Tonnage
How AC capacity is measured — one ton equals 12,000 BTU/hour of cooling.
Tonnage is the cooling capacity of an air conditioner, where one ton equals 12,000 BTU per hour — a unit that dates back to how much ice it would take to provide the same cooling. Residential systems typically run 1.5 to 5 tons. Bigger isn't better: an oversized unit cools the air quickly, shuts off, and never runs long enough to pull humidity out, leaving a home cold but clammy while the [compressor](/glossary/compressor) wears out from short-cycling. The right size comes from a Manual J load calculation — which weighs your climate, insulation, windows, and air leakage — not from a square-footage guess or matching whatever was there before. Tonnage is the single biggest driver of [AC replacement cost](/guides/central-ac-replacement-cost).