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Start with the axes and defaults
Flexbox is a one-dimensional layout model: it lays items out in a row or column, rather than controlling rows and columns as a two-dimensional grid does. The main axis follows flex-direction; the cross axis runs perpendicular to it. These axes are not simply synonyms for horizontal and vertical: writing direction affects what row means.
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Most properties fall into two groups. Container properties set the flow and alignment of the layout; item properties control an individual child’s size, visual order, or alignment.
| Role | Properties | What they control |
|---|---|---|
| Container | flex-direction, flex-wrap, flex-flow, justify-content, align-items, align-content |
Axis, line wrapping, and alignment or distribution of items and lines. |
| Flex item | order, flex-grow, flex-shrink, flex-basis, flex, align-self |
Visual order, size negotiation, and an item’s cross-axis alignment. |
| Related spacing properties | gap, row-gap, column-gap |
Gutters between items or flex lines; these are useful with Flexbox but are not in the Flexbox module’s core property list. |
With only display: flex, the initial behavior is a row with no wrapping. Items do not grow, may shrink, and use an automatic flex basis; they stretch across the cross axis by default. The initial values for justify-content, align-content, and align-items are flex-start, stretch, and stretch respectively. The default flex value is 0 1 auto. See the W3C CSS Flexible Box Layout Module Level 1 and MDN’s basic concepts guide.
Choose the direction and wrapping behavior
flex-direction: choose the main axis
flex-direction sets the main axis and the direction items follow. Its initial value is row, which follows the inline direction of the writing mode—not necessarily left to right.
row: main axis follows the inline direction.row-reverse: reverses the visual flow of a row.column: main axis follows the block direction.column-reverse: reverses the visual flow of a column.
Changing from row to column also changes which dimension is the main axis. As a result, main-axis sizing and justify-content follow the column rather than the row.
flex-wrap: keep items on one line or create more
The initial value, nowrap, keeps items on one line. If they do not fit, they may shrink or overflow; they do not automatically move to another line. Set wrap to create additional lines, or wrap-reverse to reverse how those lines stack along the cross axis.
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flex-flow: set direction and wrapping together
flex-flow is shorthand for flex-direction and flex-wrap. Either component can be omitted; the omitted property takes its initial value.
.cards {
display: flex;
flex-flow: row wrap;
}That rule makes a row-oriented container whose items can wrap onto additional lines.
Control how flex items size themselves
Flex sizing is a three-part decision: flex-basis supplies an item’s starting main size, flex-grow shares positive free space, and flex-shrink governs how negative free space is handled. The flex shorthand sets all three in that order.
flex-basis: set the starting main size
flex-basis sets the initial main size used in the flex calculation. It accepts content or a size value such as 12rem or 30%. Its initial value is auto: the browser consults the item’s relevant main-size property and, where appropriate, its content sizing. It is not simply another name for width, because the main axis can be a column.
flex-grow: divide positive free space
The initial value is 0, so items do not grow by default. A positive non-negative number lets an item claim a proportional share of available positive space. For example, if two eligible items have grow factors of 1 and 2, they divide the available free space in a 1:2 ratio. That ratio describes the added space, not necessarily their final sizes: their starting bases and sizing constraints still matter.
flex-shrink: handle items that do not fit
The initial value is 1. When items exceed the available main-axis space, shrink factors influence how negative free space is removed. The calculation accounts for item bases and minimum sizes, so a factor twice as large does not guarantee exactly twice as many pixels of shrinkage. Content and minimum sizing can affect the result; equal flex values alone do not guarantee equal final widths.
flex: set grow, shrink, and basis
The longhand order is flex-grow, flex-shrink, then flex-basis. The spec’s initial value is 0 1 auto. These named forms are useful shortcuts:
| Declaration | Expansion | Effect |
|---|---|---|
flex: initial |
0 1 auto |
Does not grow, can shrink, and uses an automatic basis. |
flex: auto |
1 1 auto |
Can grow or shrink from an automatic basis. |
flex: none |
0 0 auto |
Neither grows nor shrinks from an automatic basis. |
flex: 1 |
A one-number shorthand commonly treated as growth with a zero-like basis | Can grow to take available space; do not assume it overrides content and minimum-size constraints. |
When you want equal-looking columns, flex: 1 is a common starting point, but the result still depends on content and minimum sizing. The shorthand’s serialized representation may use a percentage for the basis.
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Align items and distribute free space
Alignment property names describe axes: justify-content works along the main axis; align-items and align-self work across the cross axis for items; align-content distributes multiple lines across that cross axis.
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justify-content: place items along the main axis
This container property distributes remaining main-axis space after item sizing. Its initial value is flex-start. Common values include:
flex-start: pack items toward the main-start side.flex-end: pack them toward the main-end side.center: group them in the middle.space-between: put free space between items, with no extra space at the ends.space-around: distribute space around each item, leaving half-sized spaces at the ends relative to the gaps between items.space-evenly: distribute equal spaces between items and at both ends.
If items grow enough to consume the available space, justify-content has little or no free space left to distribute. Modern Box Alignment also provides logical alignment forms; choose them when you want alignment expressed relative to writing direction rather than a physical side.
align-items: align items across a line
align-items sets the default cross-axis alignment for items in a flex line. Its initial value is stretch. The Flexbox property table also lists flex-start, flex-end, center, and baseline. Stretching applies when the item’s cross size is automatic and other constraints permit it.
align-self: override alignment for one item
Set align-self on an individual flex item to override its parent’s align-items value. Its initial value, auto, defers to the parent. For example, one item can be centered while its siblings stretch:
.toolbar {
display: flex;
align-items: stretch;
}
.toolbar__action {
align-self: center;
}align-content: distribute multiple lines
align-content distributes flex lines along the cross axis. It matters when the container has multiple lines and extra cross-axis space; it has no meaningful line-distribution effect for a single line. Its initial value is stretch. Flexbox values include flex-start, flex-end, center, space-between, space-around, and stretch.
For example, a wrapped container needs enough cross-axis room for the lines to be distributed visibly. align-items aligns items within a line; align-content distributes the lines themselves. MDN summarizes the cross-axis relationship: “Flex items can be aligned in the cross axis of the current line of the flex container, similar to justify-content but in the perpendicular direction.” W3C CSS Flexible Box Layout Module Level 1.
Set practical spacing with gap
gap sets gutters between flex items and, in a wrapped layout, between flex lines. Use row-gap and column-gap when you need different spacing by row or column; gap is the shorthand. These properties belong to the broader CSS alignment and spacing family, not the Flexbox module’s 12-property reference list.
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display: flex;
gap: 1rem;
}A gap belongs between adjacent items or lines, rather than adding space to each item’s outside edge. Margins remain useful when spacing belongs to a particular box or when you need behavior that a uniform gutter cannot express.
Understand visual order and source order
order: change visual placement
order assigns a flex item to an integer ordering group. Its initial value is 0; items with the same value retain their relative order. Items with lower values are placed before those with higher values in the visual layout.
row-reverse, column-reverse, and order change visual placement, not the document’s source order. Keep the DOM order meaningful for assistive technology and sequential navigation; visual reordering can create a mismatch between what sighted users see and the order other users encounter.
Build a small layout, changing one decision at a time
This example starts with three items. Each step changes one property so the effect is easier to identify.
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.demo {
display: flex;
}
.demo > div {
padding: 1rem;
border: 1px solid;
}The direct children line up on a single row, do not grow, may shrink, and stretch across the cross axis when sizing permits.
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2. Add a consistent gutter
.demo {
display: flex;
gap: 1rem;
}The items now have a gutter between them without attaching margins to each child.
3. Distribute remaining main-axis space
.demo {
display: flex;
gap: 1rem;
justify-content: space-between;
}With items that do not grow to fill the row, space-between places the remaining space between them.
4. Let the items grow
.demo > div {
flex: 1;
}The items can now grow to use available space. Because growth consumes free space, justify-content may have little left to distribute.
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.demo {
display: flex;
flex-flow: row wrap;
gap: 1rem;
align-content: space-between;
}If items wrap into multiple lines and the container has extra cross-axis room, align-content distributes those lines. Without multiple lines and available cross-axis space, it will not produce a visible distribution.
6. Align just one item differently
.demo > div:last-child {
align-self: center;
}The final item uses its own cross-axis alignment rather than the container’s default.
Quick choices for common Flexbox tasks
| Goal | Starting point | What to check |
|---|---|---|
| Center items horizontally in a row | display: flex; justify-content: center; |
In a column, the main axis is vertical instead; writing direction also affects row start and end. |
| Center items across the cross axis | align-items: center; |
This centers items within a line; it does not distribute multiple lines. |
| Center one child across the cross axis | align-self: center; |
Apply it to the flex item, not its parent. |
| Make a row of items wrap | flex-flow: row wrap; |
Check that the container has a constrained main-axis size so wrapping can occur. |
| Space items evenly with a gutter | gap: 1rem; |
Use justify-content separately if you also need to distribute leftover main-axis space. |
Why flex items may not wrap
- The container still has
flex-wrap: nowrap. Setflex-wrap: wrapor useflex-flow: row wrap. - The container has enough main-axis space for the items at their current sizes. Wrapping occurs when items cannot fit on a line under the layout constraints.
- The main axis is not the one you assumed. Check
flex-direction; a column’s main axis is not the row direction. - The children are not direct flex items. Only a flex container’s direct children become flex items.
For property definitions and the complete Flexbox model, consult the MDN Flexbox guide, its basic concepts, and the W3C specification.
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