Window units run from 5,000 BTU (≈150 sq ft) to 24,000 BTU (≈1,000+ sq ft). Oversizing a window unit is the most common mistake — it cools fast but leaves the room damp.
Estimate only — confirm with a professional load calculation.
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Window units come in tighter capacity steps than any other AC type (5,000 / 6,000 / 8,000 / 10,000 BTU and up), which makes right-sizing easy and oversizing tempting. Resist the bigger box: a window unit's dehumidification depends on long run cycles, and an oversized one turns a bedroom cold and clammy — the classic short-cycling problem in miniature. Check three things besides BTU: the chassis width against your window opening, the plug and circuit (above 15,000 BTU many units need a dedicated 20 A circuit — the ampere calculator covers it), and the CEER efficiency rating, where a couple of points is real money over a six-month season. Beyond 24,000 BTU a window unit stops being the economical answer — compare a mini-split before buying two.
A 10×12 bedroom (120 sq ft) in a moderate climate needs about 5,000–6,000 BTU. Add roughly 10% if it's sunny or on the top floor.