15em

15em is a CSS length unit in the em family. It represents 15 times the font size of the element to which the property is applied. Because em is relative, 15em is not a fixed pixel value; it depends on the element’s own font size, which may be inherited or changed by parent elements. As a result, em-based sizes can cascade and compound in nested elements, leading to scalable yet potentially unpredictable results.

Computing 15em depends on the element’s font-size. If the root font size is 16px and an element has a font-size of 16px (1em), then 15em equals 240px. If the element’s font-size is 20px (1em = 20px), 15em equals 300px. If a child element has a font-size defined as 2em relative to a parent, the child’s final font size becomes 32px; a property set to 15em on that child would compute to 480px.

Because 15em scales with the element’s font size, it can be very large in nested contexts or small on tightly constrained layouts. This behavior makes em useful for typography-anchored or proportionally scaled layouts, but it can complicate responsiveness and precision. For more predictable sizing that is independent of nesting, rem units (root ems) are commonly recommended.

Common use cases for 15em include padding, margins, widths, or gaps where the designer wants spacing to scale with text size. The practical value of 15em thus depends on the current font sizing and the layout’s nesting structure.