CSS Grid Layout: Percentages vs. `fr` Units – What's the Difference?

The Difference Between Percentage and fr Units in CSS Grid Layout

When configuring a grid layout with CSS, the choice of units for specifying column and row sizes can impact the layout's behavior. Two commonly used units are percentages and fractional units (fr). Let's examine the key differences between these units in the context of CSS Grid Layout.

fr Units

The fr unit represents fractional units of the free space available within the grid container after accounting for grid gaps. Essentially, it allows columns or rows to take up a proportional amount of the remaining space. The free space is distributed equally among the columns or rows with fr values.

This ensures that the elements' sizes scale responsively as the grid container resizes. Grid gaps, defined by grid-column-gap and grid-row-gap, are subtracted from the available space before calculating the fr values.

Percentage Units

Percentage units, on the other hand, specify the width of columns or rows as a percentage of the entire grid container, including grid gaps. This means that the elements' sizes are fixed, and they do not adjust dynamically based on the container's size.

As a result, it's possible to run into situations where the total width of the columns, including grid gaps, exceeds the grid container's width. This can cause the layout to overflow the container.

Example

To illustrate the difference in behavior, consider the following grid layout:

.grid {
  display: grid;
  grid-template-columns: repeat(12, calc(100% / 12));
  grid-column-gap: 10px;
  grid-row-gap: 10px;
}

In this case, percentage units are used to define the column widths. As a result, the total width of the columns, including the grid gaps (11 gaps with a gap of 10px), exceeds the grid container's width, causing the layout to overflow.

However, if we switch to using fr units instead:

.grid {
  display: grid;
  grid-template-columns: repeat(12, 1fr);
  grid-column-gap: 10px;
  grid-row-gap: 10px;
}

The columns now take up proportional amounts of the available free space, and the grid gaps are subtracted from the total space before calculating the fr values. This results in a responsive layout that adjusts dynamically based on the grid container's size.

Conclusion

Choosing the appropriate unit for your grid layout depends on the desired behavior. Percentage units are suitable when you want fixed sizes, while fr units are beneficial when you need responsive layouts that scale according to the available space. Understanding the difference between these units and their impact on the layout is crucial for creating efficient and visually pleasing CSS Grid layouts.

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