Stop Guessing. Start Sealing.
A planner for custom shims and weatherstripping around your window AC unit. Get exact cutting dimensions, material lists, and a printable template in minutes.
Open the PlannerYour Window & AC Setup
Your Shim Plan
Side Shims
Cut two pieces, one for each side.
Top Shim
Fills the space above the unit.
Bottom Shim
Only needed if there is a gap below.
Weatherstripping
Total length of foam tape needed.
Security Bar
Recommended bar length for inside the frame.
Material List
- Enter your dimensions above to see the material list.
Window-Specific Notes
How This Planner Works
Pick Your Window Type
The planner adjusts its advice based on how your window opens. Double-hung windows are the most common, but casement and sliding windows need different approaches. Casement windows often need the shim on the hinge side to prevent the window from being forced open.
Enter Dimensions
Measure the inside of your window frame where the AC unit sits. Then measure the AC unit itself. The difference between these numbers tells the planner how big your shims need to be. Use a tape measure and write down the numbers to the nearest quarter inch.
Measure the Gaps
With the AC unit installed and the window closed, measure the space on each side. Even small gaps let in hot air, bugs, and rain. The planner uses these numbers to tell you exactly how wide each shim should be and how much weatherstripping to buy.
Cut, Install, Seal
Use the cutting template to mark your shim material. Cut with a jigsaw or hand saw. Sand the edges. Place the shims, then apply foam weatherstripping around the edges. Add a security bar if your unit is low to the ground.
What We Assume
- Your window frame is rectangular and in good condition.
- The AC unit sits inside the window opening (not a through-the-wall unit).
- You have basic tools: tape measure, saw, drill, and a level.
- Shim material is exterior-grade plywood or hardboard.
- Measurements are in inches. If you measure in centimeters, convert first.
Common Mistakes (and How to Avoid Them)
Over-Tightening the Side Panels
Most AC units come with accordion side panels. People crank them tight against the frame, which warps the panels and actually creates gaps. The panels should be snug but not forced. Use shims to fill the remaining space instead of relying on the panels alone.
Ignoring the Tilt
Window AC units need a slight backward tilt (about 1/4 inch) so condensation drains outside. If your shims are all the same height, the unit may tilt forward and drip on your floor. The planner accounts for this in the side shim heights.
Using the Wrong Weatherstripping
Thin V-strip weatherstripping looks neat but compresses over time and stops sealing. Use medium-density foam tape (1/4" to 3/8" thick) for the best long-term seal. Replace it each season if you take the unit out for winter.
Skipping the Security Bar
An unsecured AC unit can be pushed in from outside in minutes. A simple 1x2 wood bar cut to fit inside the frame, screwed into the sash track, stops this. The planner tells you the exact length to cut.
Blocking the Drain Holes
Some people seal the bottom of the unit completely, which blocks the condensation drain. Leave the back bottom edge unsealed or use thinner weatherstripping there so water can escape.
Not Checking the Window Weight Limit
Older windows and sashes have weight limits. A 12,000 BTU unit can weigh 80 pounds or more. Check that your window frame and sash can handle the load before installing. If in doubt, add a support bracket underneath.
Troubleshooting by Window Type
Casement Windows — The crank handle gets in the way
Remove the crank handle if possible, or position the AC unit on the side opposite the hinges. You may need an L-shaped shim to clear the hardware. Some people build a small shelf inside the frame to support the unit.
Sliding Windows — The track is too shallow
Sliding window tracks are often only 1/2 inch deep. Use a flat bar or angle iron screwed into the track to create a wider ledge for the AC unit to sit on. Shim the top gap generously since these frames flex more.
Awning Windows — The unit wants to slide forward
Awning windows open from the bottom, so the sash pushes against the front of the AC unit. Use a bracket or cleat at the bottom of the frame to prevent forward movement. The security bar is especially important here.
Old Wood Frames — The frame is not square
Older homes often have frames that have shifted over time. Measure the width at the top, middle, and bottom. Use the smallest measurement for the AC unit fit, and shim the larger gaps. A flexible foam backer rod works well for irregular gaps.
Questions People Ask
- What if my window type is not listed?
- Pick the closest match and use the custom fields. The gap-based calculations work for any rectangular frame. For arched or irregular windows, measure at the top, middle, and bottom separately and use the largest gap for each side.
- What material should I use for shims?
- Exterior-grade plywood between 1/4 inch and 3/4 inch works for most shims. For narrow gaps, hardboard or rigid foam insulation works too. Avoid pressure-treated lumber because it can off-gas in summer heat.
- How do I measure the gaps correctly?
- With the AC unit installed and the window sash closed against it, use a tape measure or ruler to find the space between the AC unit and the frame on each side. Measure at the top and bottom of each gap and use the larger number.
- Do I really need a security bar?
- If your AC unit is on the first or second floor, yes. A simple wooden bar or metal bracket across the inside frame prevents the unit from being pushed in from outside. The planner tells you the length needed.
- Can I use this for a portable AC with an exhaust hose kit?
- Yes. Treat the exhaust hose panel as your AC unit. The gap measurements and shim plan work the same way. Just enter the panel dimensions instead of the AC unit size.