Understanding and Utilizing CSS Units: px, em, rem, %, vh, and vw

5 min readSep 4, 2023

Cascading Style Sheets (CSS) are a fundamental part of web development, allowing designers and developers to control the layout and styling of web pages. One crucial aspect of CSS is the choice of units for specifying dimensions and sizes. Four commonly used units for this purpose are pixels (px), em, rem, and percentages (%). In this article, we will explore when and where to use each of these units to create responsive and flexible web designs.

1. Pixels (px)

Pixels, denoted as “px,” are the most straightforward unit of measurement in CSS. One pixel represents a single dot on a screen. When you specify an element’s size in pixels, it will always be that exact size regardless of the user’s device or screen size. Pixels are an absolute unit, and their use can lead to fixed layouts that may not adapt well to various screen sizes and resolutions.

Use Cases for Pixels (px):

Border and outline thickness: When you want to ensure a consistent and unchanging border or outline size regardless of screen dimensions.
Text size: Use pixels for text when precise control over font size is required, such as for pixel-perfect designs.

.element {
    width: 200px;
    border: 1px solid #000;
    font-size: 16px;
}

2. Relative Units: em and rem

Relative units, namely em and rem, offer a flexible approach to web design by scaling elements based on the parent element or root element, respectively. These units are ideal for creating responsive designs that adapt to different screen sizes.

2.1. em

The “em” unit is relative to the font size of the parent element. For example, if a parent element has a font size of 16px, 1em is equivalent to 16px. If a child element within that parent is set to 2em, it will be twice the size of the parent’s font size (32px in this case).

Use Cases for em:

Font size: When you want text to scale proportionally with the parent element’s font size.
Padding and margin: Use em for spacing within an element to maintain relative proportions.

.parent {
    font-size: 16px;
}

.child {
    font-size: 2em; /* 32px */
    margin-bottom: 1em; /* 16px */
}

2.2. rem

The “rem” unit, short for “root em,” is relative to the font size of the root element (usually the <html> element). This provides a more predictable and consistent way to control element sizes throughout your layout.

Use Cases for rem:

Layout and spacing: Use rem for consistent spacing and layout control that isn’t influenced by nesting.
Responsive design: Rem units are particularly useful for creating designs that scale uniformly across various screen sizes.

html {
    font-size: 16px; /* Sets a baseline */
}

.element {
    font-size: 1.5rem; /* 24px */
    margin: 1rem; /* 16px */
}

3. Percentage (%)

Percentage units are relative to the parent element’s size. For example, if you set an element’s width to 50%, it will be half the width of its parent container. Percentages are versatile units for creating responsive and fluid layouts.

Use Cases for Percentage (%):

Fluid layouts: To create flexible designs that adapt to different screen sizes and orientations.
Sizing images: When you want images to scale proportionally within their containers.

.container {
    width: 80%; /* Adjusts to parent container's width */
}

.image {
    max-width: 100%; /* Ensures images don't exceed their container width */
}

4. Viewport Height (vh) and Viewport Width (vw)

Viewport height (vh) and viewport width (vw) units are relative to the height and width of the viewport (the visible area of the browser window), respectively. These units are particularly valuable for creating responsive designs that adapt to the user’s screen size, making them essential for mobile and responsive web development.

Use Cases for Viewport Height (vh) and Viewport Width (vw):

Full-screen elements: To create elements that occupy a specific percentage of the viewport’s height or width, ensuring they adapt to different screen sizes.
Typography: For text elements that need to scale with the viewport size, such as headlines and hero banners.

.header {
    height: 100vh; /* Fills the entire viewport height */
}

.hero-text {
    font-size: 5vw; /* Scales with the viewport width */
}

Conclusion
The choice of CSS units — pixels (px), em, rem, percentages (%), viewport height (vh), and viewport width (vw) — is a critical consideration in web development, as it directly impacts the responsiveness and adaptability of your designs. Each unit has its own strengths and best-use scenarios:

Pixels (px): Pixels are absolute units suitable for elements where fixed and precise sizing is required. However, they may not be the best choice for responsive design since they don’t adapt to varying screen sizes.
Relative Units (em and rem): Em and rem units offer a flexible and scalable approach to design. Em units are relative to the parent element’s font size, making them ideal for text and proportional spacing within elements. Rem units, on the other hand, are relative to the root element’s font size and provide more predictable and consistent control over layout and sizing.
Percentage (%): Percentage units are relative to the parent element’s size, making them perfect for creating fluid and responsive layouts. They are especially useful when designing responsive web pages that need to adapt gracefully to various screen sizes and orientations.
Viewport Height (vh): The “vh” unit is relative to the viewport’s height. It’s particularly handy for elements that need to scale based on the height of the user’s screen. This unit is often used for creating full-height sections or elements that adapt to the screen size.
Viewport Width (vw): The “vw” unit is similar to “vh” but relative to the viewport’s width. It’s useful for creating elements that scale based on the width of the viewport, making it a valuable tool for responsive design, especially in conjunction with media queries.

The key to effective web design is knowing when and where to use these units appropriately. While there are no strict rules, the following guidelines can help:

Use pixels (px) sparingly for specific elements like borders, outlines, and fixed text sizes in situations where precision is essential.
Employ em units for text, padding, and margin sizes when you want them to scale proportionally with their parent elements.
Leverage rem units for consistent spacing and layout control that isn’t influenced by nesting, facilitating the creation of responsive designs.
Utilize percentage units (%) for building fluid and adaptive layouts, particularly for containers, images, and elements that should adapt to different screen sizes.
Consider using “vh” and “vw” units for creating elements that adapt to the viewport’s height and width, respectively, making them valuable for responsive and full-screen designs.

Ultimately, the choice of CSS units should align with your design goals and the type of layout you want to achieve. By understanding the strengths and use cases of each unit, you can create web designs that provide a seamless and user-friendly experience across a wide range of devices and screen resolutions.

Hope it was helpful :)