How to Install AC Window Unit? | Step-By-Step Fit Guide

Installing a window AC unit takes two people, about an hour, and a snug fit in a double-hung window.

Mounting an air conditioner in a window seems like a one-person job until you lift one. Most window AC units weigh 50 to 80 pounds, and the official guides from GE Appliances and Home Depot both say the same thing: this is a two-person install. The process itself is straightforward — confirm the window fits, mount the rails and side panels, lift the unit into the sill, and seal every gap.

What You Need Before You Start

Check three things before you lift anything. First, measure the window’s clear opening. Most window AC units fit standard double-hung windows with clear opening widths between 23 and 37 inches, and the sash must open far enough to allow a clear vertical opening of about 13 inches. Second, confirm a three-prong wall outlet is within reach of the power cord — the unit must plug directly into the wall, never an extension cord or power strip. Third, set aside the hardware that came in the box, including the top mounting rail, side curtains, screws, and foam gaskets.

You will also want a drill for pilot holes, a screwdriver, and weatherstripping or foam strips for sealing gaps.

The Core Step-By-Step Installation

GE Appliances and Home Depot publish nearly identical sequences. Here is the condensed version that works for most double-hung windows.

1. Mount the top rail. Attach the top mounting rail to the top of the unit using the included Type-A screws. The exact screw count depends on the model — some take three, some take four.
2. Add the side curtains. Slide the accordion side panels into the channels on both sides of the unit. Do this before lifting the AC into place.
3. Lift the unit into the sill. With two people, rest the AC on the bottom window sill, centering it left to right. Keep the bottom mounting rail against the back edge of the sill.
4. Close the window behind the top rail. Lower the window sash so it sits snugly behind the top mounting rail’s upper flange. This locks the unit in place vertically.
5. Tilt it slightly outward. The unit should be level or tilted just a few degrees downward toward the outside so condensation drains properly. Never tilt it inward — that sends water into your room.
6. Extend the side curtains. Pull the accordion panels outward until they touch the window frame’s sides, sealing the gap on each side.
7. Secure with screws. Drill pilot holes, then drive the Type-B screws through the side curtains into the window sash and sill. Install the window lock bracket (the L-shaped metal piece) so the sash can’t be pushed up from outside.
8. Seal the gaps. Fit the foam top gasket between the top rail and the lowered sash, and add foam strips or weatherstripping around any remaining gaps to block air and insects.

When you are done, the unit should feel solid when you push on it from the top and sides. If it rocks or slides, tighten the screws or add a support bracket.

Common Installation Mistakes and How to Avoid Them

The mistakes that wreck window AC installs are almost always the same four. Installing alone when the unit is heavy — the manuals repeatedly flag this as a two-person job, and dropping a 70-pound AC is a fast way to damage the unit or the window frame. Skipping the included brackets and screws, which leaves the unit unstable. Omitting the foam sealing strips, which turns the install into an air leak and an insect highway. And using an extension cord or power strip instead of a direct wall outlet, which is a known fire hazard at the current a window AC draws.

If your window is a single-hung or sliding style rather than a standard double-hung, the shape of the side panels and the way the sash closes may differ slightly, but the same fit-and-seal logic applies. Measure carefully, and if the unit doesn’t fit the opening, swap it for a size your window can actually accept.

Once the unit is mounted and sealed, it should run quietly enough to hear the compressor cycle and cool the room without rattling. If you are shopping for a unit for a camper or a trailer, where windows are a different shape entirely, our tested roundup of AC window units for campers points you to models that fit those openings.