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.
| Recommended unit | — |
| Tonnage equivalent | — |
Window units come in tight capacity steps (5,000 / 6,000 / 8,000 / 10,000 BTU and up), which makes right-sizing easy — and oversizing tempting. Resist the bigger box: dehumidification needs long run cycles, and an oversized unit leaves a room cold and clammy (short-cycling in miniature).
Check three things besides BTU:
| BTU | Approx room | Circuit |
|---|---|---|
| 5,000–6,000 | 100–250 sq ft (bedroom) | Standard outlet |
| 8,000–10,000 | 300–450 sq ft | Standard outlet |
| 12,000–14,000 | 500–650 sq ft | Often dedicated |
| 15,000–18,000 | 700–1,000 sq ft | Dedicated 20 A |
| 24,000 | ~1,000–1,400 sq ft | Dedicated 230 V |
Beyond ~24,000 BTU a window unit stops being economical — compare a mini-split before buying two. For a room with no suitable window, a portable works but delivers only ~50–60% of its box rating.
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.
Yes — it's the most common window-unit mistake. An oversized unit cools fast, then short-cycles before it can wring humidity out of the air, leaving the room cold and clammy. Right-size, or size slightly under, for a dry, comfortable room.
Around 24,000–25,000 BTU (about 2 tons), enough for ~1,000–1,500 sq ft. Units above ~14,000 BTU usually need a dedicated 240 V (US) circuit rather than a standard wall socket — check before you buy.
A window unit cools more efficiently per BTU because it vents heat straight outside. A portable loses roughly 40–50% of its box rating through the exhaust hose and case (the SACC derate), so it needs a much bigger label number for the same room. See the portable AC calculator.
capacity ratings per AHRI 210/240 and ENERGY STAR sizing guidance — AHRI, ENERGY STAR. See our full methodology.