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How to Calculate the Right AC Size
Getting the wrong size air conditioner wastes money in both directions. Too small and it runs constantly, driving up your electric bill without ever reaching temperature. Too large and it short-cycles — turning on and off repeatedly — which wears out the compressor and leaves you with clammy, humid air.
The formula is simple: Square footage × 20 BTU = baseline. Then adjust for your specific conditions.
Adjustment Factors
Sunny rooms — Add 10-15%. South and west-facing windows get pounded by afternoon sun.
High ceilings — Add 10-20%. More air volume means more work for the compressor.
Kitchens — Add 10%. Stoves, ovens, and dishwashers pump heat into the room.
Multiple occupants — Add 600 BTU per person beyond 2. Bodies are space heaters.
Shaded rooms — Subtract 10%. Trees and overhangs are free insulation.
Why Mini-Splits Beat Window Units
Window air conditioners are cheap upfront but expensive to run. A typical window unit has a SEER rating of 10-12. A modern mini-split runs at 20+ SEER — meaning it uses 40-50% less electricity for the same cooling.
Mini-splits are also whisper-quiet (21 dB vs 50+ dB for window units), don't block your window, and double as heaters in winter. The upfront cost pays for itself within 2-3 years in energy savings.
Quick Reference: Room Size to BTU
Room Size
BTU Needed
Recommended
150-300 sq ft
6,000-8,000
12,000 BTU unit (oversized for reliability)
300-550 sq ft
8,000-12,000
12,000 BTU unit
550-900 sq ft
12,000-18,000
18,000 BTU unit
900-1,400 sq ft
18,000-24,000
24,000 BTU unit
Frequently Asked Questions
How do I calculate what size AC I need?
Multiply room length × width for square footage. Multiply by 20 BTU per sq ft as a baseline. Adjust up for sunny rooms, high ceilings, kitchens, or multiple occupants. Our calculator above does this automatically.
What size mini-split do I need for 500 sq ft?
A 12,000 BTU mini-split handles up to 550 sq ft in standard conditions. For sunny or high-ceiling rooms, step up to 18,000 BTU to avoid the unit running constantly.
Is a mini-split cheaper than central AC?
Yes. Mini-splits cost $540-$1,300 installed DIY vs $3,000-$7,000+ for central AC professional installation. They're also 30-40% more energy efficient since there's no ductwork losing cool air in the attic.
Can I install a mini-split myself?
Yes — many modern mini-splits come pre-charged with refrigerant and include all mounting hardware. You'll need basic tools and about 4-6 hours. No HVAC license required for pre-charged units.
What does SEER2 rating mean?
SEER2 measures cooling efficiency. Higher = cheaper to run. A 20 SEER2 unit uses about 40% less electricity than a 12 SEER window unit. The minimum federal standard is 14 SEER2 for new units as of 2023.