Mastering wrapping of flex items

The initial value of the flex-wrap property is nowrap. This means if a set of flex items is too wide for their flex container, they will overflow it. To make them wrap once they are too wide, add the flex-wrap property with a value of wrap, or use the shorthand flex-flow with values of row wrap or column wrap. Items will then wrap onto new lines when they overflow their container.

In this example, there are ten flex items with a flex-basis of 160px that can grow and shrink. Once there is not enough space to place another 160 pixel item in a row, a new flex line is created. New lines are created as needed until all of the items are placed. As the items can grow, they will expand to fill each row completely. If there is only one item on the final line it will stretch to fill the entire line.

The same thing happens with flex columns. To wrap and create new columns, the container needs to have a height. In the case of columns, items stretch vertically to fill each column completely.

Wrapping works as you might expect when combined with flex-direction. If flex-direction is set to row-reverse then the items will start from the end edge of the container and lay themselves out in reverse ordered lines.

Note that the reversing is only happening in the inline, row direction. We start on the right then go onto the second line and again start from the right. We aren't reversing in both directions, starting from the bottom coming up the container!

As we have seen from the above examples if our items are allowed to grow and shrink, when there are fewer items in the last row or column then those items grow to fill the available space.

There are no flexbox features to tell items in one row to line up with items in the row above — each flex line acts like a new flex container. It deals with space distribution across the main axis. If there is only one item, and that item is allowed to grow, it will fill the axis just as if you had a single item flex container. If you want layout in two dimensions then you probably want grid layout.

This example demonstrates the difference, using CSS grid layout to create a layout with as many columns of at least 160px as will fit, distributing the extra space between all columns. We use the same HTML as the flexbox wrapped row example above but set display: grid on it. Instead of the flex shorthand, which has no effect outside of flexbox, we set the item's minimum width and ability to grow directly on the container with grid-template-columns. With CSS grid, the last item stays in its grid cell; grid items don't stretch when there are fewer of them in the last row.

This is the difference between one and two-dimensional layouts. In a one-dimensional layout method like flexbox, we only control the row or column. In a two-dimensional grid layout, we control both at the same time. If you want the space distribution row by row, use Flexbox. If you don't, use CSS grid.

Flexbox-based layouts can be forced to line up as grid systems, but that is not the intended purpose of flexbox. If you assign percentage widths to flex items — either using flex-basis or by adding a width to the item itself and leaving the value of flex-basis as auto — you can give the impression of a two-dimensional layout.

In this example, flex-grow and flex-shrink have been set to 0 to make inflexible flex items. The flexibility is controlled via percentages.

This technique allows you to line up flex items on the cross-axis. However, when you catch yourself adding widths to flex items in this way or adding empty flex items to take up space, it is a good indication you may want to switch to CSS grid layout for that component.

To create gaps or gutters between flex items, use the gap property directly on the flex container to create a fixed space between adjacent flex items. The gap property is a shorthand for row-gap and column-gap. These properties specify the size of the gutters between rows and columns within grid, flex, and multi-column layouts.

The gap property is not the only thing that can add space between items. Margins, padding, justify-content, and align-content can also increase the size of the gutter, affecting the actual size of the gap.

To see how the gap property differs from margin in both axes, try changing the gap value in the container .box and adding a margin value to the .box > * rule in the stylesheet below. Click the "Reset" button to revert to the previous values.

The flexbox specification details what should happen if a flex item is collapsed by setting visibility: collapse on an item. See the MDN documentation for the visibility property. The specification describes the behavior as follows:

"Specifying visibility:collapse on a flex item causes it to become a collapsed flex item, producing an effect similar to visibility:collapse on a table-row or table-column: the collapsed flex item is removed from rendering entirely, but leaves behind a "strut" that keeps the flex line's cross-size stable. Thus, if a flex container has only one flex line, dynamically collapsing or uncollapsing items may change the flex container's main size, but is guaranteed to have no effect on its cross size and won't cause the rest of the page's layout to "wobble". Flex line wrapping is re-done after collapsing, however, so the cross-size of a flex container with multiple lines might or might not change." - Collapsed items

This behavior is useful if you want to target flex items using JavaScript to show and hide content for example. The example in the specification demonstrates one such pattern.

In the following live example, I have a non-wrapped flex container. The third item has more content than the others yet is set to visibility: collapse; therefore, the flex container is retaining a strut of the height required to display this item. If you remove visibility: collapse from the CSS or change the value to visible, you will see the item appear, and the space is redistributed between non-collapsed items; the height of the flex container should not change.

When dealing with multiple-line flex containers however you need to understand that the wrapping is re-done after collapsing. So the browser needs to re-do the wrapping behavior to account for the new space that the collapsed item has left in the inline direction.

This means that items might end up on a different line to the one they started on. In the case of an item being shown and hidden it could well cause the items to end up in a different row.

I have created this behavior in the next live example. You can see how the stretching changes row based on the location of the collapsed item. If you add more content to the second item, it changes row once it gets long enough. That top row then only becomes as tall as a single line of text.

If this causes a problem for your layout it may require a rethinking of the structure, for example putting each row into a separate flex container in order that they can't shift rows.

In previous live example, try using visibility: hidden or display: none instead of visiblity: collapse. Using visibility: hidden, the item is made invisible but the box is kept in the formatting structure, so it still behaves as if it were part of the layout. When you use display: none, the item is completely removed from the formatting structure. Not only is it invisible but the structure is removed as well. This means counters ignore it and things like transitions do not run.