How To Install A Window AC Unit – Complete Guide

| Last Updated: August 23, 2021

As temperatures start soaring, we're all looking for ways to escape from the heat.

One of the easiest ways to do this is to install a Window air conditioner (AC) unit in your home.

Even if you're new to DIY, if you're well-prepared, the installation is a breeze.

Project Metrics

Here's what to expect when installing a window AC. 

Working Time

25 minutes

Prep Time

15 minutes

Skill Level


Material Cost

This depends on the unit's size, but you can expect to spend between $140 to $200.

Supplies Needed to Install a Window Air Conditioner

Apart from the supplies provided by the manufacturer, you’ll also need the following tools:

  • Measuring tape

  • Cordless drill 

  • Screwdriver

  • Pencil

  • Utility knife/scissors

  • Level (Optional)
  • How to Install a Window AC Unit

    1. Check Your Tools

    One of the tricks to being good at DIY is having the right tools. Before you get started, make sure that you have all the tools and materials that you'll need.

    2. Assess and Prepare the Window

    Most AC's don't have long power cords, so make sure that the window is close to a plug point. Clean the window by removing all dust and dirt.

    If your AC came with a weatherstrip, you could install it now. The weatherstrip helps to seal small gaps, but it isn’t necessary for your unit's functioning.

    Using your tape measure, measure the width of the window. Find the center, and mark it on the window sill with a pencil.

    3. Unbox the AC

    Remove the unit and all its parts from the packaging. Make sure all the pieces you need are there.

    4. Install the Top Rail

    Most units come with a top rail that must be attached to them. The top rail is a safety feature. To prevent the screws from getting stripped, you can screw them in manually. Be sure to tighten them all.

    5. Attach the Side Panels

    The two accordion-style panels will be marked “left” and "right.”' Slot each one into the correct side of the AC and secure with screws. Push the panels flat against the AC.

    6. Insert the AC

    Two people need to complete this step.

    Together, lift the AC and place it on the bottom window sill. The center of the AC must be where you made your pencil mark at step three.

    While one person holds the AC in place, the other should check that the back of the unit is at a slight tilt. The tilt ensures that any water that’s collected can be drained. Close the window now.

    7. Secure the AC

    Stretch out the panels on the side of the AC towards the window sill on either side. Use the locks and screws that the manufacturer provided to mount the panels.

    You’ll most likely need to drill holes in the window sill to complete this step.

    8. Start Cooling

    Plug in your unit, and don't forget to make sure that the power is on! Enjoy the cool air.

    How to Install a Window AC Unit in a Casement/Horizontal Sliding Window

    With some basic DIY, you can fit your standard AC into a horizontal sliding window. To secure the AC, you’ll need to build a frame for it. You can do this as follows:

    1. Build your bottom frame before inserting the unit into the window. Use a 2x2 cross piece that is the same length as the window to act as the window frame bottom. You’ll screw the bottom of the air conditioner into this instead of the window frame.

    2. Use another 2x2 cross piece and attach it to a 1x4 crosspiece (with screws) to create a window ledge. You can use adhesive velcro to attach your new ledge to the existing ledge. It must line up with your new window frame bottom.

    3. When you've completed the bottom frame, you can insert the window AC.

    4. Add the top of the frame. The top fills the gap above the AC. For this, you can cut a piece of plexiglass to size.

    5. To make sure that the space around the plexiglass is airtight, you can seal it with duct tape.

    6. If there’s still a gap where the windows connect, you can use a piece of foam core board (cut to size) to fill it.

    7. Check the space around the side of the unit. If there are gaps once you extend the panels, you’ll need to seal them. You can use pieces of foam to do this.

    How to Install a Window Air Conditioner in a Travel Trailer

    AC units can't fit into the regular windows of a travel trailer, so you’ll need to modify them. You can make two adapters out of plywood to slide into the top and bottom of the window. These adaptors will secure the unit and fill up the space between the window and the AC.

    How to Install a Window Air Conditioner Without Screws

    An easy way to install an AC without screws is to use dowels. You would need two dowels.  The first one fits between the outer window and the inner window, while the second dowel goes between the top of the window frame and the double-hung window. The dowels keep the top window open and the inside window closed. Using dowels is a quick, inexpensive way to secure your AC.

    How to Install a Window AC Unit on a Second Floor

    When installing an AC unit on the second floor, you may want to further secure it with a brace or bracket. The brace/bracket would fit at the back of the unit and help keep it steady. You can purchase one at a hardware store or online, but be sure to keep your AC measurements on hand.


    There's a lot to consider when you're thinking of installing an AC, but it's simple to do with the right tools and preparation. Remember to have someone help you because AC units are heavy, and you need to work with precision.

    People Also Ask

    Below are some questions that people commonly ask. Have a peek; there may be a question that’s crossed your mind too.

    How Do You Secure a Window Air Conditioner?

    One way that you can secure your AC is with an L bracket at the top of the open window. If you didn't get one with your unit, you can buy one online or from your hardware store. The L bracket holds the window in place by preventing it from being pushed open

    Can You Use an Extension Cord When Installing a Window AC?

    Window AC units draw a lot of electrical currents, and regular extension cords can't handle this much electricity. If you use an extension cord with your AC, it could overheat and start a fire.

    Do I Need Insulation When Installing a Window Air Conditioner?

    It's a good idea to insulate your unit so that you can prevent air leaks. If there are air leaks, the AC will use more electricity because it's working harder to keep the air around it cool. 

    Tips For Installing a Window Air Conditioner in an Apartment

    If you're considering getting a window AC for your apartment, the first thing that you'll need to do is make sure that you have permission to install one. If you do, you can follow the steps in our guide, even if you only have side sliding windows.

    My name is Bob Wells and I am a retired HVAC tech from Washington state. I am currently retired and no longer do much with HVAC, however, I feel like I have a lot of knowledge in the subject and I wanted to create a website where I could talk about what I've learned and help upcoming HVAC techs. Find more info about me and HVAC Training 101.