How to Install a Window Air Conditioner

Installing a window AC is one of the simplest home improvement tasks. Most people can do it in under 30 minutes with basic tools. Follow these steps for a safe, secure, and efficient installation.

What You Will Need

  • Window AC unit
  • Screwdriver (Phillips)
  • Level
  • Measuring tape
  • Foam insulation strips (usually included)

Step 1: Measure Your Window

Measure the width of your window opening. Most window ACs fit windows 23 to 36 inches wide. Check the unit specifications for minimum and maximum window width. Also ensure the window sill can support the weight of the unit (typically 40-80 pounds).

Step 2: Install the Mounting Bracket

Most units come with an L-shaped bracket that attaches to the window sill. This provides additional support and prevents the unit from falling. Place the bracket so that the unit will tilt slightly backward (about 1/2 inch) for proper condensation drainage.

Step 3: Place the Unit

With the lower window sash fully open, carefully lift the unit and set it on the sill and bracket. Center it in the window. Have someone help if the unit weighs more than 50 pounds.

Step 4: Extend Side Panels and Secure

Pull out the accordion-style side panels to fill the gaps on both sides. Lower the upper window sash onto the top of the unit. Most units include angle brackets to screw into the window frame, preventing the sash from being opened from outside.

Step 5: Seal and Insulate

Apply the included foam insulation strips to any gaps between the unit, side panels, and window frame. Proper sealing prevents hot air from leaking in and improves efficiency by up to 10%.

Step 6: Plug In and Test

Plug the unit directly into a grounded wall outlet. Never use an extension cord — window ACs draw 5-15 amps and can overheat a standard extension cord. Turn the unit on, test cooling, fan speeds, and thermostat.

Common Mistakes to Avoid

  • Tilting the unit forward instead of backward (causes water to drip inside)
  • Using an extension cord (fire hazard)
  • Not sealing gaps (wastes 15-20% of cooling energy)
  • Placing the unit in a window that gets direct afternoon sun
  • Forgetting to remove the unit before winter (cold air leaks, damage to seals)

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