CSS Unit Converter

REM (Root EM) units are relative to the user's browser font size setting (default 16px). If a user with visual impairments increases their browser's default font size to 24px, a website built with REMs will scale up proportionally. A website built with fixed PX values will strictly stay at the hardcoded pixel size, ignoring the user's preference and potentially making the text unreadable.

REM is relative to the root element (html), giving you a consistent global sizing system. EM is relative to the parent element. This means EMs can compound: a 2em font inside a 2em container becomes 4em relative to the root. EMs are great for component-level modularity (like padding scaling with font size), while REMs are better for global layout and typography.

The formula is Target PX / Root Font Size = REM. Assuming the standard root size is 16px:

• 16px = 1rem
• 24px = 1.5rem
• 32px = 2rem. Most developers set the root font size to 100% (default) but some use the '62.5% trick' (setting html size to 10px) to make math easier (so 1.4rem = 14px), though this is debated in modern development.

Viewport units are powerful for layouts that need to adapt to the window size, not just the content. 100vh is perfect for full-screen hero sections. vw is great for fluid typography that scales seamlessly from mobile to desktop. However, use them carefully for font sizes, as text can become too small on mobile or too massive on ultrawide monitors without clamping.

The ch unit represents the width of the character '0' (zero) in the current font. It is incredibly useful for setting max-widths on text containers to ensure optimal readability. Research suggests standard reading lines should be 45-75 characters long. Setting max-width: 65ch creates this perfect typographic measure automatically.

CSS pixels are actually 'logical' pixels, not physical device pixels. On a high-DPI screen (like a Retina display), 1 CSS pixel might equal 2, 3, or even 4 physical hardware pixels. Our converter deals with standard CSS units, which is exactly what you need for writing stylesheets, regardless of the screen density.

Container query units are the evolution of responsive design. While vw/vh relate to the viewport, cqw/cqh relate to a specific parent container marked as a query container. This allows you to build truly modular components (like a card) that look good whether they are in a narrow sidebar or a wide main content area.

Yes, modern browsers support sub-pixel rendering (e.g., 14.5px). However, the result depends on the screen technology. Some engines might round to the nearest whole pixel, while others use anti-aliasing to simulate the fraction. For consistent, predictable layouts, it's often safer to stick to whole pixels or use flex/grid for distribution.

Points (pt) are a print-centric unit where 72pt = 1 inch. In web design, they are rarely used for screen layouts but are crucial when creating Print Stylesheets (@media print). If you want your web page to print perfectly on physical paper, define your print styles using pt, cm, or in.

On mobile browsers (like Safari iOS), the URL bar comes and goes, changing the viewport height dynamically. A heavily debated issue is that 100vh often includes the area covered by the URL bar, causing content to be cut off. The solution is using the new dynamic viewport units: dvh (Dynamic Viewport Height), lvh (Large), and svh (Small) to handle these UI states correctly.