How to Measure a Basement Window for an AC Unit | Get The Right Fit

A basement window AC unit needs a perfect fit that starts with accurate measuring: take the smallest width from three points across the inside of the frame and the full height from the windowsill to the raised sash.

One wrong measurement and your AC either won’t fit the basement window or leaves gaps that leak cool air. The fix isn’t complicated — it’s about measuring the right spots the right way. Here’s the exact protocol for measuring a basement window so your window air conditioner installs cleanly and cools effectively.

What You Need Before You Measure

Grab a steel tape measure (a cloth one bends and gives a false reading), a pencil, and paper to record three width numbers. You’ll also need the AC model’s specs on hand — the box lists minimum and maximum window dimensions, and you need your numbers ready to compare.

Most window AC units work only with double-hung windows where the bottom sash slides upward. A sliding, casement, or single-hung basement window may not accept a standard unit without modifications. If you’re unsure about your window style, check TCL’s compatibility guide for the details.

How to Measure Basement Window Width for an AC Unit

Open the lower sash completely. Place the tape measure end inside the vertical track (the jamb) on one side and extend it straight across to the opposite jamb. Do not include the sash itself or any trim — you want the clear opening width.

Take this measurement at three spots: the bottom, the middle, and the top of the opening. Old basements shift and settle, so those numbers will likely differ slightly. Write all three down, then circle the smallest one. That is the width your AC must fit through. An average of the three would get you a unit that jams on the tightest spot.

How to Measure Basement Window Height for an AC Unit

Raise the lower sash as high as it will go. Place the tape measure end on the flat top surface of the windowsill (the ledge you’d set a plant on). Extend it straight upward to the bottom edge of the raised sash. This gives you the clear opening height — the space the AC unit has to sit in vertically.

This height must exceed the AC’s minimum height requirement listed on the box. If the clearance is tight, measure twice — a unit that barely clears on paper often bumps the sill when you slide it in.

Measuring Sill Depth and Checking Obstructions

Measure from the interior window stool (the wood or vinyl lip inside) to the outer edge of the sill or storm window frame. Some basement windows have a deep sill, some barely an inch. The sill depth tells you whether the AC’s base will sit flat or need a support bracket.

Also look for obstructions: a crank handle, a center mullion, or a security bar can block a standard AC. If your basement window has a crank mechanism, you likely have a casement window — and standard window ACs won’t work without swapping the window first.

BTU Calculations: What Size AC Does Your Basement Need?

Basements run cooler than upper floors, but the BTU formula still applies. Multiply the room’s square footage (length × width) by 25, then add 1,000 BTUs for each window and 400 BTUs for every person who regularly uses the space.

Here’s how the math works for common basement sizes:

An undersized unit runs constantly and never catches up; an oversized one cycles on and off so fast it can’t pull humidity out of the air. Stick close to the calculated number for a basement that feels cool and dry.

Common Measurement Mistakes That Ruin the Fit

The most frequent error is including the window trim or frame thickness in the measurement — that instantly buys you an AC too wide to fit. Measuring only once is the second-most common mistake; you need three width readings because no basement window is perfectly square.

Another killer: not raising the sash fully before measuring height. If the sash is halfway up when you measure, the real clearance will be several inches shorter and your new AC won’t slide in. And bending the tape measure when measuring width (common in tight basement openings) introduces an inch of error easily. Keep the tape straight and level.

Window Compatibility and Model Requirements

Most standard window AC units are engineered for double-hung windows with a bottom sash that raises and lowers. TCL provides specific width and minimum height charts for its models, and GE’s installation guides list exact window dimensions per unit. Always check the minimum and maximum window dimensions in your specific model’s installation guide before buying.

If your basement window is smaller than typical, you may need a compact or “low-profile” AC designed for tight openings. The unit’s casing height must clear the raised sash, and the width must land between the unit’s minimum and maximum listed width — not close to it, but inside it. A unit at the maximum width leaves no room for the accordion side panels, and those panels seal the gap to keep hot air out.

Once your measurements confirm a fit, you’re ready to compare models. Our roundup of the best ACs for basement windows shows which units handle small frames and humid basements best, tested for real-world fit and performance.

Installation Steps After Measurement

Once you’ve confirmed the AC fits your window dimensions, follow this sequence:

Set the AC in the window opening centered left to right. Lower the sash so it rests on the top of the unit’s casing. Tilt the AC 3 to 5 degrees downward toward the outside — this lets condensation drain properly instead of pooling inside. Secure the unit by screwing the casing into the window sash and fastening the side panels to the window frame.

Seal gaps with foam weather stripping between the lower and upper sash and apply caulk at the edges. Plug the unit into a grounded wall outlet — never an extension cord unless it’s rated for AC use. Test the seal by running your hand around the edges; if you feel air, add more stripping.

Window AC units are deceptively heavy. Never install alone — use a helper to lift and set the unit, and inspect the window for rot or damage before hanging weight on it. A rotten sill can let the whole unit drop.

Final Measurement and Installation Checklist

Run through this list before you lift the unit. Every missed check is a drafty basement or a returned AC.