Not sure what size air conditioner to buy? Enter the room’s length and width — or tap a typical room — and the ceiling height. Then tell the calculator how sunny the room is, how many people usually use it and whether it’s a kitchen. It looks up the cooling capacity ENERGY STAR recommends for that floor area, applies the standard adjustments, and rounds up to a common window-unit size. You’ll also see the result in kilowatts and tons. Switch to Metric to enter meters.

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Estimate only. Top-floor rooms, poor insulation, big west-facing windows or a very hot climate may need a size up. Don't oversize by much — an AC that's too big cools fast but leaves the room damp.

How it works: the base size comes from the ENERGY STAR room air conditioner chart (for example 150–250 sq ft → 6,000 BTU, 450–550 sq ft → 12,000 BTU). Then: −10% for a heavily shaded room, +10% for a very sunny one, +600 BTU for each person beyond two, and +4,000 BTU for a kitchen. Ceilings above 8 ft add load in proportion to height (a rule of thumb, not part of the chart). 1 ton = 12,000 BTU/hr and 1 kW ≈ 3,412 BTU/hr. Portable ACs also list a lower DOE (SACC) rating — compare that number when shopping for one.