Worked example
A 150 sq ft room (9 ft ceiling) cooled by a 1.5 ton AC would take about 1.17 minutes to drop 6°C — for the air alone, with no ongoing heat gain.
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Room Cooling Time Calculator
Theoretical minimum time to cool your room's air
Theoretical minimum time
1.17 min
This is a theoretical minimum — how fast the AC could remove the heat already in the room's air, assuming no new heat enters. Real rooms keep gaining heat through walls, windows, the roof and occupants while the AC runs, so actual pull-down time is longer than this figure, sometimes by a wide margin on a hot day.
How this is calculated — and its real limit
Heat to remove. Q (BTU) = room volume (ft³) × 0.075 (air density, lb/ft³) × 0.24 (specific heat of air, BTU/lb·°F) × temperature drop (°F) — the standard sensible-heat formula, using real physical constants.
AC's effective cooling rate. We take the AC's rated BTU/hr (tonnage × 12,000) and apply a 75% sensible heat ratio, since some of an AC's capacity goes to removing humidity rather than lowering temperature.
What this deliberately leaves out. Walls, windows, the roof, sunlight and people all add heat to a real room continuously — this calculator only accounts for the air that's already there. That's why it's labelled a theoretical minimum, not a promise of real-world performance.
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Frequently asked questions
Why does the real room take longer to cool than this estimate?
This calculator answers a narrower question: how fast could the AC remove the heat already in the room's air, if no new heat came in. In reality, walls, windows, the roof and anyone in the room keep adding heat while the AC runs, so actual pull-down always takes longer — often much longer on a hot day or in a poorly insulated room.
Why is this still a useful number?
It's a genuine physics-based floor, not a guess — useful for comparing scenarios (a bigger AC vs a smaller one, a bigger drop vs a smaller one) even though the absolute real-world time will be higher.
What assumptions does this use?
Standard air density (0.075 lb/ft³) and specific heat of air (0.24 BTU/lb·°F) — textbook physical constants — plus an assumed 9 ft ceiling height and a 75% sensible heat ratio (the share of an AC's capacity that goes to temperature cooling rather than dehumidification), typical for split ACs.
Is my AC undersized if the real room takes much longer than this?
Not necessarily — this gap is expected and doesn't by itself mean your AC is undersized. If cooling is consistently slow or the AC never quite reaches the set temperature on hot days, check our AC tonnage calculator to confirm the unit is sized correctly for the room.