Buying the wrong size air conditioner is one of the most common and expensive HVAC mistakes. An undersized unit runs constantly and never cools properly. An oversized unit cools too fast, leaving behind humidity and causing short-cycling that wears out the compressor.
What is BTU?
BTU stands for British Thermal Unit — it's the measurement of how much heat an air conditioner can remove per hour. A higher BTU means a more powerful unit. Matching BTU to room size is critical for efficiency and comfort.
Basic BTU Rule of Thumb
Start with 20 BTU per square foot of living space. Then adjust based on the factors below.
| Room Size (sq ft) | Recommended BTU |
|---|---|
| 100–150 | 5,000 BTU |
| 150–250 | 6,000 BTU |
| 250–350 | 8,000 BTU |
| 350–550 | 10,000 BTU |
| 550–700 | 12,000 BTU |
| 700–1,000 | 14,000–18,000 BTU |
Factors That Adjust Your BTU Needs
- Add 10% if the room gets strong direct sunlight
- Subtract 10% if the room is heavily shaded
- Add 600 BTU per additional regular occupant beyond 2 people
- Add 4,000 BTU if the unit will be used in a kitchen
- Higher ceilings (over 8 ft) require more BTU — add 10–15%
Central AC vs Window Units
For whole-home cooling, central AC is measured in tons (1 ton = 12,000 BTU). A typical 2,000 sq ft home needs 2.5–3 tons. Window units and mini-splits use BTU directly and are sized per room or zone.
Calculate the Right AC Size for Your Space
Enter your room dimensions and get an instant BTU recommendation.
Use AC Size Calculator →SEER Rating: Why Efficiency Matters
SEER (Seasonal Energy Efficiency Ratio) measures how efficiently an AC uses electricity. A higher SEER means lower energy bills. Current minimum is SEER 14 in the US. SEER 20+ units cost more upfront but save significantly on electricity in hot climates.