Interior Heating & Cooling Cooling

How to Install a Window Air Conditioner

With these simple tips, it's a breeze to install a window air conditioner quickly and securely.
Bob Vila Avatar
how to install a window ac unit

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For homeowners and renters who don’t want the bother (or expense) of installing a central air conditioning system, window air conditioners are terrific options for keeping cool in warmer months. Though window air conditioners are heavy and you may worry about attaching them securely enough that they don’t fall out of the window, installing them is not as intimidating as it may seem at first. In fact, those who have window ACs are likely to become adept at installing them, given that most people remove them at the end of the summer and reinstall them the following year when the weather warms up again.

Bear in mind, however, that not all window designs are meant to accommodate such a large, unwieldy box. In this guide, learn exactly how to install a window AC unit with our pro tips for DIYers.

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Before You Install a Window AC Unit

While installing a new window AC unit is not a difficult task, there are a few things to keep in mind before you begin. Before you purchase a window air conditioner (or try to install one), it’s important to understand how—or whether—it will fit inside your window. If you have yet to purchase a window air conditioner, first research whether the sizes and types of windows you have are compatible with these bulky appliances. If they are not compatible, a portable air conditioner may be a better fit for your home. Ahead, learn more about portable AC units, and how to match and size a window air conditioner properly.

how to install a window ac unit

Portable vs. Window AC: Which is Better for Your Needs?

If you’re considering a window air conditioner because you need a cooling solution for your home that isn’t as elaborate or expensive as central air conditioning, portable air conditioners should also be on your radar. Here’s what to think about as you decide which type of air conditioner is best for your situation.

A portable air conditioner is a freestanding appliance that sits inside a room. This type of air conditioner plugs into a standard outlet and has a hose that extends through a nearby window through which the unit ejects hot air. Since you do not need to mount the air conditioner to a window wall, portable air conditioners are pretty easy to move. They are good choices if you want an AC unit that you can transport from room to room.


  • Easy to move from room to room
  • Requires no window installation


  • Takes up floor space
  • Can be noisy
  • Must be near a window to vent

A window air conditioner mounts between the sash and the sill of a window, allowing it to compress air that it draws in from outside, removing the warm air and sending it back outside while blowing the remaining cool air into the room. These AC units are mounted to the window with sturdy hardware to ensure they are secured in place. Though installing a window unit isn’t complicated, it’s a more involved process than setting up a portable AC unit.


  • Saves space by sitting in the window
  • Directs some noise outside the window
  • More efficient than portable air conditioners


  • Can be difficult to install and remove
  • Noisier than a central AC unit

Window Compatibility

The window unit must be able to fit the window size and type. Most window air conditioners are designed to be compatible with double- or single-hung windows; with a few adjustments, many models can also be installed in sliding windows.

Window air conditioning units indicate the minimum and maximum width window they can fit as well as the minimum opening of the window. The window opening should be wider than the unit but still narrow enough to fit the maximum width of the air conditioner’s baffles. The window should also open wide enough to allow the air conditioning unit to sit between the sash and the sill. Be sure to measure your window and research products’ dimensions carefully before deciding on a window air conditioner.

RELATED: The Dos and Don’ts of Installing a Window Air Conditioner 

how to install a window ac unit

Choosing the Right Window Air Conditioner for Your Room Size

Choosing the right size air conditioner is key to cooling a room. If you get too small an air conditioner, it won’t be able to maintain a cool temperature in the room. Too large an air conditioner will waste energy and drive up your electricity bill.

An air conditioner’s cooling capacity is measured in British Thermal Units (BTUs). Power output ranges from 5,000 BTUs for small air conditioners, and up to 20,000 BTUs for larger ones. To determine the size of air conditioner that best suits your room, first determine the room’s square footage by multiplying its length by its width. Then multiply that number by 20 or 25, which is the number of BTUs required to cool a square foot of living space. For example:

  • A 10-foot by 20-foot room is 200 square feet, and would require a 4,000- to 5,000-BTU unit.
  • A 20-foot by 25-foot room is 500 square feet, and would require a 10,000- to 12,500-BTU unit.

7 Easy Steps to Installing a Window Air Conditioner

After selecting the right window air conditioning unit for your room and window size, it’s time to cool down—your room, that is. First, gather the tools and materials listed above so they’re close at hand—you don’t want to pick up a bulky piece of machinery only to realize that your screwdriver is in the other room.

STEP 1: Measure your window.

Before you shop for a window air conditioner, measure the height and width of the window’s opening. The air conditioner you purchase should have at least 2 inches of wiggle room on either side of the housing so the unit’s baffles can be extended.

how to install a window ac unit

STEP 2: Assemble the unit.

Once you’ve purchased and unpacked an appropriately sized air conditioner, you’re ready to install it. First, attach any rails, flanges, or accordion-style panels to the unit’s housing. Follow the manufacturer’s instructions, and use the screws provided.

From here on, you’ll need a helper: Two pairs of hands are best for all but the very smallest air conditioners.

STEP 3: Prepare your window.

Open the window wide enough to accommodate the height of the air conditioner. Next, lift the unit and rest it on the bottom of the window frame. Have your helper hold the unit in place while you see to the remaining tasks.

STEP 4: Place the unit in the window.

Most window air conditioners are designed with two flanges: one that runs along the top of the unit, another along the bottom. These flanges make installation easier and improve the air conditioner’s stability.

Position the bottom flange so that it abuts the windowsill. Next, lower the window sash (which you raised in Step 2) until its bottom rail meets the top flange on the unit. The air conditioner should now be held in place by the top window sash. Still, ask your helper to stabilize the unit until you’ve completed the next step.

Depending on the style of window and the size of the AC unit, you may also need to install a window AC unit mount, such as Ivation’s Window Air Conditioner Mounting Support Bracket. To confirm whether this device is required for your installation, consult the air conditioner manufacturer’s instructions—and, if applicable, your building’s regulations about air conditioner installation.

how to install a window ac unit

STEP 5: Screw in the angle brackets.

Your air conditioner probably came with one or two small angle brackets that secure the two window sashes together. These brackets are essential—they prevent the sashes from slipping apart or the window from being opened accidentally. Both situations are dangerous, and could cause the air conditioner to fall out of the window.

Place the angle bracket against the top sash where it meets the top of the bottom sash. With a pencil, mark where the screws should go, drill pilot holes, and tighten the screws using a screwdriver. Extend the accordion-style panels (which you attached to the unit in Step 2) and secure them to the window using the manufacturer-provided screws. At this point, make sure that all screws that came with the unit have been secured according to the unit’s instructions.

STEP 6: Attach the panels.

Window air conditioning units come with accordion-shaped side panels that can extend and retract to fit different window widths. These panels close the gaps on either side of the unit, preventing air and insects from passing into the home.

Once the air conditioning unit is securely mounted to the window, extend the panels on either side so they cover the entire gap. If there are still small cracks and gaps around the panel, use foam weather stripping to create an airtight fit. Once you’re satisfied with the panel’s positioning, locate the small mounting holes on the panel. Use a drill to create small pilot holes in the sash that align with the mounting holes. Attach the panel to the sash using two screws and a screwdriver.

If for some reason your AC unit does not come with an air conditioner screen, you may need to purchase a window AC installation kit. Alternatively, you can also construct a window AC frame out of wood or plastic sheeting to cover the gap.

STEP 7: Insulate the opening with a window AC seal.

The last step is to seal the opening between the upper sash and the lower sash, which has been raised to accommodate the unit.

Your air conditioner should have come with at least one foam insulating strip. Cut it to length, then fit it snugly into the gap between the lower sash and the glass panes of the top sash.

If your air conditioning unit didn’t come with an insulating strip, consider buying one and installing it as well.

how to install a window ac unit

Additional Tips for Installing a Window AC Unit

If you choose to remove the air conditioning unit before winter, remember to store it upright in a dry location.

If your air conditioner came with L-brackets, be sure to put these in place before lifting the unit into the window.

Never leave an air conditioner in the window without securing it. If it is not secured, the unit could fall out of the window, causing injury. A loose air conditioner unit might also make your home susceptible to burglary.

Final Thoughts

By following the instructions above, you should be able to install a window air conditioner safely and securely. If you’re purchasing a new air conditioner, make sure to carefully measure the window to ensure a proper fit.

Keep in mind that omitting any steps in the installation can create a hazard. For example, you might be tempted to forego securing an air conditioning unit to the window with screws and hardware in order to avoid drilling into the window or to make seasonal un-installation easier. Resist the urge to skip this step. An air conditioner weighs upwards of 60 pounds and can cause serious injury should it fall out of a window because it isn’t properly secured.

FAQs About How to Install a Window Air Conditioner

If you’re wondering how air conditioners work or why they need a window, then read on for answers to these and other commonly asked questions about these appliances.

Q: Can you use a window air conditioner without a window?

No. You should not use a window air conditioner without a window. Window air conditioners must be mounted to a window opening in order to properly vent warm air outside the home.

Q: How do window air conditioners work?

An air conditioner works by pulling warm into the unit and cooling it over coils chilled by refrigerant. The coils extract heat and moisture from the air, which is vented back outside. A built-in fan circulates the remaining chilled air into the room, cooling the indoor temperature.

Q: Can you plug a window AC unit into a regular outlet?

Yes. Most window AC units have a three-prong plug that fits into a standard 120-volt outlet. Units with a BTU rating higher than 15,000 require a 220-volt circuit, which requires a different type of plug or a hardwire connection.

Q: Should I remove my window AC unit in winter?

Yes. You should remove your window AC unit before the arrival of snow, ice, and freezing temperatures that can damage the unit. Removing the AC unit will also prevent cold air from entering the room. Store the air conditioner in a basement or attic during the off season.

Q: Why do window air conditioners need to be vented?

Window air conditioners work by removing heat and moisture from the air it draws into the unit. That air must be vented outside for the air conditioner to cool the room.