If you've ever asked a contractor, "What size AC do I need?" and gotten a quick answer based only on your square footage, you're not alone. It's the most common way homes get sized, and it's also one of the biggest reasons homeowners end up with a system that short-cycles, runs constantly, or leaves certain rooms too hot or too cold. The truth is, tonnage isn't a whole-house guess. It's a room-by-room calculation that depends on how much heat each space gains, not just how bi