Window AC Size Calculator
Find the right window AC unit for your room. Enter square footage, ceiling height, sun exposure, and number of occupants to determine the BTU capacity you need.
Window AC Sizing Formula
Window air conditioner sizing starts with room square footage multiplied by 20-25 BTU per square foot for standard 8-foot ceilings. A 200-square-foot room needs 4,000-5,000 BTU as a baseline. Adjust for ceiling height by multiplying by (actual height / 8). A 10-foot ceiling increases the requirement to 5,000-6,250 BTU because you're cooling 25% more air volume.
Sun exposure shifts the load. South- and west-facing rooms gain heat from afternoon sun, adding 10-15% to cooling needs. Shaded rooms (trees blocking windows, north-facing) reduce the load by 10%. Each additional person beyond the first adds 600 BTU of body heat. A 300-square-foot sunny living room with 3 people needs (300×25×1.15) + (2×600) = 9,825 BTU, rounding to a 10,000 BTU unit.
Window ACs come in standard sizes: 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, and 18,000 BTU. Always round up to the next available size. A calculated 7,500 BTU requirement calls for an 8,000 BTU unit. Manufacturers rate BTU at specific test conditions (95°F outside, 80°F inside, 50% humidity). Actual performance varies slightly with local climate, but sizing based on calculated BTU ensures adequate capacity.
Efficiency and Operating Cost
Window AC efficiency is measured by EER (Energy Efficiency Ratio) or CEER (Combined Energy Efficiency Ratio). EER divides cooling output (BTU/hr) by power input (watts). A 10,000 BTU unit using 1,000 watts has an EER of 10. Higher EER means lower electricity bills. Budget units score 8-9 EER; Energy Star models hit 10-12 EER. Each EER point cuts operating cost by roughly 10%.
CEER accounts for standby power draw when the unit is off but plugged in. It's 1-5% lower than EER but more accurate for total energy cost. Look for CEER ratings on Energy Star labels. A 12 CEER unit costs about $50 per season to run in moderate use (8 hours/day, $0.13/kWh); a 9 CEER unit costs $65, saving $15/season with the efficient model.
Inverter window ACs (variable-speed compressors) adjust output to match cooling demand instead of cycling on/off. They save 20-30% energy compared to fixed-speed units but cost $100-$200 more. Payback is 3-5 years. Inverter models also dehumidify better and run quieter. For bedrooms and living rooms where you spend hours, inverter ACs improve comfort; for occasional-use rooms, standard units suffice.
Installation and Performance Tips
Proper installation maximizes window AC performance. Seal gaps around the unit with foam weatherstripping or side panels to prevent hot air infiltration. Tilt the unit slightly outward (1/4-inch drop toward the exterior) so condensate drains outside instead of pooling inside. Install on the shadiest wall if possible; units in direct sun lose 5-10% efficiency from radiated heat.
Window AC placement affects airflow. Mount at least 3 feet off the floor and away from furniture that blocks the front grille. Cool air falls, so high mounting distributes better. Avoid placing heat-generating appliances (lamps, TVs, stoves) near the AC; they fool the thermostat into running longer. Close doors to isolate the room; cooling an open floor plan with a window AC wastes energy.
Maintenance is simple but critical. Clean or replace the filter every 2-4 weeks during cooling season. Clogged filters reduce airflow 10-30%, forcing the compressor to work harder and slashing efficiency. Vacuum the coils (visible behind the filter) once a season to remove dust buildup. Store units indoors over winter or cover them to prevent weather damage. A well-maintained window AC lasts 10-12 years; neglected units fail in 5-7 years.
Frequently Asked Questions
How many BTU do I need per square foot?
Start with 20-25 BTU per square foot for 8-foot ceilings. Add 10% for sunny rooms, subtract 10% for shaded rooms, and add 600 BTU per occupant beyond the first person.
What size window AC for a bedroom?
A typical 12×12-foot bedroom (144 sq ft) needs 5,000-6,000 BTU. A master bedroom (200-300 sq ft) needs 8,000-10,000 BTU. Sunny rooms or high ceilings push you to the higher end.
Can a window AC be too big?
Yes. Oversized units cool quickly but don't run long enough to dehumidify, leaving rooms clammy. They also cycle on/off frequently, wasting energy. Match the unit closely to calculated BTU.
What BTU for a living room?
Living rooms range from 250-500 sq ft. A 15×20-foot room (300 sq ft) needs 8,000-10,000 BTU. Open-concept rooms sharing space with kitchens may need 12,000-14,000 BTU.
How does ceiling height affect AC sizing?
Higher ceilings increase room volume, requiring more cooling. A 10-foot ceiling adds 25% to the load compared to 8 feet. Use the height factor adjustment in this calculator.