What Size Air Conditioner Do I Need?

"What size AC do I need?" is the most important question to answer before buying. The right size keeps you comfortable and saves energy. The wrong size — too small or too large — costs more and performs worse. Here's exactly how to calculate the right size for your space.

Step 1: Measure Your Room

1. Measure the length of the room in feet
2. Measure the width of the room in feet
3. Multiply: Length × Width = Square footage

Example: A room 15 ft long × 12 ft wide = 180 sq ft

Step 2: Find Your Base BTU

Step 3: Adjust for Your Conditions

Add or subtract BTU based on these factors:

• Sunny room (south/west facing): Add 10%
• Heavily shaded room: Subtract 10%
• More than 2 people regularly in the room: Add 600 BTU per extra person
• Kitchen: Add 4,000 BTU
• High ceilings (above 9 ft): Add 10–20%
• Poor insulation / older home: Add 10%
• Very hot climate (consistently above 95°F): Add 10%

Worked Examples

Example 1: Small Bedroom

• Room: 12 ft × 10 ft = 120 sq ft
• Base BTU: 5,000
• Sunny room: +500 BTU
• Result: 5,500 BTU → Choose a 6,000 BTU unit

Example 2: Master Bedroom

• Room: 18 ft × 16 ft = 288 sq ft
• Base BTU: 7,000
• Shaded room: −700 BTU
• Result: 6,300 BTU → Choose a 6,000–7,000 BTU unit

Example 3: Open-Plan Living Room

• Room: 25 ft × 20 ft = 500 sq ft
• Base BTU: 12,000
• South-facing, sunny: +1,200 BTU
• 3 people regularly: +600 BTU
• Result: 13,800 BTU → Choose a 14,000 BTU unit

Recommended Units by Size

• Small rooms (up to 150 sq ft): Amazon Basics 5,000 BTU Window AC
• Medium rooms (up to 350 sq ft): BLACK+DECKER 8,500 BTU Portable AC
• Large rooms (up to 500 sq ft): Whynter 14,000 BTU Dual Hose Portable AC
• Very large rooms (up to 1,020 sq ft): Frigidaire 24,000 BTU Smart Inverter Window AC
• Whole room / permanent install: Senville LETO 24,000 BTU Mini Split

Why Not Just Buy Bigger?

An oversized AC short-cycles — it cools the air temperature quickly but doesn't run long enough to remove humidity. The result is a room that feels cold and clammy rather than comfortably cool. It also wears out the compressor faster and costs more to buy and run. Always match BTU to room size rather than going bigger "just in case."

Five minutes of measuring and calculating saves you from years of an AC that doesn't perform the way you expect. Get the size right and everything else follows.