How to Install a Window AC Unit

Open the window fully and measure the inside width of the opening from jamb to jamb, then measure the vertical height of the opening. Compare these measurements to the AC unit's dimensions on the box — the unit must fit within the window width, and the accordion side panels must be able to span any remaining gap. For room sizing, calculate your room's square footage (length x width) and multiply by 20 BTU — a 150 sq ft bedroom needs roughly 5,000 BTU, while a 350 sq ft living room needs 8,000-10,000 BTU. Oversized units cool too fast without dehumidifying, leaving the room clammy.

Remove the window screen by pressing the tabs and sliding it out of the track — set it aside somewhere safe so it doesn't get damaged. Inspect the window frame, sill, and sash for any rot, cracks, or warping — the frame must be structurally sound to support 40-80 lbs of AC unit. Clean the windowsill of dirt and debris so the unit sits flat. If your window has a storm window, remove or raise that as well.

Before lifting the unit into the window, attach the accordion-style side panels (also called curtain panels) to the left and right sides of the AC housing. Slide each panel into its channel and secure it with the provided screws — most units have two screws per side. The panels will extend later to fill the gap between the unit and the window frame. If your unit came with foam strips for the top of the side panels, attach those now too.

For units over 50 lbs, second-floor or higher windows, or older window frames, install an exterior support bracket before placing the AC. Most brackets mount to the exterior wall below the window using lag screws driven into the wall studs or masonry anchors. Position the bracket so its top edge is level with or just below the windowsill, then level it with a torpedo level and tighten the fasteners. The bracket takes the unit's weight off the window sash and prevents the worst-case scenario — the unit falling out.

With a helper, carefully lift the AC unit and set it on the windowsill, centering it in the opening. The back of the unit (the side with the condenser coils and fins) should extend outside, and the front (with the controls and vents) faces into the room. Most units are designed so that roughly one-third of the body sits inside and two-thirds hangs outside. Slowly lower the window sash down onto the top of the AC unit — there should be a flange or lip on the unit where the sash rests securely.

Your AC unit likely came with one or two small metal L-brackets and screws. These brackets lock the upper and lower window sashes together so the window can't be accidentally opened from outside (a security concern) and can't slide up off the unit. Position one bracket so it bridges the gap where the two sashes meet, drill a pilot hole through the bracket into the upper sash, and drive the provided screw. Do the same on the other side if two brackets are included. The sashes should now be firmly locked together.

Pull the accordion side panels outward until they meet the window frame on each side, closing the gap completely. Secure each panel to the window frame with the provided screws — most panels have pre-drilled holes at the end. Then apply the foam weatherstrip along the top edge where the lower sash meets the upper sash's glass — cut it to length with scissors and press it firmly into the gap. This foam strip prevents warm outside air from leaking in around the top of the unit, which would make the AC work harder and waste energy.

Place a torpedo level on the top of the AC unit from front to back. The unit should tilt very slightly toward the outside — about 1/4 inch lower in the back than the front. This slight slope ensures that condensation drains out the back of the unit instead of pooling inside or dripping into your room. If the unit is perfectly level or tilts inward, shim the front edge with thin wood shims between the unit and the windowsill until you achieve the correct slope. Check the drain hole or channel on the back of the unit to confirm it's not blocked.