flex-basis property CSS Reference

Definition and Usage

The CSS flex-basis property specifies the flex basis which is the initial main size of a flex item.

  • Initial auto
  • Applies to flex items, including in-flow pseudo-elements
  • Inherited no
  • Percentages refer to the flex container's inner main size
  • Media visual
  • Computed Value as specified, with lengths made absolute
  • Animatable yes, as a length, percentage or calc(); when both values are lengths, they are interpolated as lengths; when both values are percentages, they are interpolated as percentages; otherwise, both values are converted into a calc() function that is the sum of a length and a percentage (each possibly zero), and these calc() functions have each half interpolated as real numbers.
  • Canonical order the length or percentage before the keyword, if both are present


Formal syntax: <'width'>
flex-basis: value
flex-basis: inherit


Computed value returns a percentage of the parent flex container main size property. Negative values are invalid.
absolute length
Defined as a number followed by a absolute unit such as px, mm or pt. Negative values are invalid.
Refers to the default value of the property.
The computed value of flex-basis equals the computed value of the main size of the flex item.

Note: Writing Modes Level 3 specification will introduce the following new keywords: min-content, max-content, fill-available and fit-content.

Note: flex-basis determines the size of the content-box unless specified otherwise using box-sizing.


element { 
flex-basis: 18em;


Desktop browsers

Feature Firefox (Gecko) Chrome Internet Explorer Opera Safari
Basic support 18.0 (18.0) (behind a pref)
22.0 (22.0)
21.0-webkit Not supported 12.10 Not supported

Mobile browsers

Feature Firefox Mobile (Gecko) Android IE Phone Opera Mobile Safari Mobile
Basic support NA NA Not supported 12.10 Not supported

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