In CSS, choosing the appropriate unit for achievingresponsive design,andand usabilityis crucial.

px、emandremare the three most commonly used length units. Their main difference lies inthe reference benchmarkbeing different.

1. px (Pixel - Pixel)

pxis anabsolute unit(on screen displays). It represents a physical pixel point on the screen.

  • Characteristics: Fixed size, does not change with browser settings or parent element font size.
  • Advantages: Precise, easy to control, suitable for scenarios requiring "pixel-level" alignment (such as border lines, small icons).
  • Disadvantages: Lacks flexibility, not conducive to responsive layout. If the user enlarges the font in the browser, layouts usingpxmay not adjust accordingly.

2. em (Relative to Element - Relative Unit)

emis arelative unit, and its reference is thecurrent element'sorparent element'sfont size (font-size)。

  • Calculation rules：
  • If used onfont-size, the reference is theparent element'sfont size.
  • If used on other properties (such aswidth、padding、margin), the reference is thecurrent element'sown font size.
  • Disadvantage: compound stacking effect. If multiple layers are nested and all useem, the calculation becomes very complex.

Example:
If the parent element is20px, and the child element is set tofont-size: 1.5em;, then the actual size of the child element is .

3. rem (Root em - Root Relative Unit)

remis also arelative unit, but it only references theroot element(i.e., the<html>tag)'s font size.

  • Calculation rule: No matter how deeply elements are nested,1remalways equals<html>offont-size。
  • Advantages：
  • High predictability: Avoidsemthe stacking and nesting problem of em.
  • Responsive-friendly: Just modifyhtmlthe root element's font size, and the entire page'sremlayout will scale proportionally.
  • Default value: Most browsers' default root font size is16px, so by default1rem = 16px。

Core Difference Comparison Table

Unit Type Reference Applicable Scenarios
px Absolute unit Physical screen pixel Borders, shadows, elements that do not need scaling
em Relative unit Current/parent element font size Local component scaling (e.g., button margins change with text size)
rem Relative unit <html>Root element font size Overall page layout, spacing, responsive typography

Summary and Recommendations

  1. First choice:remFor typography and layout: This ensures that your webpage can scale gracefully when users adjust the browser font size.
  2. Second choice:pxFor fixed details: For example,1pxborder lines, which you don't want to become blurry or disappear due to scaling.
  3. Use sparingly:emOnly for local use: Only consider using em when an element's Padding or Margin must scale synchronously with the element's text size (e.g., adaptive buttons).em。

A small tip:
For ease of calculation, many developers sethtmlthe root element's font size to62.5%(i.e., ). This way, when writing code,1.4remdirectly corresponds to14px, simple and intuitive.