When designing responsive websites, one of the most important decisions you’ll make is whether to use REM or PX units for defining sizes, from text to spacing and layout. Each unit offers its own advantages, depending on how flexible or fixed you want your design to be across different devices and screen sizes. In this guide, we’ll break down the differences between REM and PX, explore when to use each, and provide a practical analogy to help you better understand the concept.

What is PX?

PX (Pixels) are a fixed measurement unit. When you use PX, the size you specify will remain constant regardless of the screen size or resolution. For example, setting a font size to 16px will ensure that the text always appears as 16 pixels tall on any screen.

Advantages of PX:

• Precision: PX units offer precise control over the size of elements, making them ideal for pixel-perfect designs.
• Consistency: Because PX is a fixed size, it ensures that elements will look the same across devices (though not necessarily the same size due to screen density).

Example of PX in CSS:

css

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h1 {
 font-size: 24px;
 padding: 20px;
}


In this case, the heading will always be 24 pixels tall, and the padding around it will always be 20 pixels, no matter the screen size.

What is REM?

REM (Root EM) is a scalable, relative unit of measurement based on the root font size of the HTML document (usually set to 16px by default in most browsers). This means that 1rem is equal to the size of the root font. REM is responsive because it allows the size of elements to scale dynamically based on the user’s preferences or changes to the root size.

Advantages of REM:

• Scalability: When you use REM, changing the root font size will automatically adjust the size of all elements that use REM.
• Accessibility: REM ensures better accessibility, allowing users to adjust the text size through browser settings, which improves readability for visually impaired users.
• Consistency: It maintains consistent proportional scaling across different devices and screen sizes.

Example of REM in CSS:

css

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h1 {
 font-size: 1.5rem; /* equivalent to 24px if the root is 16px */
 padding: 1rem;     /* equivalent to 16px if the root is 16px */
}


In this example, the heading’s size will adjust proportionally if the root font size changes, making the design more flexible.

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Real-World Analogy for REM vs PX

To make this clearer, here’s a practical analogy:

• PX (Pixels) is like using fixed-sized bricks. Every brick (or pixel) is the same size no matter where you use it. Whether you build a small or large house, the bricks are rigid and unchanging. This works well for precise designs but doesn’t adjust if you need to make your house larger or smaller.
• REM (Root EM) is like using adjustable, flexible bricks. The size of the brick depends on the foundation of the house (the root element). If you build a larger house, the bricks automatically adjust in size to maintain balance, scaling up or down with the foundation. This allows for more responsive, fluid designs that adapt across different screen sizes.‍

When to Use PX vs REM?

Knowing when to use PX or REM depends on the needs of your project and how much flexibility you require in your design.

Use PX for:

• Pixel-Perfect Precision: When exact sizing is necessary, such as for small UI components like buttons, borders, and icons.
• Fixed Layouts: If you’re working on a project where the design will primarily be viewed on one screen size (e.g., a kiosk or fixed display).

Use REM for:

• Responsive Layouts: When you want the design to adapt fluidly to different screen sizes, such as mobile, tablet, and desktop devices.
• Text Sizes: For better accessibility, using REM for font sizes allows users to scale the text through their browser settings.
• Scalable Components: When you want padding, margins, or spacing to adjust proportionally as the root font size changes.

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‍REM to PX Conversion Guide

Most browsers use a default root font size of 16px unless explicitly changed. So by default, 1rem = 16px. Here’s a quick reference to convert REM to PX and vice versa:

REM ValueEquivalent in PX1rem16px1.5rem24px2rem32px0.75rem12px

To calculate:

• From PX to REM: Divide the pixel value by the root font size (usually 16px).
  • Example: 32px ÷ 16px = 2rem
• From REM to PX: Multiply the REM value by the root font size (usually 16px).
  • Example: 1.5rem × 16px = 24px

Example of a Combined REM and PX Layout

In practice, you can combine both PX and REM in your design, depending on the context.

css

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body {
 font-size: 16px;
}

h1 {
 font-size: 2rem; /* 32px */
}

button {
 padding: 10px;  /* Fixed size for precise spacing */
 font-size: 1rem; /* Scalable with the root */
}


In this example, we use REM for scalable text (so that changing the root size will adjust the text proportionally) and PX for the button padding, where precision is needed.

Conclusion:

When designing responsive websites, using REM over PX offers greater flexibility, accessibility, and adaptability. While PX units are precise and reliable, REM’s scalability makes it ideal for designs that need to adapt seamlessly across different devices and user settings. Understanding when and how to use these units is a key skill in creating websites that work well for everyone, no matter the screen size or resolution.

By using the analogy of fixed and adjustable bricks, the difference between PX and REM becomes clearer: one offers stability and precision, while the other provides flexibility and scalability.

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