Window unit BTU basics
Retail window ACs sell in 5,000, 8,000, 10,000, 12,000, and 15,000+ BTU steps. Calculate load first — then buy the next size up, not down. A 150 sq ft bedroom often lands near 5,000–6,000 BTU calculated; retail minimum is often 8,000 BTU.
Oversizing window units cycles on/off without dehumidifying well — sticky feeling despite cold air. Right-sizing matters more in humid climates.
ENERGY STAR charts vs this calculator
ENERGY STAR room-AC tables map square footage to BTU bands for average rooms. Use them as a baseline, then adjust for ceiling height, sun, occupants, and kitchen load with the window AC calculator.
A west-facing room with tall ceilings often needs one retail step above the chart. A shaded, well-insulated bedroom may sit at the low end.
When to skip window AC
Large open rooms, sunrooms, and additions with poor insulation may need 12,000+ BTU window units that are loud and block egress windows. Compare mini-split vs window AC before buying the biggest box at the store.
Rentals that ban brackets, or HOAs that ban visible units, may force a portable — size with the same BTU math, then expect a higher nominal rating. See portable AC vs window AC.