In searching for a case for my laptop, I see product specs that state their case will hold a 16" laptop. This is a diagonal measurement, but I really want to know the width and depth. If laptops these days are built using a proportionate length and depth standard then there should be a math equation to use the diagonal length to determine the exact width and depth. If not then how do you determine the width and depth of a laptop if all they give you is the diagonal measurement?
Unless the case is hard case, you should be fine. Usually the material that cases is made out of stretchable material or has Velcro straps to secure even larger (thicker) laptops.
There is no specific proportion that all laptop manufactures follow. If your laptop is more than 3” thick, consider buying a laptop sleeve or a non-hard case that is an inch or two larger than the laptop size you have.
In general, I'm sure that any store you purchase a laptop case from will let you return it if it doesn't fit your specific needs.