BTU Calculator · Practical guide
What Size Air Conditioner Do I Need?
Size an air conditioner to the room's air volume, then adjust for insulation, sun, and climate.
At a glance
Start With the Room's Volume
Air conditioner size is measured in BTU per hour, and the right capacity scales with how much air the room holds. This calculator uses 25 BTU per cubic foot: multiply length × width × ceiling height, then × 25. A 20×15 living room with 8-foot ceilings holds 2,400 cubic feet and calls for about 60,000 BTU/hr of cooling under average conditions — roughly 17.6 kW of equivalent electrical power (60,000 ÷ 3,412).
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.
Typical Room Sizes at 8-Foot Ceilings
| Room Area | Volume | Cooling BTU/hr |
|---|---|---|
| 100 sq ft | 800 cu ft | 20,000 |
| 150 sq ft | 1,200 cu ft | 30,000 |
| 200 sq ft | 1,600 cu ft | 40,000 |
| 250 sq ft | 2,000 cu ft | 50,000 |
| 300 sq ft | 2,400 cu ft | 60,000 |
| 400 sq ft | 3,200 cu ft | 80,000 |
| 500 sq ft | 4,000 cu ft | 100,000 |
These figures assume average insulation, moderate sun, and a mixed climate. Adjust upward for poor insulation (+20%), heavy sun (+15%), or a hot climate (+15%), and downward for good insulation or heavy shade (−10% each).
Bigger Is Not Better
An oversized air conditioner cools the room so fast it shuts off before removing humidity — a problem called short cycling — leaving the space cold but clammy and wearing out the compressor early. An undersized unit runs constantly and never quite catches up on hot afternoons. Aim for the estimate that matches your adjusted load, and when you land between two unit sizes, the slightly smaller one usually dehumidifies better in humid regions.
When to Call a Professional
For a single window or portable unit, the calculator's estimate is a solid shopping guide. For central systems, multi-room layouts, or homes with unusual features like cathedral ceilings or large glass walls, a Manual J load calculation from an HVAC contractor accounts for details no rule of thumb can capture.
Frequently Asked Questions
How do I convert BTU to tons of cooling?
One ton of cooling equals 12,000 BTU/hr, so divide the BTU estimate by 12,000. A 60,000 BTU/hr load corresponds to a 5-ton system.
Does a higher ceiling mean a bigger unit?
Yes. Cooling load tracks air volume, so a 10-foot ceiling needs 25% more capacity than an 8-foot ceiling over the same floor area.
What happens if my AC is too big?
It short cycles: the room reaches temperature before the coil dehumidifies the air, so the space feels damp and the compressor wears faster. Correct sizing improves both comfort and equipment life.
Should two rooms share one unit?
Only if air can circulate freely between them. Closed doorways isolate rooms, so size a unit for each enclosed space or use a multi-zone system.