HVAC Calculators

Every other material on this site rewards rounding up. Air conditioning punishes it. An oversized system satisfies the thermostat before it has run long enough to pull moisture out of the air, so it short-cycles — leaving a house that is cold and clammy while the compressor wears out faster than it should.

That is the opposite of the intuition most people bring to sizing, and it is why the tonnage calculator here rounds to the nearest half ton rather than up. A system that runs almost continuously on the hottest day of the year is not struggling; that is what correct sizing looks like.

A proper residential load calculation is ACCA Manual J. It accounts for orientation, window area and glazing type, insulation values, air-tightness, duct losses, internal gains and your local design temperature — and it produces a room-by-room result that a square-footage estimate cannot.

These tools take floor area and apply the adjustments that move the answer most: climate, ceiling height, sun exposure, occupancy and insulation quality. That is enough to sanity-check a contractor's proposal or scope a project. It is not enough to specify equipment for a whole-house replacement, and no page here pretends otherwise.

Room-by-room figures also do not simply add up to a whole-house size. Rooms peak at different times of day, so a house total is meaningfully less than the sum of its rooms.

Correctly sized equipment connected to undersized ductwork performs like undersized equipment. Most residential air handlers are rated to deliver their nominal airflow at half an inch of external static pressure, and a high-MERV filter in a slot built for a cheap one can consume half that budget on its own.

The duct and static pressure calculators exist for that reason. Poor airflow shows up as weak cooling, frozen coils and high bills rather than as an obvious fault, so it goes undiagnosed for years.