CSS Units Best Practices for Webflow: When to Use Pixels, REMs, EMs, VH, VW, and more

Choosing the right CSS unit is crucial for creating responsive, accessible, and scalable web designs. CSS units fall into two main categories:

• Absolute units stay fixed regardless of screen size, zoom, or user settings (e.g., px).
• Relative units adapt based on context, making them more flexible for responsive design (rem, em, %, vw, vh).

This guide will help you understand when to use each CSS unit and the tradeoffs involved.

When to Use Pixels (px)

Pixels (px) is the only true absolute unit in CSS, remaining fixed and unaffected by external factors such as screen size or user settings. If you need an element to stay consistent in size and not adjust based on other variables, using pixels is a reliable choice.

However, it's important to ensure that your content doesn’t overflow, especially on smaller screens. Pixels are particularly useful for setting a max width on elements like containers, where a fixed, non-responsive size is desired for maintaining layout integrity across various devices.

Best Use Cases:

• Borders, icons, and fine-tuned UI elements
• Ensuring elements don’t change with font-size adjustments
• Tradeoff: Can break accessibility and responsiveness

When to Use REMs and EMs for Typography

rem (Root EM)

• Based on the root font-size (typically 16px by default)
• Scales consistently across the design
• Ideal for accessible typography

em

• Based on the font-size of the parent element
• Useful for modular designs where components scale together

Why Avoid Mixing px and rem?

One common mistake is using px for padding and rem for text size. This causes issues when users adjust font sizes, making the design look inconsistent.

When to Use Percentages (%)

Percentages are ideal for defining an element's size relative to its parent container, allowing for flexible and adaptive layouts. This approach ensures that elements scale proportionally as the parent container changes in size, making it an essential tool in responsive design.

Percentages are commonly used in grid-based designs and layouts, where the goal is to create flexible, fluid arrangements that adjust seamlessly to different screen sizes and orientations. By using percentages, designers can maintain consistency and alignment across various devices while preserving the structure of the design.

Example:

‍When to Use Viewport Units (vh, vw, vmin, vmax)

Viewport units (vh, vw, vmin, vmax) are ideal for creating responsive designs that adapt to the size of the user's screen. Use vh and vw to set dimensions relative to the viewport's height and width, ensuring elements scale fluidly.

vw (Viewport Width) & vh (Viewport Height)

• 1vw = 1% of the viewport width, 1vh = 1% of the viewport height
• Useful for full-width layouts, hero sections, and fluid typography

vmin & vmax

• vmin: The smaller value between vw and vh
• vmax: The larger value between vw and vh


Advanced Viewport Units: SVH, DVH, LVH, SVW, DVW, LVW

Modern viewport units offer better control over dynamic UI changes:

• SVH/SVW (Small Viewport Height/Width): Accounts for the smallest possible viewport size, considering mobile UI elements.
• DVH/DVW (Dynamic Viewport Height/Width): Adjusts with dynamic elements like address bars in mobile browsers.
• LVH/LVW (Large Viewport Height/Width): Uses the largest available viewport size.

Example:

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Best Practices for Combining CSS Units

Typography

Use rem (root em) units for font sizes to improve accessibility, as they allow users to adjust text size based on their preferences or device settings. Additionally, leverage vw (viewport width) for fluid scaling of typography, enabling text to adjust dynamically to different screen sizes, ensuring readability and consistency across devices. This approach enhances both usability and design flexibility.

Spacing & Layouts

Use percentages (%) for fluid containers to ensure your design adapts seamlessly to varying screen sizes. For full-screen sections, leverage viewport units like vh (viewport height) and vw (viewport width) to create layouts that dynamically adjust based on the size of the user's screen, ensuring a responsive experience across all devices. This approach allows for more flexible and scalable designs.

Responsive Typography

Using clamp() for responsive typography ensures text remains readable on all screen sizes. It allows for a minimum and maximum font size while scaling based on the viewport width, preventing text from becoming too large or too small. This approach improves user experience without needing media queries.‍


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Conclusion

Choosing the right CSS unit depends on the context:

• Fixed sizes? Use px.
• Scalable and accessible typography? Use rem or em.
• Responsive layouts? Use %, vh, vw.
• Advanced responsiveness? Consider dvh, lvh, or svh.

Understanding how CSS units interact with other elements helps create flexible, accessible, and future-proof designs.