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 |
Window, portable, or mini-split
Window units remain the most efficient dollar-for-BTU option and the simplest to install, at the cost of blocking a window and a fair amount of noise indoors. Portable units are the least efficient of the three, because the exhaust hose radiates heat back into the room and single-hose designs pull conditioned air out of the space to cool the condenser. Choose portable only where a window unit genuinely cannot be fitted.
Ductless mini-splits cost several times more installed and are far quieter and more efficient, with the compressor outside and only a small refrigerant line entering the wall. They also heat, often efficiently enough to serve as primary heating in mild climates. For a room you occupy daily and year round, the running cost difference recovers a meaningful part of the installation over a decade.
Whichever type, check the electrical requirement before buying. Larger window units and most mini-splits need a dedicated circuit, and some need 240 volts. Discovering that after delivery is a common and annoying detour.
SEER2, EER, and what the ratings mean
SEER2 is the current seasonal efficiency measure for central and mini-split systems, replacing the older SEER under updated test procedures, so a SEER2 number is slightly lower than the SEER figure for the same equipment and the two cannot be compared directly. Higher is better, and the practical range runs from the code minimum around 14 to over 20 for premium variable-speed equipment.
Room air conditioners are rated with CEER, which combines cooling efficiency with standby power. For a unit that runs most of the summer in a hot climate, paying for a higher rating returns real money; for a spare bedroom used a few weeks a year, it usually does not. The Energy Star threshold is a reasonable shortcut when you do not want to model payback.
Efficiency ratings assume the unit is correctly sized and installed. A high-rated system that is oversized, or one with a poorly sealed window kit, will underperform a modest unit fitted properly, which is why sizing deserves more attention than the rating on the box.
Why oversizing makes a room less comfortable
An air conditioner does two things: it lowers temperature and it removes humidity. The second only happens while the coil runs long enough for moisture to condense on it and drain away. An oversized unit satisfies the thermostat quickly and shuts off before that occurs, so the room reaches the set temperature while staying damp, which feels clammy and cold rather than comfortable.
Short cycling also wears equipment faster, since compressor starts cause more wear than running time does, and it produces noticeable temperature swings as the unit hunts around the setpoint. A correctly sized unit runs long, steady cycles on a design day, which is exactly what you want.
This is why the sensible response to a room that will not cool is rarely a bigger unit. Check for air leaks, insulation, and solar gain through windows first. Shading a west-facing window or sealing an attic bypass often does more than several thousand extra BTU would, and costs less to fix.
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.