Measuring an alcove
This is a measurement method, not a product specification.
An alcove is rarely a single width. Openings change with trim, skirting, a shelf that is narrower at the back, and a floor that is not parallel to the wall.
Measure the clear width at the height where the television will sit, and again near the top of the cabinet if the opening tapers. Use the smaller clear width for a straight-in comparison. Measure clear height from the surface the set rests on, or from the floor if it is a tall opening, up to the lowest obstruction. Measure depth from the front of the opening to the back, and note anything that sticks out: a cable plate, a mantel return, a window handle.
Write the unit next to each reading. If you are unsure of a reading, keep an uncertainty rather than entering a hopeful exact value. This site’s space check starts from 5 mm of uncertainty on a space reading unless you change it. That default is a product assumption, not a statement about your tape.
Compare those readings with the configuration you will use: cabinet only on a mount is not the same rectangle as the set on its stand. A positive difference on width does not answer ventilation, socket projection or whether the set can be rotated into the opening. If the manufacturer’s required clearance is not in the record, the geometric comparison still is not an installation approval.
Take the smallest clear dimension you found, not the average. Averaging a wide front and a narrow back will hide the collision.