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What is the difference between auto and 100% widths in CSS?

Asked on Sep 08, 2025

Answer

In CSS, the difference between "auto" and "100%" for widths lies in how they calculate the size of an element based on its parent container and content. "Auto" allows the browser to determine the width based on content and other properties, while "100%" makes the element fill the entire width of its parent.

Example Concept: "Auto" width allows the element to adjust based on its content and other styling, such as padding or margins, while "100%" width forces the element to stretch to the full width of its parent container. "Auto" can result in a width less than the parent if the content is smaller, whereas "100%" will always match the parent's width, minus any padding or borders.

Additional Comment:

Using "auto" is useful when you want the element to adapt naturally to its content size.
"100%" is ideal when you need the element to span the full width of its parent, often used in fluid layouts.
Consider box-sizing properties, as they can affect how widths are calculated with padding and borders.
Be mindful of parent container constraints, as they directly impact how "auto" and "100%" behave.

✅ Answered with CSS best practices.

Ask any question about CSS here... and get an instant response.

What is the difference between auto and 100% widths in CSS?

Asked on Sep 08, 2025

Answer

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