CSS layout: when to use flexbox and when to use grid

What it is

MDN describes flexbox as a one-dimensional layout model: a flex container lays its items out along a main axis (row or column), distributes free space with flex-grow, flex-shrink and flex-basis, and can wrap into several lines with flex-wrap, each line laid out on its own. Grid is two-dimensional: grid-template-columns and grid-template-rows define tracks (with the fr unit, repeat() and minmax()), items are placed into cells or named areas, and content that does not fit the explicit grid creates implicit tracks. Both share gap and the box-alignment properties such as align-items and justify-content.

Why it matters

Most layout defects, such as overflowing text, columns that never line up, or content that jumps at some viewport width, come from choosing the wrong model. Wrapped flex lines do not align items across lines, so card grids built with flexbox drift. Grid with fixed pixel tracks does not adapt to content or narrow screens.

How to apply

• Content decides the size (navigation bars, button rows, tag lists, vertical centring of one item): flexbox.
• The structure decides the size (page shell with header, sidebar and main; forms with aligned labels; card grids where rows must align): grid.
• Responsive card grid without media queries: grid-template-columns: repeat(auto-fill, minmax(16rem, 1fr)).
• Let flex items shrink: an item shrinks only down to its min-content size, so long unbreakable strings force overflow; set min-width: 0 (or overflow-wrap: anywhere) on such items.
• Use gap for spacing between items rather than margins on children, which leak to the edges.
• Nest freely: grid for the page, flexbox inside components; check the result at a narrow phone width and at a wide width.

Pitfalls

justify-content acts on the main axis only; centring in both directions needs align-items too, or place-items: center on a grid. A percentage height on a flex child requires a definite height on the parent. order and row-reverse change the visual order but not the DOM order that keyboard focus and screen readers follow, which can make a page confusing to navigate.