What Size AC Does My Home Need?

Short answer: The right size comes from a Manual J load calculation, not a rule of thumb. It weighs square footage, insulation, windows, and sun. In Florida an oversized unit cools before it dehumidifies.

Square footage is a starting point, not the answer

The old rule of one ton per 400 to 600 square feet gets you in the ballpark, but it is not what should end up on your install paperwork. Two identical floor plans on the same street can need different tonnage once you account for which way the windows face, how much shade the lot has, ceiling height, and how well the attic is insulated. The right number comes from a Manual J load calculation, which runs those variables through software to output a real cooling load.

Why oversizing hurts in a humid climate

In Central Florida, humidity control matters as much as temperature. An oversized system cools the air quickly and shuts off before it has pulled the moisture out, leaving a home that feels cold and clammy at the same time and inviting musty odors. An undersized system runs constantly and still cannot keep up on the hottest afternoons, wearing itself out early. Correct sizing is what lets a system run longer, steadier cycles that actually dehumidify.

What a real load calculation measures

• Conditioned square footage and ceiling height.
• Window area, type, and orientation — west-facing glass adds significant afternoon load.
• Insulation R-values and air infiltration.
• The duct system's condition, because leaks change how much capacity reaches the rooms — see why one room stays hotter.

Getting the number right once

Ask any contractor quoting a replacement whether they are running a Manual J or matching the old nameplate — the answer tells you a lot about the install you are about to get. Every Chilly Billy replacement estimate includes the load calculation, and it is free. If you are still deciding whether to replace at all, start with repair or replace your AC.