CSS box-sizing Property Reference

The CSS box-sizing property is a standard property in CSS Box Sizing. Review its formal syntax, computed behavior, and practical declarations.

Updated 2026-08-12

box-sizing overview

The CSS box-sizing property is a standard property in CSS Box Sizing. Review its formal syntax, computed behavior, and practical declarations.

The property is not inherited by default. Use an explicit global keyword when inheritance is intended.

Syntax and formal definition

box-sizing formal syntax

Field	Value
Formal syntax	content-box \| border-box
Initial value	content-box
Applies to	allElementsAcceptingWidthOrHeight
Inherited	No
Percentages	no
Computed value	asSpecified
Animation type	discrete
Media	visual
Specification group	CSS Box Sizing
Status	Standard

Practical examples

These declarations demonstrate valid ways to set box-sizing. The initial and inherit global keywords remain available even when a property-specific value is preferable.

box-sizing live example

CSS

.reset-example {
  box-sizing: initial;
}

.inherit-example {
  box-sizing: inherit;
}

@supports (box-sizing: initial) {
  .supported {
    box-sizing: initial;
  }
}