If a grid item is positioned into a column that is not explicitly sized by grid-template-columns, implicit grid tracks are created to hold it. This can happen either by explicitly positioning into a column that is out of range, or by the auto-placement algorithm creating additional columns.

<length>

Is a non-negative length.

<percentage>

Is a non-negative <percentage> value relative to the block size of the grid container. If the block size of the grid container is indefinite, the percentage value is treated like auto.

<flex>

Is a non-negative dimension with the unit fr specifying the track's flex factor. Each <flex>-sized track takes a share of the remaining space in proportion to its flex factor.

When appearing outside a minmax() notation, it implies an automatic minimum (i.e. minmax(auto, <flex>)).

max-content

Is a keyword representing the largest maximal content contribution of the grid items occupying the grid track.

min-content

Is a keyword representing the largest minimal content contribution of the grid items occupying the grid track.

minmax(min, max)

Is a functional notation that defines a size range greater than or equal to min and less than or equal to max. If max is smaller than min, then max is ignored and the function is treated as min. As a maximum, a <flex> value sets the track's flex factor. As a minimum, it is treated as zero (or minimal content, if the grid container is sized under a minimal content constraint).

fit-content( [ <length> | <percentage> ] )

Represents the formula min(max-content, max(auto, argument)), which is calculated similar to auto (i.e. minmax(auto, max-content)), except that the track size is clamped at argument if it is greater than the auto minimum.

auto

As a maximum represents the largest max-content size of the items in that track.

As a minimum represents the largest minimum size of items in that track (specified by the min-width/min-height of the items). This is often, though not always, the min-content size.

If used outside of minmax() notation, auto represents the range between the minimum and maximum described above. This behaves similarly to minmax(min-content,max-content) in most cases.

Note: auto track sizes (and only auto track sizes) can be stretched by the align-content and justify-content properties. Therefore by default, an auto sized track will take up any remaining space in the grid container.

grid-auto-columns = 
  <track-size>+  

<track-size> = 
  <track-breadth>                                   |
  minmax( <inflexible-breadth> , <track-breadth> )  |
  fit-content( <length-percentage> )                

<track-breadth> = 
  <length-percentage>  |
  <flex>               |
  min-content          |
  max-content          |
  auto                 

<inflexible-breadth> = 
  <length-percentage>  |
  min-content          |
  max-content          |
  auto                 

<length-percentage> = 
  <length>      |
  <percentage>  

HTML

<div id="grid">
  <div id="item1"></div>
  <div id="item2"></div>
  <div id="item3"></div>
</div>

CSS

#grid {
  height: 100px;
  display: grid;
  grid-template-areas: "a a";
  gap: 10px;
  grid-auto-columns: 200px;
}

#grid > div {
  background-color: lime;
}

Result

• Related CSS properties: grid-auto-rows, grid-auto-flow, grid
• Grid Layout Guide: Auto-placement in grid layout - sizing rows in the implicit grid
• Video tutorial: Introducing Grid auto-placement and order