Central AC size starts with the home heat gain, expressed in BTU per hour or tons (1 ton = 12,000 BTU/hr). Rules of thumb (20 BTU per sq ft) screen candidates; the final size follows an ACCA Manual J room-by-room load and Manual S equipment selection.
Article
AC Sizing Guide: Tons, BTU, and Manual J
Rules of thumb screen candidates; Manual J and Manual S pick the winner.