What size window air conditioner do I need?

Size by BTU to the room's square footage: roughly 5,000 BTU for up to 150 sq ft, 8,000 BTU for 300 to 350 sq ft, 12,000 BTU for 550 sq ft, and 14,000 to 18,000 BTU for large open rooms. Add about 10 percent for a sunny room or a kitchen. Oversizing is a real mistake — an AC that is too big cools fast then shuts off before it dehumidifies, leaving the room cold and clammy. Match the BTU to the space.

Do I need a support bracket for a window AC?

For anything but the smallest units, yes. A support bracket that mounts to the sill and braces the unit from below carries the weight and takes strain off the window frame, and many building codes and landlords now require one, especially above the ground floor. A heavy unit balanced on the sill alone can shift or fall. Brackets are inexpensive and install in minutes — skipping one is the biggest safety error people make.

Which way should a window AC tilt?

Tilt the unit very slightly toward the outside — about a quarter inch lower at the back — so the condensate water drains outdoors rather than pooling inside the unit and dripping into your room. Most units are designed for this small outward pitch. Never tilt it inward. A level or inward-tilted unit collects water, overflows indoors, and can breed mold. Check with a level after mounting.

Can one person install a window air conditioner?

For a small 5,000 BTU unit in a low window, carefully, yes — but two people is strongly recommended for anything larger or above the ground floor. The dangerous moment is balancing the unit on the sill while raising the window and fitting the brackets, when a slip could drop the unit outside or pinch your hands. A second person steadies the unit so you can secure it, which is why nearly every guide calls for a helper.

How do I seal the gaps around a window AC?

Extend the accordion side panels to fill the window width and screw them to the sash, then seal the perimeter and the gap where the upper and lower sashes meet. Foam weatherstrip (usually included) goes between the sashes; foam tape or a window-AC insulation panel seals the sides. Sealing gaps keeps cooled air in, hot air and bugs out, and stops the unit working harder than it needs to — it noticeably improves efficiency.

How to Install a Window Air Conditioner: A Safe DIY Guide

Written & researched by Derek Holt Home & Appliances Editor

Updated August 2026

Wide view of a person adjusting a wall thermostat in a warm hallway, in soft early-morning light

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Installing a window air conditioner is a quick two-person job — plan on 30 to 45 minutes, and no permit. Size the unit to the room by BTU, mount a support bracket, seat the unit with a slight outward tilt so it drains outside, and seal the gaps. The two things that matter most are a proper support bracket (the unit can fall without one) and a helper for the moment you balance it on the sill. This is a mechanical install, not an electrical one, though the unit needs a properly rated outlet.

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How to Install a Window Air Conditioner — The Home Depot

Tools and materials

A support bracket sized for your unit and sill — the key safety item.
A screwdriver or drill, a level, and the foam weatherstrip and side panels that come with the AC.
Foam tape or a window-AC insulation panel to seal the sides.
A helper. Non-negotiable for larger units or upper floors.
The right-size unit. If a plug-in cooler suits the room better, compare our best tower fan and portable options.

Golden-hour view of a person adjusting a wall thermostat in a warm hallway

Step 1: Size the unit and check the outlet

Match the BTU to the room (about 5,000 BTU per 150 sq ft, scaling up). Confirm the window opening fits the unit's minimum and maximum width. Check the outlet: larger units (over about 12,000 BTU) may need a dedicated 115V or a 230V circuit — read the plug type and do not use an extension cord, which is a fire risk with an AC's draw.

Step 2: Mount the support bracket

Install the support bracket on the windowsill per its instructions, leveling it and setting the slight outward pitch so the unit will drain outside. Tighten it to the sill so it bears the unit's weight. This bracket is what keeps a heavy unit from stressing the frame or falling — fit it before the AC goes anywhere near the window.

Step 3: Set the unit in the window

Open the window fully. With your helper steadying the unit, lift it onto the bracket and slide it into the opening so the bottom flange sits behind the sill and the unit rests on the bracket. Keep a firm grip until it is seated — this is the moment a unit can slip outward. Lower the window sash down behind the top flange to lock it in place.

Step 4: Secure and confirm the tilt

Screw the top flange to the window sash and, where provided, the sash lock bracket that stops the window being raised. Put a level on the unit and confirm the slight outward tilt (about a quarter inch lower at the rear). Adjust the bracket if it reads level or tilts inward — proper drainage depends on this.

Low-angle view of a person adjusting a wall thermostat in a warm hallway, in warm evening lamplight

Step 5: Seal the gaps and test

Extend the accordion side panels to fill the window width and screw them to the sash. Fit the foam weatherstrip between the upper and lower sashes where they overlap, and seal the side panels with foam tape or an insulation panel. Plug the unit into its proper outlet, switch to cool, and confirm cold air and drainage outside. A well-sealed install cools faster and cheaper.

Common mistakes

Skipping the support bracket. The biggest safety error — the unit can fall.
Installing alone. Use a helper for the sill-balancing moment.
Tilting the unit inward or level. It must pitch slightly outward to drain.
Oversizing the BTU. Too big cools without dehumidifying — clammy and short-cycling.
Using an extension cord. A fire risk with an AC's current draw. Plug into a proper outlet.

When to call a pro

The install itself is DIY, but call an electrician if the unit needs a dedicated or 230V circuit you do not have — new circuits and any panel work are licensed-electrician and permit jobs, jurisdiction-specific. For a very heavy unit on an upper floor, or a window that will not support a bracket safely, a professional install is the safer call.

How to Install a Window Air Conditioner