🏠How many tons of central air does this house need?

Roughly 1,600 to 2,000 sq ft of conditioned space in a typical, reasonably insulated house. The spread is the honest part: the same 3 tons covers 2,000 sq ft in a well-sealed modern build and struggles at 1,600 in a 1960s house with single glazing and no wall insulation. The rate per square foot is a starting point, not a specification.

Yes, exactly. The unit dates from the ice trade: one ton of refrigeration is the rate of cooling produced by a ton of ice melting over 24 hours, which works out at 12,000 BTU per hour. It has nothing to do with the weight of the equipment, which is a surprisingly common assumption.

It short-cycles: the house hits the thermostat setpoint in a few minutes, the compressor stops, and the moisture that a longer run would have condensed out stays in the air. You end up with a cold, damp house, higher bills than a correctly sized system, and compressor wear from repeated starts. Oversizing is the most common fault found in existing installations.

Include any space with supply and return registers that you actually condition. A finished, ducted basement counts. An unfinished basement with no registers does not, and neither does an attached garage. Counting them adds a half ton to the estimate and pushes you into a unit that will short-cycle for the next fifteen years.

It changes how the system is designed more than how it is sized. Total conditioned area is still the input, but heat rises, so the upper floor peaks later and hotter and the lower floor overshoots. The fix is zoning, damper balancing or a separate system per floor — not more tons.

Sometimes for good reasons — Manual J counts west-facing glass, duct losses through an unconditioned attic and infiltration that no rule of thumb can see. Sometimes for a bad one: sizing generously means fewer callbacks in a heatwave and a larger invoice. Asking to see the Manual J output, and which rooms drove it, separates the two quickly.