What's the difference between em and rem?

Both scale relative to a font size, but em is relative to the *parent* element's font size (so it can compound when nested), while rem is always relative to the root (html) element's font size. This calculator computes both the same way, using the base font size you provide — for em, that means the immediate parent's size, and for rem, the root's size.

That's the standard CSS reference pixel density used by browsers, which is where the fixed 1pt = 1.333px relationship comes from (96px ÷ 72pt per inch).

Why should I convert px to rem for accessibility?

Rem units scale properly when a user changes their browser's default font size, while fixed px values don't — using rem for font sizes and spacing helps your layout respect a visitor's accessibility preferences.

What base font size should I use if I'm not sure?

16px is the default browser font size in virtually every modern browser unless a page explicitly overrides it — use that as your base unless you know your project sets a different root font size.

Does this work for converting spacing values, not just font sizes?

Yes — the same px/em/rem/pt relationships apply whether you're converting a font size, a margin, a padding value, or any other CSS length.

Measurement & Unit Converters Cluster

CSS Unit Converter

Convert between px, em, rem and pt.

TL;DR: CSS Unit Converter is a free, browser-based tool that lets you convert between px, em, rem and pt.

Value

From

Base font size (px)

About the CSS Unit Converter

The CSS Unit Converter converts measurements between metric (SI), imperial, and specialized scientific unit standards with high-precision conversion factors.

How to Use the CSS Unit Converter

Enter the numeric quantity you want to convert.
Select the source unit from the dropdown list.
Select the target unit you wish to convert into.
View the exact converted value, scientific notation, and mathematical conversion formula.

Mathematical Formula & Methodology

Here is the standard mathematical equation used to calculate this result accurately:

\text{rem} = \frac{\text{px}}{\text{Root Font Size (16px)}}, \quad \text{vw} = \frac{\text{px}}{\text{Viewport Width}} \times 100

Formula Variables Explained:
Converts web design typography units across Pixels (px), REM, EM, Points (pt), Percent (%), Viewport Width (vw), and Viewport Height (vh).

Step-by-Step Worked Example

24px with default 16px base font size = 1.5rem; 32px = 2.0rem.

Expert Tips for CSS Unit Converter

Exact Standard Factors: Uses international bureau standards (e.g. 1 inch = exactly 25.4 mm; 1 pound = 0.45359237 kg).
Scientific Notation: Automatically switches to scientific notation ( $1.25 \times 10^6$ ) for very large or microscopic values.
Multi-Unit Conversion: View simultaneous conversions across all related units in the reference table.

Related Guides & Tutorials

Text Case Converter Guide — Learn in-depth concepts, best practices, and expert tips.

100% Client-Side Privacy Guarantee

All processing runs locally inside your browser using JavaScript and HTML5 APIs. Your data, files, and inputs are never uploaded to any remote server. Complete privacy by design.

Frequently Asked Questions

: Both scale relative to a font size, but em is relative to the *parent* element's font size (so it can compound when nested), while rem is always relative to the root (html) element's font size. This calculator computes both the same way, using the base font size you provide — for em, that means the immediate parent's size, and for rem, the root's size.
: That's the standard CSS reference pixel density used by browsers, which is where the fixed 1pt = 1.333px relationship comes from (96px ÷ 72pt per inch).
: Rem units scale properly when a user changes their browser's default font size, while fixed px values don't — using rem for font sizes and spacing helps your layout respect a visitor's accessibility preferences.
: 16px is the default browser font size in virtually every modern browser unless a page explicitly overrides it — use that as your base unless you know your project sets a different root font size.
: Yes — the same px/em/rem/pt relationships apply whether you're converting a font size, a margin, a padding value, or any other CSS length.

Cite This Tool

Referencing this tool in a paper, article, or bibliography? Copy a ready-made citation below.

NexLove.org. (2026). CSS Unit Converter [Software]. https://nexlove.org/tools/css-unit-converter.html

“CSS Unit Converter.” NexLove.org, 2026, nexlove.org/tools/css-unit-converter.html.

NexLove.org (2026) CSS Unit Converter. Available at: https://nexlove.org/tools/css-unit-converter.html (Accessed: 2026).

@misc{nexlove_css_unit_converter,
  title = {CSS Unit Converter},
  author = {{NexLove.org}},
  year = {2026},
  url = {https://nexlove.org/tools/css-unit-converter.html}
}

Embed This Tool

Add the live CSS Unit Converter tool to your own website with this snippet — it loads the real, working tool in an iframe, not a static screenshot.

<iframe src="https://nexlove.org/embed/css-unit-converter.html" width="100%" height="600" style="border:1px solid #e2e8f0;border-radius:12px" title="CSS Unit Converter — NexLove.org" loading="lazy"></iframe>

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