undersized unit will run constantly, struggle to cool the room, and increase your electricity bills. An oversized unit will cycle on and off too frequently (short-cycling), wasting energy, creating temperature swings, and failing to properly dehumidify the air - leaving you with a cold but clammy room.

How BTU Requirements Are Calculated

The base calculation follows ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) guidelines, which are used by HVAC professionals nationwide:

Base BTU = Square Footage x 20 BTU

This baseline is then adjusted for real-world factors that affect cooling load:

• Occupants: Each person generates about 600 BTU of body heat. Add 600 BTU per person beyond 2
• Ceiling height: Higher ceilings mean more air volume. Multiply by (height/8) for non-standard ceilings
• Sun exposure: South and west-facing windows receive intense afternoon sun. Reduce 10% for shaded rooms, increase 10% for sunny rooms
• Insulation quality: Older buildings lose cooling faster. Reduce 10% for well-insulated, increase 10% for poor insulation
• Kitchen: Cooking appliances, refrigerators, and dishwashers add significant heat. Add approximately 4,000 BTU for kitchen applications

Understanding AC Types

Different AC types suit different installation needs:

• Window units: Self-contained, affordable, easy to install. Best for single rooms up to 12,000 BTU
• Portable AC: No installation required, can move between rooms. Less efficient than window units
• Mini-split (ductless): Quiet, efficient, requires professional installation. Ideal for additions, converted spaces, and homes without ducts
• Central AC: Cools entire home through ductwork. Most efficient for whole-house cooling. Sized in tons (1 ton = 12,000 BTU)

BTU to AC Size Guide

• 5,000-6,000 BTU: 100-250 sq ft (small bedroom, home office)
• 7,000-8,000 BTU: 250-350 sq ft (large bedroom, small living room)
• 9,000-10,000 BTU: 350-450 sq ft (living room, studio apartment)
• 12,000 BTU (1 ton): 450-550 sq ft (large living room)
• 18,000 BTU (1.5 ton): 700-1,000 sq ft (small apartment, multiple rooms)
• 24,000 BTU (2 ton): 1,000-1,400 sq ft (large apartment, small home)
• 36,000+ BTU (3+ ton): 1,400+ sq ft (full home, requires central system)

Energy Efficiency Tips

• Look for ENERGY STAR certified units for 10-15% energy savings
• Higher SEER (Seasonal Energy Efficiency Ratio) ratings mean lower operating costs
• Proper sizing is the most important factor for efficiency
• Use fans to circulate cool air and allow higher thermostat settings
• Close blinds during peak sun hours to reduce cooling load