Measuring with the Box Model

I have been studying the Box Model for calculating the space any box will take up on a site. A box in a website is just a container that holds things like the text or images. This means that if you want to put some text on the page, you will have to calculate the area the text will take up, the padding, the border and the margin space. The total area can be best shown by the illustration below:
For example, If my content is a width of 200px, I put 15px padding, a border of 5px and a margin of 5px, my content box will be a total width of 250px (200px wide + 2(15px padding) + 2(5px border) + 2(5px margin).