CSSApril 2026 · 6 min read

How to Convert Between CSS Units (PX, REM, EM, VW, %)

CSS has over a dozen length units, and converting between them is a frequent task during development. PX to REM for accessibility, VW to PX for debugging, EM to PX for specificity — the Unit Converter handles all of these conversions with adjustable base values.

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Derek Giordano

Designer & Developer

In this guide

01Absolute Units02Relative Units03Conversion Formulas04When to Use Each Unit

⚡ Key Takeaways

Convert between CSS measurement units: PX to REM, EM to PX, VW to PX, percentages.
Covers absolute units.
Covers relative units.
Covers conversion formulas.
When to Use Each Unit.

Absolute Units

px (pixels) — the most common absolute unit. 1px is one device pixel (or one CSS pixel on high-DPI screens). Use for borders, shadows, and fine visual details. pt (points) — 1pt = 1.333px. Print legacy. in (inches), cm (centimeters), mm (millimeters) — physical measurements, primarily for print stylesheets.

Relative Units

rem — relative to the root font size (default 16px). 1rem = 16px. Use for font sizes and spacing. em — relative to the parent element's font size. 1em = parent's font-size. Use for component-internal spacing. % — relative to the parent element's dimension. width: 50% = half the parent's width. vw/vh — 1vw = 1% of the viewport width. 100vw = full viewport width. ch — width of the '0' character. Useful for constraining line lengths.

💡 Tip

Stick to animating transform and opacity for smooth 60fps performance. These properties are handled by the GPU compositor and skip expensive layout recalculations.

Conversion Formulas

PX to REM: px ÷ root-font-size. 24px ÷ 16 = 1.5rem. REM to PX: rem × root-font-size. 1.5rem × 16 = 24px. VW to PX: vw × viewport-width ÷ 100. 50vw at 1440px viewport = 720px. EM to PX: em × parent-font-size. 1.5em with 20px parent = 30px. The Unit Converter handles all of these with adjustable base values.

⚠ Warning

Avoid animating width, height, top, or left. These trigger layout recalculations on every frame and can drop performance below 60fps.

When to Use Each Unit

rem for font-size, margin, padding, max-width — scales with user preferences. px for border, box-shadow, outline — fine details that shouldn't scale. em for padding/margin inside components — scales with the component's font size. vw/vh for full-viewport layouts and fluid typography. % for width inside containers. ch for max-width on text containers (e.g., max-width: 65ch for optimal line length).

Frequently Asked Questions

How do I convert PX to REM?+

Divide the pixel value by the root font size (default 16px). 24px ÷ 16 = 1.5rem. Use the Unit Converter for instant conversion with any base size.

What is the difference between REM and EM?+

REM is relative to the root (html) font size and is consistent everywhere. EM is relative to the parent element's font size, so it compounds when nested — 1.2em inside 1.2em becomes 1.44em equivalent.

When should I use VW units?+

Use vw for viewport-relative sizing: full-width elements (100vw), fluid typography (combined with clamp()), and viewport-height sections (100vh). Avoid for text sizing alone — it doesn't account for user font preferences.

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Derek Giordano

Written by the creator of Ultimate Design Tools. BA in Business Marketing.

📚 References & Further Reading

⚡ Try the free Size Chart Generator →