CSS box-sizing

The CSS box-sizing property can be set to:

• ▪ content-box or
• ▪ border-box

content-box [default]

When this value is set and you set the width of an element, you are setting the width of the content box. The core of the onion. The total width of the onion will be this width plus the width of the 3 other layers.

* {
   margin: 0;
   padding: 0;
}

div {
   width: 120px;
   padding: 20px;
   border: 4px solid;
   margin: 30px;
}

border-box

When this value is set and you set the width of an element, you are setting the width of the border box. This is made up of the onion core and 2 of the layers. The total width of the onion will be this width plus the width of outer-most layer.

* {
   margin: 0;
   padding: 0;
}

div {
   box-sizing: border-box;
   width: 120px;
   padding: 20px;
   border: 4px solid;
   margin: 30px;
}

border-box is more commonly used than content-box. Use the snippet below to set it on all elements.

*, ::before, ::after {
   box-sizing: border-box;
}

Working with Margins

When working with margins, their size can be influenced by things other than the margin value. See consistent / varying gaps.

References

• box-sizing