BTU Calculator · Practical guide
Window Air Conditioner BTU Chart by Room Size
Match your room's dimensions to a BTU rating, then adjust for sun exposure, insulation, and climate.
At a glance
The Sizing Chart
Window air conditioners are sold by BTU per hour, and the capacity you need scales with the air volume of the room. Using this calculator's rule of 25 BTU per cubic foot with standard 8-foot ceilings, average insulation, moderate sun, and a mixed climate, here is what common room sizes call for:
| Room Size | Example Dimensions | Cooling BTU/hr |
|---|---|---|
| 100 sq ft | 10 × 10 ft | 20,000 |
| 144 sq ft | 12 × 12 ft | 28,800 |
| 200 sq ft | 10 × 20 ft | 40,000 |
| 300 sq ft | 15 × 20 ft | 60,000 |
| 400 sq ft | 20 × 20 ft | 80,000 |
| 500 sq ft | 20 × 25 ft | 100,000 |
Interactive tool
BTU Calculator
Enter your own values, then compare the result with the examples below.
BTU Estimate
Results are estimates. Consult an HVAC professional for precise sizing.
How to Adjust the Chart
Three conditions move the estimate. Add 20% for poor insulation, 15% for heavy afternoon sun, and 15% for a hot climate; subtract 10% each for good insulation and a shaded room. A 12×12 bedroom that needs 28,800 BTU/hr at baseline jumps to about 33,100 with heavy sun, and to roughly 45,700 if it is also poorly insulated in a hot climate. Enter your actual room above to get the adjusted figure directly.
Reading the Chart Against Store Shelves
Real window units cluster around 5,000 to 25,000 BTU/hr, and manufacturers print their own square-footage ratings on the box using different assumptions than this volume-based rule. Treat both as estimates: if the chart and the box disagree, compare which assumptions — ceiling height, sun, insulation — each one makes. The calculator above lets you test your exact conditions rather than a generic one.
Beyond Raw Capacity
Once you know the BTU range, check three things before buying: the unit's EER or CEER efficiency rating (higher is cheaper to run), whether your window width and outlet circuit support the unit, and noise ratings if the room is a bedroom. A correctly sized, efficient unit beats a larger bargain unit every summer it runs.
Frequently Asked Questions
What room size does a 5,000 BTU unit cover?
Under this calculator's volume rule (25 BTU per cubic foot, 8-ft ceilings), 5,000 BTU/hr covers only about 25 square feet. Manufacturer box ratings often claim 100–150 sq ft because they use a different per-square-foot rule — compare assumptions before trusting either number.
Should I round up to the next BTU size?
Round up only when conditions justify it — heavy sun, poor insulation, or a hot climate. Otherwise the next size up risks short cycling and poor humidity removal.
Do portable air conditioners use the same chart?
The room load is the same, but portable units vent through a hose and lose some effective capacity. Check the unit's newer SACC rating rather than the older BTU figure for a fairer comparison.
Can one window unit cool two rooms?
Rarely well. Airflow through a doorway is weak, so the far room stays warm. Size a unit for the room it sits in, or consider a ductless mini-split for multiple rooms.