What Css Units To Use

CSS Programming

When it comes to styling a website, CSS units play a crucial role in determining the size, spacing, and positioning of elements on the page. As a web developer, I’ve spent countless hours experimenting with different units and figuring out which ones work best for specific scenarios. In this article, I’ll share my personal insights and recommendations on what CSS units to use and why.

The Almighty Pixel

Let’s start with the most commonly used unit: the pixel (px). A pixel is a relative unit that represents a single dot on a screen. It provides absolute control over element dimensions and positioning, making it a reliable choice for creating precise layouts. However, using pixels can be limiting in terms of responsiveness, especially when designing for different screen sizes. Remember, what looks great on a large desktop might not translate well to a mobile device.

Em and Rem for Scalability

For a more scalable and flexible approach, I highly recommend using relative units like em and rem. Both em and rem are based on the font size of the parent element, making them ideal for creating responsive designs. The key difference between the two is that em is based on the font size of the immediate parent element, while rem is based on the root (html) font size.

Em units are particularly useful when you want an element’s size to be relative to its parent’s font size. For example, if the parent’s font size is 16 pixels, setting an element’s size to 2em would result in a size of 32 pixels. On the other hand, rem units allow you to establish a standard reference point throughout your entire website. By setting the root font size to a percentage of the viewport width, you can easily scale your design proportionally across different devices.

Flexible Layouts with Percentages

Another unit that works well for creating flexible layouts is the percentage (%). This unit sets an element’s size relative to its parent’s size. For example, if you set an element’s width to 50%, it will occupy half of its parent’s width. Percentages are particularly useful when you want elements to automatically adjust their size based on available space.

Viewport Units for Device Independence

If you’re aiming for full device independence and responsiveness, viewport units can be a valuable addition to your CSS toolbox. Viewport units are relative to the size of the browser window or viewport, making them ideal for creating designs that adapt seamlessly to different screen sizes. The most commonly used viewport units are vw (viewport width) and vh (viewport height).

For example, setting an element’s width to 50vw would make it occupy 50% of the viewport width, regardless of the device’s actual screen width. Viewport units are particularly useful when dealing with full-screen elements, overlays, or elements that need to span across the entire screen.


Choosing the right CSS units can greatly impact the flexibility, responsiveness, and scalability of your website. While pixels provide precise control, relative units like em, rem, percentages, and viewport units offer more adaptability and responsiveness to different screen sizes. As a web developer, I often find myself combining different units to achieve the desired effect. Experimentation is key, so don’t be afraid to try out different units and see what works best for your specific design needs.