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CSS: px vs em

CSS, Web Development2 min read

When it comes to sizing elements in CSS, choosing the appropriate unit is crucial for achieving responsive web design. Two commonly used units are pixels (px) and ems (em). In this article, we will explore the differences between them and discuss their best use cases.

Pixels (px)

Pixels (px) are an absolute unit of measurement in CSS. They provide a fixed size regardless of the device or browser settings. When you specify a size in pixels, it will always render at that exact size, which can be advantageous in certain scenarios.

Let's consider an example where we want to set the font size of a paragraph to 16 pixels:

1p {
2 font-size: 16px;

In this case, the text inside the <p> element will always be rendered at a size of 16 pixels, regardless of the user's device or any accessibility settings they may have enabled. This makes pixels useful when precise control over element sizes is required.

However, the downside of using pixels is that they do not scale with the user's preferences or screen size. If a user has increased their default font size, the text specified in pixels might become too small for comfortable reading. Additionally, on high-density screens, such as those found on many mobile devices, content specified in pixels can appear smaller and less crisp.

Ems (em)

Ems (em) are a relative unit of measurement in CSS. They are based on the font size of the parent element or, in some cases, the element itself. This makes ems a versatile option for responsive design since they can adapt to different contexts.

Let's modify the previous example to use ems instead of pixels:

1p {
2 font-size: 1em;

Now, the font size of the paragraph will be equal to the font size of its parent element. Suppose the parent element has a font size of 16 pixels; in that case, the paragraph text size will also be 16 pixels.

One advantage of using ems is that they scale proportionally when the font size changes. If the user increases their default font size, elements specified in ems will adjust accordingly, providing a better reading experience. Ems are particularly useful when designing accessible websites that cater to users with varying visual needs.

Choosing between px and em

The choice between pixels (px) and ems (em) depends on the specific requirements of your web design project. Here are some guidelines to help you make an informed decision:

  1. Use pixels (px) when precise control over element sizes is necessary, such as for fixed-width layouts or when aiming for pixel-perfect designs.
  2. Use ems (em) for flexible and responsive designs, especially when dealing with typography. Ems allow elements to scale with the user's preferences, providing a better user experience across different devices and accessibility settings.
  3. Consider combining both units strategically. For example, you can use pixels for container widths and ems for font sizes inside those containers to achieve a balance between control and adaptability.

Remember that modern web development often involves building responsive websites that work well on various devices and screen sizes. Using a combination of pixels and ems can help create adaptable and visually appealing designs.

In summary, understanding the differences between pixels (px) and ems (em) is crucial for effective web design. While pixels provide precise control over element sizes, ems offer flexibility and responsiveness. By using the appropriate unit based on your design goals, you can create visually pleasing websites that cater to a wide range of users.