How do font-size, line-height, and actual height interact to determine element height in CSS?

Font-Size, Line-Height, and Actual Height Demystified

In CSS, understanding how font-size and line-height relate to element height can be a challenge. This article aims to demystify these concepts and provide straightforward rules to guide your styling efforts.

Font-Size: Containing Characters

Font-size, as its name suggests, sets the size of the font, determining the height of the box in which all characters (including ascenders and descenders) must fit. However, it does not directly affect the actual height of an element.

Line-Height: Shaping the Line

Line-height defines the height of the line box, effectively setting the vertical spacing between lines. Its role extends beyond mere line spacing, though. In cases where the inline element's height is not explicitly specified, the line-height becomes the default height of that inline element.

Actual Height: A Balancing Act

The actual height of an element is influenced by several factors:

• Line-height of the line box: The line box's height is typically determined by the line-height of its content. However, if the parent block also has a line-height, that value comes into play. The parent block's line-height sets a minimum height for the line box, ensuring that there is sufficient space for potential ascenders and descenders.
• Line-height of inline elements: If the inline element's height is not defined, the line-height becomes its default height. However, if the inline element's line-height is smaller than that of the parent block, its actual height will be smaller than the line box's height.
• Vertical-align: This property specifies the vertical alignment of the inline element within the line box. For instance, aligning the inline element to the top of the line box would result in an actual height equal to the inline element's line-height.

Example: Breaking Down the Height

Consider a span with a font-size of 40px and a line-height of 40px. Intuitively, one might expect the span to have an actual height of 40px. However, due to character ascenders, the actual height instead measures as 45px. This is because the browser allocates extra space above and below the ascenders and descenders to ensure they fit within the line box, which has a height of 40px thanks to the line-height property.

Additional Considerations:

• If the inline element's line-height is smaller than its font-size, the actual height will be smaller than the line box's height.
• Conversely, if the inline element's line-height exceeds the line box's height, the actual height will be dominated by the inline element's line-height.

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