What Size Aircon for Bedroom? | BTU Chart & Formula

For the full breakdown, see our best Aircon For Bedroom guide.

Getting the air conditioner size wrong is the most common bedroom cooling mistake. A unit that’s too small runs constantly without ever catching up, while one that’s too large short-cycles, wastes electricity, and leaves the room clammy. The right size cools quietly and efficiently, and it starts with one measurement and one simple formula.

The One Formula That Picks Your BTU Size

The official process endorsed by the U.S. Department of Energy’s Energy Star program and every major manufacturer (Frigidaire, Carrier) is the same. Measure the bedroom’s square footage, multiply it by 20, then adjust for room conditions that change the load. The starting point is always the square footage.

Bedroom Size to BTU: Quick Reference Table

Step-by-Step: Finding Your Exact BTU Number

Measure the square footage

Measure the room’s length and width in feet, then multiply them. For a rectangular 12×14 foot bedroom, that’s 168 square feet. If the room is L-shaped, break it into rectangles and add the areas together. Convert inches to fractions (e.g., 3 inches = 0.25 feet) before multiplying.

Calculate the base BTU

Multiply the square footage by 20. 168 sq. ft. × 20 = 3,360 BTU. That’s the base; now adjust it upward or downward for real-world conditions.

Apply the adjustments

• Sun exposure: Very sunny south- or west-facing room? Add 10%. Heavily shaded by trees or awnings? Subtract 10%.
• Occupancy: Add 600 BTUs for each person beyond the first two. If you share the bedroom or have a child’s room where friends sleep over, this matters.
• Ceiling height: Standard 8-foot ceilings don’t require adjustment. Vaulted ceilings of 10–12 feet need 10–20% added to the total.
• Upper floors: If the bedroom is on the second floor or higher, step up one or two sizes because heat rises.

Round to the nearest standard size

After running the numbers, round down to the nearest 1,000 BTU. A calculation of 5,700 BTUs points you toward a 6,000 BTU unit, not a 5,000.

Climate Changes the Math

The 20 BTU per square foot rule works well for moderate U.S. climates. If you live in a consistently hot region (the Southwest, Deep South, or inland California), a more conservative estimate of 25 BTUs per square foot is safer. Energy Star’s guidance still starts with 20; hot-climate residents should lean toward the upper end of the range in the table.

Other Factors to Consider Before Buying

Window compatibility and electrical requirements

A 5,000–9,000 BTU window unit typically runs on a standard 115V household outlet and fits most single-hung or double-hung windows. Larger units, 12,000 BTUs and up, are heavier — they may require a window bracket for structural support and often need a dedicated 230V circuit. Verify your home’s electrical layout before purchasing.

Insulation and window quality

Poorly insulated bedrooms with single-pane windows lose cool air fast. If your room has drafty windows or thin walls, add up to 1,000 BTUs per window to compensate. The table’s BTU ranges already account for typical bedroom construction; you adjust from there.

FAQs

Can I use a larger unit to cool a bedroom faster?

No. An oversized unit short-cycles — it cools the air quickly but shuts off before removing humidity, leaving the room clammy and uncomfortable. The correct BTU size for your square footage works best.

What if the bedroom is connected to a hallway without a door?

Measure the hallway’s square footage and add it to the bedroom total. Conditioned air will escape into the open space, so the AC needs to handle the combined volume, not just the bedroom.

Does the unit placement in the window matter for sizing?

No, but placement affects efficiency. Install the unit where nothing blocks the airflow — pull curtains or furniture at least 18 inches away on both sides. The size calculation stays the same regardless of where in the window the unit sits.