PX to EM/REM Converter

About CSS Unit Converter

CSS offers several unit types for defining sizes, spacing, and typography on the web. Choosing the right unit is crucial for building responsive, accessible websites that look great on every screen. The most common CSS length units are pixels (px), em, rem, points (pt), and percentages (%). Each serves a different purpose, and understanding when to use which one can dramatically improve your workflow and the quality of your layouts.

Understanding CSS Units

Pixels (px) are the most familiar unit in web design. One CSS pixel is an abstract reference unit that usually maps to one device pixel on a standard screen, though high-DPI displays may map it to two or more physical pixels. Pixels provide precise, predictable control and are ideal for borders, shadows, fixed-width elements, and situations where exact sizing matters. However, pixel-based typography does not scale when users adjust their browser's default font size, which can create accessibility problems.

EM is a relative unit based on the computed font size of the parent element. If a parent has a font size of 16px, then 1em equals 16px for its children. The compounding nature of em means that nested elements multiply their sizes, so 1.2em inside 1.2em actually computes to 1.44 times the grandparent size. This makes em particularly useful for component-level spacing that should scale with the component's own text size, such as padding inside buttons or margin between a heading and its subtitle.

REM (root em) works like em but is always relative to the root element's font size, which defaults to 16px in most browsers. Because rem does not compound through nesting, it provides consistent, predictable sizing across your entire page. Most modern CSS methodologies recommend rem for font sizes and major layout spacing. If a user changes their browser's default font size from 16px to 20px, all rem-based values scale proportionally, making your site more accessible without any additional code.

Points (pt) come from print typography, where 1 point equals 1/72 of an inch. On screens, 1pt equals approximately 1.333px (at the standard 96 DPI). Points are mainly used when generating print stylesheets or working with designs that originated in print tools like Adobe InDesign or Microsoft Word. For screen-only work, px or rem are generally better choices.

Percentage (%) is relative to the parent element's property value. For font sizes, 100% equals the parent's font size. Percentages are essential for fluid layouts, responsive images, and flexible grid systems. Combined with max-width and min-width, percentages form the backbone of responsive web design.

When to Use PX vs EM vs REM

Use rem for font sizes, padding, and margins in most situations. It provides consistency, scales with user preferences, and avoids the compounding issues of em. Use em for component-internal spacing that should scale with the component's own font size. Use px for fine details like borders, box shadows, and elements that should remain a fixed size regardless of font settings. Use % for widths, flexible containers, and fluid layouts. Use pt primarily in print stylesheets.

FAQ

Q: What is the default base font size in browsers?
Most browsers default to a base font size of 16px. This means 1rem and 1em (at the root level) both equal 16px. Users can change this default in their browser settings for accessibility reasons, which is why using relative units like rem is important for inclusive design.

Q: What is the difference between em and rem?
Both are relative units, but em is relative to the parent element's font size while rem is relative to the root element's font size (the html element). This means em values compound when elements are nested, while rem values stay consistent regardless of nesting depth. For example, 1.5em inside a 1.5em parent computes to 2.25 times the grandparent's font size, but 1.5rem always equals 1.5 times the root font size.

Q: Should I use px or rem for font sizes?
Use rem for font sizes in almost all cases. Rem-based font sizes respect the user's browser font size preference, making your site more accessible. Pixels override user preferences, which can make text unreadable for users who rely on larger default fonts. The exception is very small fixed UI elements like badge counts where exact sizing is critical.

Q: How do I convert px to rem in my head?
With the default base of 16px, divide the pixel value by 16. For example: 24px / 16 = 1.5rem, 14px / 16 = 0.875rem, 32px / 16 = 2rem. Some developers set the root font size to 62.5% (10px) so that 1rem equals 10px, making mental math easier: 16px = 1.6rem, 24px = 2.4rem.

Q: Is my data safe when using this tool?
Yes. All conversions are performed entirely in your browser using client-side JavaScript. No data is transmitted to any server. You can verify this by disconnecting from the internet and confirming the tool still works perfectly.