The screen size on a laptop sleeve is a category, not a complete fit measurement. Computers with the same screen diagonal can differ in body width, depth and thickness. Measure the machine you intend to carry before choosing a sleeve by its label.
Close the laptop and measure its outside dimensions using the seller's stated method where available. Include an attached shell if you plan to leave it in place. Record the figures with units so that a later comparison does not confuse centimeters and inches. The sleeve's internal measurements are the relevant starting point; its external dimensions mainly help you decide whether it fits inside another bag.
Read how the sleeve opens and closes. A zipper may need clearance around the computer, and a flap can have a different fit arrangement. Do not assume that an internal measurement exactly equal to the laptop's width guarantees an easy closure. Look for the maker's compatibility guidance or ask for clarification when the listed dimensions leave no obvious room for the intended use.
Decide what belongs inside the sleeve. A charger, mouse and notebook add bulk, even if an accessory pocket is provided. Check the pocket's position and whether the contents will press against the laptop when carried. A separate pouch may suit bulky accessories better than forcing everything into one package. Compare the loaded arrangement rather than the neat empty photograph.
Think about the sleeve's role in your normal carrying system. If it will sit inside a backpack, compare the combined external size with the backpack compartment. If you intend to carry it alone, look at the closure and any handles specified for that purpose. Padding and material descriptions can help you compare construction, but do not treat an unspecified protective claim as a guarantee against damage.
Imagine a hypothetical slim laptop with a fitted shell. The screen category matches a sleeve, but the shell adds enough thickness to make closing awkward. Another sleeve may provide suitable room without leaving the machine excessively loose. The useful comparison is between these complete arrangements, not simply between two identical screen-size labels.
On receipt, inspect the sleeve and try the fit gently under the retailer's assessment conditions. Do not force the zipper around the computer or load the accessory pocket before checking basic clearance. Confirm that the loaded sleeve also fits its intended bag. Keep the successful dimensions and model in your notes. That record makes a future replacement easier and avoids repeating an uncertain purchase based only on a device's screen diagonal.