A full Manual J models the building envelope in detail: wall, roof and floor assemblies with their R-values; window areas, orientations and glazing types; air infiltration; duct location and leakage; internal gains from people and appliances; and local design temperatures from ASHRAE data. The output is a sensible and latent load for each room, summed for equipment selection.
Tools like ours apply the same load factors in simplified form — climate-based BTU per square foot adjusted for ceilings, sun, occupancy, insulation quality and roof exposure. That's accurate enough to sanity-check a quote, shortlist equipment, or catch a badly mis-sized recommendation. It is not a substitute for the full envelope model when you're buying a central system: a Manual J can legitimately differ from a rule-of-thumb estimate by a half ton or more on unusual homes.
Any new central system, any home with unusual glazing or cathedral ceilings, any persistent comfort complaint with correctly functioning equipment. Ask the contractor for the Manual J report — code requires it, and a contractor who sizes by square footage alone is guessing with your money.
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In the US, the International Residential Code (section N1103.7) requires equipment sizing per ACCA Manual J/S for new installations, and most states adopt it. Enforcement varies, which is why square-footage guessing persists.
Independent Manual J services typically run $99–$300 for a single-family home; many quality contractors include it in a replacement quote.
Put the numbers to work: AC tonnage calculator · bill calculator · model number decoder.