Why should I use rem instead of px for font sizes?

rem units are relative to the root (html) font size. If a user increases their browser's default font size for accessibility, rem-based sizes scale up with it. px sizes are fixed and do not respect user font size preferences, which is an accessibility issue.

1rem equals the root font size, which browsers default to 16px. So 1rem = 16px, 1.5rem = 24px, 0.75rem = 12px. If you change the root font size in your CSS (html { font-size: 18px; }), 1rem = 18px.

What is the difference between rem and em?

rem is always relative to the root (html) element's font size, making it predictable regardless of nesting. em is relative to the current element's parent font size, which compounds with nesting — a nested element with font-size: 1.5em inside a parent with font-size: 1.5em results in 2.25× the root size.

When should I use vh/vw units?

Viewport units (vh/vw) are useful for full-screen sections (height: 100vh), fluid typography that scales with window width, and positioning elements relative to the viewport rather than their containing element.

How do I convert a Figma design in px to CSS rem?

Divide the Figma pixel value by your root font size. With a 16px root, a 24px design value converts to 24/16 = 1.5rem. The tool does this automatically — enter the px value and copy the rem equivalent.

All Tools›UI / CSS›

CSS Units Converter

Free · No Server

Convert between px, rem, em, %, pt, vw and vh instantly. Configure base font size and viewport.

Convert Value

▶Context settings (base font, viewport, parent)

Root font-size (px)

Parent font-size (px)

Viewport width (px)

Viewport height (px)

Equivalent Values

16px

px — pixels

1rem

rem — root em

1em

em — local em

100%

% — percent (of parent)

12.0003pt

pt — points

1.11111vw

vw — viewport width

1.77778vh

vh — viewport height

Tailwind spacing (exact)

p-4 · m-4 · gap-4 → 16px

Fluid clamp() generator

Min size (px)

Max size (px)

Min viewport (px)

Max viewport (px)

font-size: clamp(1rem, 0.8239rem + 0.7512vw, 1.5rem);

Scales smoothly between viewports — paste straight into any CSS property (font-size, padding, margin…).

What is Css Units Converter?

The CSS Units Converter on AiTechWorlds converts between all CSS length units — px, rem, em, vh, vw, vmin, vmax, pt, and cm — instantly, with support for custom root font size and viewport dimensions. CSS has many length units, each suited for different purposes: px for fixed sizes, rem for scalable typography relative to the root font size, em relative to the parent font size, and viewport units for responsive layouts. Converting between them manually requires knowing the base font size and viewport size. This tool handles all conversions simultaneously: enter a value in any unit and see all equivalents update at once. Set your project's root font size (default 16px) and viewport dimensions to get accurate results for your specific context. All conversion runs in your browser with no server call. AiTechWorlds built this for frontend developers who constantly juggle CSS units in responsive design work.

How to Use

1
Set context values
Enter your root font size (default 16px) and viewport width/height for accurate conversions.
2
Enter a value
Type a value in any unit field — px, rem, em, vh, vw, or others.
3
See all equivalents
All other unit fields update instantly to show the equivalent value.
4
Copy any unit
Click to copy the value in the specific unit you need.

Common Use Cases

Frontend developers converting design spec pixel values to rem for scalable CSS
CSS developers computing viewport unit equivalents for responsive breakpoints
Developers migrating CSS from px-based to rem-based typography systems
Mobile developers converting design mockup values to responsive vw/vh units
Accessibility engineers ensuring font sizes are set in relative units for user scaling

Key Features

Converts between px, rem, em, vh, vw, vmin, vmax, pt, pc, cm, mm, and in
Configurable root font size (default 16px)
Configurable viewport width and height
Enter any unit and all others update simultaneously
Common design size presets
100% browser-based
Copy any converted value with one click

🔒

100% Private — No Server Required

All processing happens directly in your browser. No data is uploaded, stored, or transmitted to any server.

Frequently Asked Questions

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Last reviewed on June 1, 2026 by the AiTechWorlds Tools Team. All processing runs locally in your browser.

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AiTechWorlds

What is Css Units Converter?

How to Use

Set context values

Enter your root font size (default 16px) and viewport width/height for accurate conversions.

Enter a value

Type a value in any unit field — px, rem, em, vh, vw, or others.

See all equivalents

All other unit fields update instantly to show the equivalent value.

Copy any unit

Click to copy the value in the specific unit you need.

Common Use Cases

Frontend developers converting design spec pixel values to rem for scalable CSS

CSS developers computing viewport unit equivalents for responsive breakpoints

Developers migrating CSS from px-based to rem-based typography systems

Mobile developers converting design mockup values to responsive vw/vh units

Accessibility engineers ensuring font sizes are set in relative units for user scaling