What’s the difference between em, rem, px, and % units in CSS?
Asked on Aug 29, 2025
Answer
Understanding the differences between em, rem, px, and % units is crucial for responsive and scalable web design. Each unit serves a specific purpose and behaves differently in CSS.
Example Concept:
- px (pixels): An absolute unit that represents a fixed size on the screen. It does not scale with user settings or parent elements.
- em: A relative unit that is based on the font-size of the element's parent. It scales according to the inherited font size, which can lead to compounding effects.
- rem (root em): Similar to em, but it is relative to the root element's font-size (usually the element). This avoids compounding and provides more predictable scaling.
- % (percent): A relative unit that is based on the parent element's dimensions. It is commonly used for widths and heights to create flexible layouts.
Additional Comment:
- Use px for precise control when exact dimensions are necessary.
- Use em for scalable elements that need to adapt to their parent's font size.
- Use rem for consistent scaling across the entire page, avoiding the compounding effect of em.
- Use % for responsive layouts that adjust to parent container sizes.
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