Proper air conditioning sizing is one of the most important steps in a home comfort project, yet many bids still rely on quick rules of thumb. You may hear “one ton per 500 square feet,” but that approach ignores insulation quality, window efficiency, ceiling height, how airtight the home is, and the design temperatures for your climate. Two houses with the same floor plan can need different equipment sizes if one has a vented attic with leaky ducts and the other has upgraded insulation and double-pane windows.
The industry standard for residential cooling loads is a Manual J calculation using ACCA-approved software. Manual J estimates how much capacity the home needs on a design day, expressed in BTUs per hour and converted to tons (one ton equals 12,000 BTU/hr). A qualified contractor inputs wall assemblies, window types, orientation, internal gains, and local weather data. That result should drive equipment selection—not habit or guesswork.
Oversizing is a frequent problem. An AC that is too large cools the thermostat quickly but runs in short cycles, which limits moisture removal and can leave rooms feeling damp. Frequent cycling also adds wear on the compressor and fan. Undersizing leaves the system running near continuously on peak days and may not hold comfort during the hottest afternoons.
When comparing contractor bids, ask how tonnage was determined. A reputable proposal should reference a load calculation or clearly explain assumptions about your home. If two bids differ by a full ton with no explanation, that gap deserves scrutiny before you sign.