Measure the room and window before shopping, then match the unit's cooling capacity to the room rather than choosing the biggest model. DOE and ENERGY STAR warn that an oversized room AC can cost more and remove less humidity. Confirm the unit fits the window and installation instructions, then compare energy use, controls and filter access.
Start with two measurements: the room you need to cool and the window where the unit will sit. Then choose capacity and features that fit those conditions.
Match capacity to the room
Measure the room’s length and width and calculate its floor area. The Department of Energy provides room-area guidance for selecting a room air conditioner’s cooling capacity, expressed in Btu per hour. Its chart is based on an 8-foot ceiling, so use its additional guidance for a high ceiling or a room that does not match the assumptions.
Avoid choosing extra capacity as a shortcut. DOE and ENERGY STAR explain that an oversized unit can cost more, use more energy and cool the room before it has removed enough humidity. When a room opens into another space or gets unusual sun exposure, check the full DOE guidance and the manufacturer’s sizing advice.
Check window fit and installation
- Measure the clear opening, not just the outside of the window frame.
- Confirm the unit supports your window style and opening dimensions.
- Check the product instructions for brackets, side panels, sill requirements, clearance and electrical needs.
- Consider how the unit will be secured and sealed, and whether you can remove or store it when the cooling season ends.
The right cooling capacity is not useful if the unit cannot sit securely in the window. Read the installation guide before you buy, especially for sliding, casement or unusually narrow windows.
Compare the features you will use
ENERGY STAR lists models with different sizes, styles and controls. Compare the current product details for energy use, thermostat, timer, fan direction, remote access and filter removal. A washable, accessible filter is easier to keep on a regular cleaning routine. Variable-speed models can offer different sound and operating characteristics, so compare the manufacturer’s specifications for the actual model.
For other room-comfort decisions, see our smart thermostat basics and dehumidifier size guide. Browse appliances for more home equipment guides.
Sources
Common questions
How do I choose a window air conditioner size?
Measure the floor area and use the Department of Energy room-size guidance to find a starting capacity. Its chart assumes an 8-foot ceiling, so check the additional advice if your room has a higher ceiling or unusual layout.
Is a bigger window AC better?
No. DOE and ENERGY STAR say an oversized unit can cost more, use more energy and cool the room before removing enough humidity.
What should I measure before buying a window AC?
Measure the opening and check the window style, clearance, outlet and included mounting hardware against the product's installation guide.