I am running a test for my students, and I asked them to calculate the width of a box using the standard box model and box-model:border-box. I've gone over this in class, but not enough. They were Googling it and they can't understand how to do it from reading the instructions on W3 Schools. Reading it myself, I can understand why. (Some of it is a semantic question about whether the margin is part of the size of the box.) Is it possible to write your definition more clearly? Could I offer some help?