Get the size wrong and the room stays clammy all summer, no matter how low you crank the dial. In tonnage terms, that’s roughly 0.42 ton, since one ton equals 12,000 BTU/h. The catch is that three conditions quietly change the number: shade, sun, and who’s in the room.
Match the square footage to the table, then adjust for real conditions. Here’s the playbook.
How Do You Calculate BTU From Square Feet?
Multiply the room’s length by its width in feet, then look up that area on ENERGY STAR’s sizing chart. A 10-by-10 room lands at 100 square feet. For rooms that aren’t perfectly rectangular, take each section separately and add the totals — 9 inches converts to 0.75 feet, so a 10-foot-9-inch by 9-foot-6-inch room works out to about 102 square feet.
ENERGY STAR’s chart assumes an 8-foot ceiling. Higher ceilings mean the same floor space encloses more air, so the agency advises considering a unit with greater capacity. Match the measured area to the nearest band rather than inventing a BTU-per-square-foot formula of your own.
Once you know your band, factor in the conditions.
| Room Size | ENERGY STAR Rating | Best For |
|---|---|---|
| 100–150 sq ft | 5,000 BTU/h | Small bedroom, home office |
| 150–250 sq ft | 6,000 BTU/h | Standard bedroom with closet space |
| 250–300 sq ft | 7,000 BTU/h | Large bedroom, small studio |
| 300–350 sq ft | 8,000 BTU/h | Studio apartment, big primary bedroom |
| 350–400 sq ft | 9,000 BTU/h | Open loft, living room corner |
| 400–450 sq ft | 10,000 BTU/h | Combined living and dining area |
| 550–700 sq ft | 14,000 BTU/h | Whole-floor studio |
What Changes Your BTU Number?
Three adjustments matter most.
Don’t surge past the band to cover that gap. Buy the closest manufacturer-rated capacity and let the thermostat work; oversizing is its own problem, not a safety margin. A unit with more cooling power than the room needs can chill the air before pulling enough moisture out, leaving the space feeling damp and clammy. An undersized unit simply never catches up. Both extremes waste money and comfort — matching the chart is the goal.
Portable units need their own math. Check whether a portable model’s label reports DOE-tested capacity, and size up for a very sunny room, a kitchen, or high ceilings. When you’re ready to compare models at this capacity, these tested air conditioners under $100 are a practical starting point. For the underlying sizing data, ENERGY STAR’s room air conditioner sizing guidance is the source to trust.
What Should You Check Before You Buy?
Capacity is only half the decision. Before checkout, confirm the unit’s electrical requirements — plug type, voltage, amperage, and circuit needs — against your home’s wiring and the manufacturer’s installation instructions. A dedicated circuit is not optional on most units in this range.
Install a window unit exactly per the manufacturer’s instructions so it’s supported, sealed, and draining condensate outside. Never block the indoor or outdoor airflow grilles, and for portables, use the supplied exhaust hose and window adapter — a sloppy exhaust setup dumps heat and moisture right back into the room. Check the product label for refrigerant and installation requirements, and don’t modify sealed refrigeration parts. Finally, verify local building, rental, and window-installation rules before you drill or mount anything.
Common Questions
Is 5,000 BTU enough for a 100-square-foot bedroom?
Per ENERGY STAR’s sizing table, 5,000 BTU/h is the listed recommendation for rooms from 100 to 150 square feet with an 8-foot ceiling. A sunny room may need slightly more capacity; a heavily shaded one needs less. The table is the reliable baseline, and most 5,000 BTU/h window units handle a standard bedroom well.
Should I buy a bigger air conditioner just to be safe?
No. Oversizing can leave a room feeling cold and damp, because the unit cools the air before it removes enough moisture. Undersizing leaves you warm. Your goal is a unit matched to the room’s square footage, adjusted for sun, shade, occupants, and kitchen use.
Why does a portable unit show less BTU than a window unit?
That number is typically lower than older figures, even when the actual cooling power hasn’t changed. Compare like-for-like and check the label’s test method before judging capacity.
Sources
- ENERGY STAR. “Room Air Conditioners.” Source of the official BTU sizing table and room-condition adjustments.
- Natural Resources Canada. “Room Air Conditioners Infographic.” Supports sizing-band and capacity context.