16. Box sizing

We can define the total width or height of an element counting or ignoring its border or padding.

box-sizing

Any border or padding is added to the total width of the element. It is the default style if not defined.

.elemento {
    box-sizing: content-box;
}

To fix this mismatch we can use border-box. The widths of the border or padding are subtracted, respecting the indicated width.

.elemento {
    box-sizing: border-box;
}

In the following example we can see an element that exceeds the desired measurement of 10rem in width.

.elemento {
    width: 10rem;
    padding: 2rem;
    border: 1rem solid black;
}

How wide is it? width 10rem + 4rem padding (both right and left have 2rem) + 2rem border (both right and left have 1rem). In total 16rem, or 256px.

On the other hand, if we use box-sizing: border-box it does adjust.

.elemento {
    box-sizing: border-box;
    width: 10rem;
    padding: 2rem;
    border: 1rem solid black;
}

It measures 10rem in total, or 160px. Measure it yourself with the Inspector!

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