Smatree
Almost every case that comes back to us was the right number and the wrong rectangle. A laptop sold as fifteen point six inches can be two centimetres narrower than another laptop sold as fifteen point six inches, because the bezel around the screen has been shrinking for a decade and the diagonal measures the glass, not the lid. Five minutes with a tape settles it.
The three numbers that matter
Width across the front edge, depth from the front edge to the hinge, and thickness with the lid closed. Take all three on the machine itself, not from a specification sheet — sheets round, and some of them quote the thickness at the thinnest point of a wedge-shaped body.
Where to put the tape
Flat on a table, lid closed, tape along the outside of the shell. Do not include the rubber feet in the thickness unless the feet are tall enough to feel; on most machines they are a millimetre and they compress.
Thickness is the one people get wrong
A case has width and depth to spare almost always, because a few millimetres of lateral slack is invisible. Thickness is where a case either closes or does not, and a thick machine in a thin shell puts the zip teeth straight onto the lid.
Add what you have added
A protective skin, a screen cover, a sticker layer, a privacy filter — they all count. A skin is usually under a millimetre and fits inside the tolerance we cut. A second hard shell already clipped to the lid is not: our patterns are drawn around the bare machine.
Ports and feet
Some machines have a port cover or a rear exhaust vent that sticks proud of the body. Measure to the furthest point of the actual object, not to the flat of the panel.
Compare against the internal dimensions
Every page here lists the internal dimensions of the case, which is the useful number — external dimensions tell you whether it fits your rucksack, internal dimensions tell you whether your machine fits it. You want your three measurements to be a few millimetres under, in all three directions.
How much clearance is right
Two to four millimetres on width and depth, one to two on thickness. More than that and the machine slides; less and you are fighting to get it out one-handed on a train table.
When two cases both look right
Take the one that names your exact model. A case that names a list of machines was measured against that list; a case that names a size was measured against a bracket. Both may fit. Only one was drawn around your rectangle.