Air Conditioner BTU Calculator
Cooling capacity for a room by size, sun, kitchen, and people.
People add heat; each person beyond two adds 600 BTU.
You need
6,000 BTU/hr
Room-unit sizing at 8 ft ceilings. Whole-home systems need a Manual J calculation.
- In tons of cooling
- 0.50 tons
- Room area
- 300 sq ft
Estimates only. Verify quantities before ordering.
How this is calculated
BTU = area x 20, multiplied by the sun-exposure factor (+10% sunny, -10% shaded), plus 4,000 for a kitchen and 600 per occupant beyond two, rounded to the nearest 500. Assumes standard 8 ft ceilings; add roughly 10% per extra 2 ft of ceiling height. One ton of cooling = 12,000 BTU/hr.
Enter the room dimensions and conditions, and this calculator sizes the air conditioner in BTU per hour using the standard rule of about 20 BTU per square foot, adjusted the way Energy Star recommends: about 10% more capacity for a sunny room, 10% less for a heavily shaded one, 4,000 BTU extra for kitchens, and 600 BTU for each regular occupant beyond two.
Resist the urge to round way up. An oversized air conditioner cools the air quickly but shuts off before it dehumidifies, leaving the room cold and clammy, and the constant starting wears the compressor. Size to the calculation, and only step up a size for rooms with high ceilings, poor insulation, or big west-facing glass.
AC sizes for common rooms
Rows assume average sun, two occupants, no kitchen, and 8 ft ceilings, rounded to the nearest 500 BTU.
| Room | BTU/hr | Tons |
|---|---|---|
| 10 x 12 ft bedroom | 2,500 BTU/hr | 0.21 tons |
| 12 x 15 ft bedroom | 3,500 BTU/hr | 0.29 tons |
| 15 x 20 ft living room | 6,000 BTU/hr | 0.50 tons |
| 20 x 25 ft open plan | 10,000 BTU/hr | 0.83 tons |
| 24 x 30 ft great room | 14,500 BTU/hr | 1.21 tons |
Common questions
How many BTU per square foot?
About 20 BTU per square foot is the standard baseline for rooms with 8 ft ceilings and average insulation, before adjustments for sun, kitchens, and occupancy. A 300 square foot room lands around 6,000 BTU; a 500 square foot space around 10,000 to 12,000.
What happens if I oversize my air conditioner?
It cools the air faster than it removes moisture, then shuts off: the room reads the target temperature but feels damp and clammy, and the short cycles wear the compressor and waste energy. Matching the load matters more than having headroom.
Do high ceilings change the BTU I need?
Yes. The 20 BTU per square foot rule assumes 8 ft ceilings; the load scales roughly with air volume, so add about 10% for every 2 ft of additional ceiling height. A 20 x 15 room with 12 ft ceilings needs about 20% more than the standard calculation.
What size AC for an open floor plan?
Measure the whole connected area, not just the spot where the unit sits, since cool air spreads across open space. For L-shaped or connected areas, add the rectangles together and use the total; if the space exceeds about 700 square feet, consider two smaller units or a mini-split over one giant window unit.