By default in the CSS box model, the
height you assign to an element is applied only to the element’s content box. If the element has any border or padding, this is then added to the
height to arrive at the size of the box that’s rendered on the screen. This means that when you set
height, you have to adjust the value you give to allow for any border or padding that may be added. For example, if you have four boxes with
width: 25%;, if any has left or right padding or a left or right border, they will not by default fit on one line within the constraints of the parent container.
box-sizing property can be used to adjust this behavior:
content-boxgives you the default CSS box-sizing behavior. If you set an element’s width to 100 pixels, then the element’s content box will be 100 pixels wide, and the width of any border or padding will be added to the final rendered width, making the element wider than 100px.
border-boxtells the browser to account for any border and padding in the values you specify for an element’s width and height. If you set an element’s width to 100 pixels, that 100 pixels will include any border or padding you added, and the content box will shrink to absorb that extra width. This typically makes it much easier to size elements.
box-sizing: border-boxis the default styling that browsers use for the
<button>elements, and for
<input>elements whose type is
Note: It is often useful to set
border-box to lay out elements. This makes dealing with the sizes of elements much easier, and generally eliminates a number of pitfalls you can stumble on while laying out your content. On the other hand, when using
position: relative or
position: absolute, use of
box-sizing: content-box allows the positioning values to be relative to the content, and independent of changes to border and padding sizes, which is sometimes desirable.