Window AC BTU Calculator: What Size Unit Do You Need?
Choose a window air conditioner by the room load, not whole-house central-AC tonnage. Start with room square footage, then account for ceiling height, sun, occupancy, kitchen heat, insulation, and local climate.
Window Air Conditioner BTU Calculator
Enter project-specific values, calculate, then change an input to compare scenarios. The result stays in this page and sends no visitor data anywhere.
Based on ENERGY STAR room-air-conditioner sizing bins with disclosed height, sun, occupancy and kitchen adjustments.
ENERGY STAR room-size foundation
The calculator uses the current ENERGY STAR room-air-conditioner capacity bins as its base. Examples include 5,000 BTU/h for up to 150 square feet, 8,000 BTU/h for 300–350 square feet, 12,000 BTU/h for 450–550 square feet, and 18,000 BTU/h for 700–1,000 square feet.
ENERGY STAR also advises reducing capacity 10% for a heavily shaded room, increasing it 10% for a very sunny room, adding 600 BTU/h for each regular occupant beyond two, and adding 4,000 BTU/h for kitchen use. Those adjustments are built into the tool.
How ceiling height is handled
The public room-size bins are area-based. The calculator transparently scales the base in proportion to ceiling height relative to eight feet as a screening volume assumption. A ten-foot ceiling therefore produces a 1.25 volume factor. This is not an ENERGY STAR correction table or an equipment requirement; unusually tall or vaulted rooms deserve a fuller load calculation.
Insulation and climate inputs
Because no universal percentage accurately represents every wall, window, leakage level, or climate, the calculator does not hide a fixed “poor insulation” or “hot climate” multiplier. It exposes separate percentage fields and defaults both to zero. Enter a change only when a project-specific assessment supports it.
For a more defensible result, calculate the room’s actual envelope, solar, and outdoor-air gains using local cooling design conditions. A west-facing room with old glass may need a different adjustment from an interior bedroom with the same floor area.
Choosing the next available unit
Manufacturers sell discrete capacities, but always rounding up can oversize a small room. Compare the estimate with the manufacturer’s stated application range, installation instructions, and available models. Oversized room units may cool quickly and cycle before dehumidifying well. Undersized units may run continuously and miss the setpoint during design weather.
Window and electrical constraints
Confirm that the window opening, sill, support method, drainage, side panels, clearances, and wall construction meet the exact installation instructions. Larger room units may require a different voltage or dedicated circuit. Do not use an extension cord, change a receptacle, or increase a breaker to make an incompatible unit operate. A qualified electrician should evaluate the existing branch circuit.
Example room
A 330-square-foot room falls in the 8,000 BTU/h base bin. If it is very sunny, the official sun adjustment raises that to 8,800 BTU/h. Two regular occupants add nothing; a third adds 600 BTU/h. Kitchen use would add 4,000 BTU/h. A nine-foot ceiling applies the calculator’s disclosed volume scaling, while insulation and climate remain at zero unless supported.
The number is still a screening estimate. Electronics, large glass, open doorways, ventilation, and adjacent unconditioned spaces can change the load.
Room AC versus central AC
This page serves one-room and window-unit intent. Do not add every room-unit result and treat it as a central-system Manual J. Central systems have coincident whole-building loads, latent load, duct effects, ventilation, and room airflow requirements. For a central starting point, use the AC Size Calculator, then obtain a professional Manual J before buying equipment.
Installation affects the real result
Air that leaks around side panels can add outdoor heat and moisture after the unit has been sized. Follow the included sealing and support instructions, keep discharge air unobstructed, and verify drainage exactly as the manufacturer requires. Lamps, curtains, and furniture near the thermostat or discharge can also distort operation. Capacity cannot compensate for a poorly sealed or blocked installation.
Humidity and duty cycle
Room units remove moisture only while the evaporator is cold and operating. A unit that is far too large may satisfy dry-bulb temperature quickly but run short cycles. In humid climates, compare product controls, fan behavior, and moisture performance; do not add capacity solely because the room feels clammy. Air leakage and outdoor moisture may be the underlying problem.
Window AC questions
What size window AC for a bedroom?
Use the bedroom area and actual conditions. Many bedrooms fall within 5,000–8,000 BTU/h bins, but sun, glass, ceiling height, occupancy, and climate can change the result.
Should I buy a bigger unit for faster cooling?
No. A substantially oversized unit can cycle quickly and reduce moisture removal. Match the documented room load and product guidance.
Does a kitchen need more capacity?
ENERGY STAR’s published room-AC guidance adds 4,000 BTU/h for kitchen use.