The Ultimate Guide to PX, EM, REM, VW, and VH

This ultimate guide to PX, EM, REM, VW, and VH is an all-encompassing resource for understanding the ins and outs of these essential units of measurement in CSS. This guide aims to provide comprehensive and accurate information to web developers, budding designers, or anyone interested in digital design.


  • If you want to maintain a fixed size for an element, pixels (PX) are your ideal choice due to their absolute measurements.
  • If your objective is to ensure an element scales in accordance with the font size, Ems (EM) is the best option as they scale based on the font size of their parent element.
  • If you're seeking a unit that can dynamically adjust with the font size, then rems (REM) is your go-to. They provide both flexibility and consistency.
  • If it's crucial for an element to adjust its size relative to the viewport width, opt for viewport width (VW). This unit is flexible and adapts to different screen sizes.
  • Lastly, if you want an element to vary its size in relation to the viewport height, choose viewport height (VH). This unit provides a responsive solution for height-related adjustments.

Creating a fascinating and engaging website is like putting together a complex puzzle, where CSS units play a crucial role. Now, you might be curious, what exactly are these units?

Think of CSS units as the ruler or tape measure in the toolbox of web developers. They come in handy when it comes to deciding the sizing for different elements on your webpage. From setting the font size that's just easy on the eyes, to dictating the layout dimensions for that seamless user experience, or defining the margins and paddings to give your content some breathing space - CSS units are your go-to tool.

What is PX?

PX, or pixel, is a static measurement unit in CSS. A pixel is the smallest visible unit of a digital image or display. One thing to remember: a pixel doesn't change size based on the parent element or the screen size. It's like the reliable old friend who never changes, no matter what.

What is EM?

EM is a scalable unit in CSS. The size of 1EM is relative to the parent element's font size. So, unlike our reliable friend pixel, EM is more like a chameleon, always adapting to its surroundings.

What is REM?

Like EM, REM is also a scalable unit. But there's a key difference: while EM refers to the size of its parent element, REM, or Root EM, relates to the root element's size. Think of it like a tree that always traces back to its roots, regardless of how expansive its branches may be.

What is VW?

VW stands for Viewport Width. It is a dynamic and responsive unit in CSS. 1VW is equivalent to 1% of the viewport's width. Think of it as a river adjusting its course based on the terrain.

What is VH?

VH is another dynamic and responsive CSS unit, representing Viewport Height. 1VH equals 1% of the viewport's height. Much like VW, it adjusts based on the screen size, behaving like a chameleon.

Comparing PX, EM, REM, VW, and VH

Let's draw some comparisons now.

PX vs EM

Pixels are fixed, while EMs adjust according to their parent element. It's like comparing an unchanging mountain (PX) to a river that adjusts its course (EM).


Much like the PX vs EM comparison, REM, like the river, adapts to the root element size, while PX remains constant.

VW and VH Compared

Both VW and VH are like siblings, adjusting their dimensions according to the viewport's width and height, respectively.

Here is a summary table:

Property Description Pros Cons
PX Think of Pixels (PX) as your steadfast companion's in-screen measurement. They don't waver, no matter the size of your viewport. They're reliable and always the same size. The downside? They lack flexibility.
EM EM is a relative unit. Set an element's font size to 16px, its width to 1em, and voila, you've got a square! They're the chameleons, changing with the font size. Yet, their changeability can sometimes be a double-edged sword.
REM REM is similar to EM, but follows the rules of the page's main or "root" font size. If you change that root font size, REM changes accordingly. It's that simple! They're the good listeners, adjusting with both font size and viewport width. However, their sensitivity to changes can sometimes be a bit of a stretch.
VW VW or viewport width, scales with your viewport. Set an element's width to 1vw, and it becomes 1% of your viewport width. They're the transformers, scaling with your viewport width. Though versatile, they're not quite as exact as Pixels.
VH VH, short for viewport height, relates to your viewport's height. Assign an element's height to 1vh, and it's 1% of the viewport height. Like VW, it scales with your viewport height. While flexible, they can't quite nail the precision of Pixels.


When to Use Which Unit

Choosing the right CSS unit is like picking the right tool for a job. For static and precise control, use PX. When you need scalability according to the parent element, use EM. REM is your go-to for scalability according to the root element. For responsive design based on viewport dimensions, use VW and VH.

Understanding PX, EM, REM, VW, and VH is a crucial aspect of mastering CSS and web design. With this comprehensive guide, you're now equipped to leverage these units in your designs, creating more responsive, flexible, and user-friendly websites.

FAQs About PX, EM, REM, VW, and VH

What is the difference between PX, EM, REM, VW, and VH?

The primary difference lies in how these units measure size. Pixels (PX) are a fixed unit, whereas EMs and REMs are relative to the parent or root element's font size, respectively. Viewport units (VW and VH) are based on the size of the viewport, offering adaptability to different screen sizes.

When should I use PX, EM, REM, VW, or VH?

Each unit has its specific uses. Pixels are generally used when fixed sizes are needed, EMs and REMs for scalability and responsiveness in typography, and VW and VH for layout elements that need to adjust to the viewport's size.

How do I convert from PX to EM, REM, VW, or VH?

Converting between these units requires understanding the relationships between them. Typically, conversions between PX, EM, and REM involve the base font size, while conversions to VW and VH involve the viewport's dimensions.

What is the advantage of using EM or REM over PX?

The primary advantage of using EM or REM over PX is flexibility. These units allow for relative sizing, which can create more responsive and accessible designs.

Why should I use VW or VH in my designs?

Using VW and VH can make your designs more responsive. These units allow elements to adapt to the viewport's size, which can be especially useful in modern web design where a multitude of screen sizes must be accommodated.

Are there any limitations or challenges in using EM, REM, VW, or VH?

While these units offer great flexibility, they also come with challenges. For example, managing complex nested elements with EM can be tricky, REM might not be supported in older browsers, and VW and VH can sometimes behave unpredictably on mobile devices.