12 × 12 ft bedroom: 5,000 BTU/h
At 144 sq ft, normal sun and two occupants, the base chart capacity remains unchanged.
Window AC size calculator
Enter one room’s dimensions, sun exposure and occupants to estimate window-AC cooling capacity in BTU per hour.
The chart behind the result
Use ENERGY STAR's room-AC chart for one room with an 8-foot ceiling. The result includes the listed sun, occupant and kitchen adjustments. A taller room receives a standard-ceiling reference requiring a separate sizing check.
| Area to cool, sq ft | Base capacity, BTU/h |
|---|---|
| 100 to under 150 | 5,000 |
| 150 to under 250 | 6,000 |
| 250 to under 300 | 7,000 |
| 300 to under 350 | 8,000 |
| 350 to under 400 | 9,000 |
| 400 to under 450 | 10,000 |
| 450 to under 550 | 12,000 |
| 550 to under 700 | 14,000 |
| 700 to under 1,000 | 18,000 |
| 1,000 to under 1,200 | 21,000 |
| 1,200 to under 1,400 | 23,000 |
| 1,400 to under 1,500 | 24,000 |
| 1,500 to under 2,000 | 30,000 |
| 2,000 to 2,500 | 34,000 |
At a shared band boundary, the tool uses the band beginning at that area: exactly 150 sq ft uses 6,000 BTU/h; exactly 250 uses 7,000. The last band includes 2,500 sq ft. Smaller or larger rooms receive no extrapolated capacity.
How sun, people and a kitchen change the figure
The sun adjustment applies to the base chart capacity: reduce it by 10% for heavy shade or increase it by 10% for a very sunny room. Then add 600 BTU/h for each regular occupant beyond two, and 4,000 BTU/h when the room is a kitchen. Normal sun exposure adds nothing.
20 × 15 ft sunny room: 10,000 BTU/h
At 300 sq ft, the base is 8,000. Very sunny exposure adds 800, and four occupants add 1,200. Total: 10,000 BTU/h, with no kitchen adjustment.
Check the unit and window before buying
BTU/h describes cooling capacity, not electrical input watts or the size of the cabinet. Check the model's permitted window opening, mounting instructions, power requirements and minimum variable-speed output. Choosing a much larger unit can reduce humidity removal; this tool does not automatically round the result up to a product size.
Do not carry this number into a portable-AC purchase: portable equipment uses a different rating context. A mini-split or whole-home system needs its own sizing method. Complex room shapes can be divided into rectangles; this version accepts one rectangular room only.
Window AC sizing questions
Does a 5,000 BTU/h result mean any 5,000 BTU unit will fit?
No. Cooling capacity and physical window fit are separate. Check the exact model's window dimensions, mounting, power and performance requirements.
Can this size cooling for the entire apartment?
No. It assumes the area entered is one room served by the window unit. Closed doors and separate rooms require a different plan.
Why is there no multiplier for insulation or a taller ceiling?
The cited chart does not publish those correction factors. This tool avoids an unsupported precision claim and flags high ceilings for a separate sizing check.
Source and verification
Primary guidance checked October 12, 2026.
- ENERGY STAR Room Air Conditioners: buying guidance
Source for the room-area chart, 8-foot-ceiling assumption and sun, occupant and kitchen adjustments. The result is a planning estimate, not a model recommendation.