When styling elements in CSS, you have two categories of units to choose from: absolute units and relative units. Here's a breakdown of each of them and how they differ:

1. Absolute Units

Absolute units are fixed units of measurement. They are not affected by other elements or screen sizes, meaning their size is constant regardless of the context in which they are used.

Common Absolute Units:

• px (pixels): A pixel is a small square on the screen. It's the most commonly used unit for fixed layouts.

  • Example:

  h1 {
    font-size: 24px;
  }
  

• pt (points): Typically used in print media, 1pt equals 1/72 of an inch.

• cm (centimeters) and in (inches): Rarely used in web design, these units are based on physical dimensions.

Advantages of Absolute Units:

• Consistency: The size will always be the same, no matter the device or screen size.
• Predictable: Ideal for creating fixed layouts where you want the design to look exactly the same across all platforms.

Disadvantages:

• Lack of flexibility: Absolute units don’t adapt well to different screen sizes or zoom settings, making them less responsive.

2. Relative Units

Relative units are flexible and scale based on the context they are used in. Their size depends on other elements, such as the parent container, viewport, or base font size.

Common Relative Units:

• em: Relative to the font size of the element it’s used on. If the parent element has a font-size of 16px, 1em equals 16px. If the parent element’s size changes, so will the size in em.

  • Example:

  p {
    font-size: 1.5em; /* 1.5 times the font size of the parent */
  }
  

• rem (root em): Relative to the font size of the root element (usually the element). This makes it more predictable compared to em.

By default, browsers typically set the root font size to 16px, unless specified otherwise. If you define a custom font size for the element, all rem values will be calculated based on that new size.

• Example:
  

  p {
    font-size: 2rem; /* 2 times the root font size */
  }
  

  • % (percent): Relative to the parent element's size. For example, width: 50% makes the element 50% the width of its parent container.

• Example:
  

  div {
    width: 75%; /* 75% of the parent's width */
  }
  

  • vw (viewport width) and vh (viewport height): These units are relative to the browser’s viewport. 1vw is 1% of the viewport's width, and 1vh is 1% of the viewport's height.

• Example:
  

  div {
    width: 50vw; /* 50% of the viewport's width */
  }
  

Advantages of Relative Units:

• Responsive design: Relative units scale automatically based on screen size, font size, or container dimensions, making your design more flexible.
• Easier to maintain: Changing the font size of the root element (using rem) can scale your entire design.

Disadvantages:

• Can be harder to control: Without a good understanding of inheritance and how sizes cascade through elements, relative units can sometimes produce unexpected results.

When to Use Absolute vs. Relative Units

• Absolute units (like px) are best when you need precise, fixed measurements. Use these when you want something to be the same size everywhere (e.g., a small logo or icon).
• Relative units (like em, rem, vw, and %) are ideal for responsive design. They allow elements to scale and adapt based on the screen size or parent element, making your design more flexible.

Example: Absolute vs. Relative Units in Practice

This is 24px text
This is 1.5rem text (24px based on root size)

In this example:

• The absolute font size (24px) will always remain the same.
• The relative font size (1.5rem) will adjust based on the root font size (16px), resulting in 24px.

Conclusion:

• Use absolute units when consistency across devices is needed, but be aware they are not responsive.
• Use relative units for more flexible and responsive designs, especially when developing for multiple screen sizes and devices.