In CSS Grid, fr assigns a flexible track a share of the space left in the grid container after fixed tracks and gaps are accounted for. Use equal values such as 1fr 1fr for equal flexible columns, or 1fr 2fr for a 1:2 flex-factor ratio. If a track refuses to shrink, its automatic minimum size may be the reason; minmax(0, 1fr) lets the track shrink further.
What does fr mean in CSS?
fr is the flexible length unit for CSS Grid tracks. The W3C CSS Grid Layout Module Level 2 specification defines it this way: “A flexible length or <flex> is a dimension with the fr unit, which represents a fraction of the leftover space in the grid container.” Read the CSS Grid specification.
The key phrase is “leftover space.” Grid first accounts for other track sizes and gaps; flexible tracks then divide the space available to them in proportion to their flex factors. So 1fr does not always mean one fraction of the container’s entire width.
How do flex factors divide space?
The number before fr is a flex factor. Equal factors divide flexible space equally; unequal factors divide it proportionally. The ratio describes the flexible tracks’ share, not a guarantee of exact final widths: content-based minimums and other track-sizing rules can affect the result.
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1fr 1fr: equal flex factors.1fr 2fr: the second track has twice the flex factor of the first.2.5fr: fractional flex-factor values are valid; the unit attaches directly to the number.
MDN documents the <flex> value and its use in Grid track sizing: MDN: <flex>.
Common ways to use fr
Three equal columns
.grid {
display: grid;
grid-template-columns: repeat(3, 1fr);
gap: 1rem;
}repeat(3, 1fr) creates three equal flexible columns. The 1rem gaps use space too, so the tracks share what remains after those gaps are accounted for. See MDN’s Grid layout basics.
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Fixed sidebar with a flexible main column
.layout {
display: grid;
grid-template-columns: 16rem 1fr;
gap: 1.5rem;
}The first column is fixed at 16rem; the second flexibly uses the space available after the fixed column and gap.
Responsive columns with a minimum width
.cards {
display: grid;
grid-template-columns: repeat(auto-fill, minmax(14rem, 1fr));
gap: 1rem;
}auto-fill makes the repeated track count depend on available space. Each track has a 14rem minimum and a flexible maximum. Choose the minimum based on how narrow an item can become while remaining usable; 14rem is an example, not a universal setting.
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For a fixed number of equal columns, use a fixed repeat count such as repeat(3, 1fr). When the number should adapt to available space, use an automatic repeat with minmax() and set a meaningful item minimum. See MDN: minmax().
Why can a 1fr track overflow or refuse to shrink?
A bare flexible track has an automatic minimum size. In effect, it behaves like minmax(auto, 1fr); the automatic minimum can reflect the minimum size of the track’s contents. An unbreakable word, wide image, or other intrinsically wide item can therefore keep the track from shrinking as far as expected. MDN’s grid-template-columns reference source describes this behavior.
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If the track should be allowed to shrink below that content-based minimum, set its minimum to zero:
.main {
display: grid;
grid-template-columns: minmax(0, 1fr);
}This removes the track’s automatic minimum; it does not automatically make overflowing content fit. The grid item may also need wrapping, a minimum-width adjustment, or deliberate overflow handling, depending on the markup and content.
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How minmax() affects a track
minmax(min, max) defines the permitted size range for a track. Its maximum can be an fr value, but its minimum cannot. For example, minmax(200px, 1fr) allows a flexible track with a 200px floor; minmax(0, 1fr) permits the track minimum to reach zero. If the maximum is smaller than the minimum, the maximum is ignored and the minimum is used. Details are in MDN’s minmax() reference.
A practical debugging sequence
- Identify the flexible tracks. Check the computed or authored
grid-template-columnsorgrid-template-rowsvalues and note which tracks usefr. - Account for space already used. Include fixed-size tracks and the grid’s gaps before reasoning about the space left for flexible tracks.
- Check the automatic minimum. Look for wide or unbreakable content that could be setting a track’s minimum size.
- Choose the intended floor. Use a meaningful minimum with
minmax(200px, 1fr), for example, if the track should not get narrower than that; useminmax(0, 1fr)when the track itself must be allowed to shrink to zero. - Inspect the grid item too. Depending on the content, wrapping,
min-width: 0, or explicit overflow behavior on the item may be needed. Check the actual markup rather than assuming one declaration resolves every overflow issue.
Compatibility
MDN marks both the <flex> value and minmax() as Baseline Widely available, with availability dates of March 2017 and October 2017 respectively. The cited references do not establish specific browser-version numbers. If your project must support a particular legacy browser, consult the live compatibility information on the relevant MDN <flex> and MDN minmax() pages.
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