How do I convert px to % manually?

Divide the pixel value by the parent dimension, then multiply by 100. For example, a 500px element inside a 1000px container is (500/1000) × 100 = 50%. This tool does the math automatically with bidirectional conversion.

Why is vertical padding calculated from width, not height?

CSS specifies all margin and padding percentages relative to the parent's width — even padding-top and padding-bottom. This is intentional and enables aspect-ratio hacks (padding-top: 56.25% creates a 16:9 box). The modern CSS aspect-ratio property does this directly.

% vs vw — which should I use?

Use % for sizing relative to the parent container (fluid grids, components inside containers). Use vw for sizing relative to the viewport (browser window) — hero sections, full-bleed images, fluid typography that should scale with screen size regardless of nesting.

px to % Converter — Free Online CSS Percentage Calculator

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Visual Layout Preview

160px

Generated CSS

width: 16%; /* 160px */

What Is the % Unit in CSS?

% (percentage) is a relative CSS unit calculated against the dimension of the containing block. For width, that's the parent's content width; for height, the parent's height (with caveats); for font-size, the parent's font-size; for padding and margin, always the parent's width — even for vertical padding. % is the original fluid-layout unit and remains essential for responsive design alongside CSS Grid and Flexbox.

px to % Converter Online — What This Tool Does

This free px to % converter performs bidirectional conversion in your browser. Enter a pixel value and a parent width (or height) to see the percentage equivalent, with a live visual preview showing the computed width.

How to Convert px to %

Enter a pixel value in the px field.
Set the Parent Width to the container the percentage will be calculated against (default 1000px).
The % equivalent appears instantly: px / parent × 100.
Or enter a % value to see the equivalent px (bidirectional).
The visual preview shows the computed pixel width inside a representative container.

When % Is Calculated From Width vs Height

width: 50% — 50% of parent's content width.
height: 50% — 50% of parent's height, but only if the parent has an explicit height. Otherwise collapses to 0.
padding-top: 50% — 50% of parent's width (not height). Used for aspect-ratio hacks before CSS aspect-ratio existed.
margin: 5% — also based on parent width for both horizontal and vertical.
font-size: 120% — relative to parent's font-size (equivalent to 1.2em).

% vs vw — When to Pick Which

% — relative to the immediate parent. Good for fluid grids and responsive components inside containers.
vw — relative to the viewport (browser window). Good for hero sections, full-bleed images, fluid typography.
Modern preference — CSS Grid / Flexbox with fr units often replaces % for layout. % remains useful for non-grid contexts and legacy code.

Tips & Tricks

Use the px to vw converter when you want sizing relative to the viewport rather than the parent.
For aspect-ratio — modern code uses aspect-ratio: 16 / 9. The old padding-top % trick still works in legacy browsers.
Combine % with max-width — width: 100%; max-width: 1200px keeps containers fluid but capped.
Test on narrow screens — % values can become impractically small on mobile; pair with media queries or min() / max() / clamp().

Related Tools

px ↔ rem Converter — typography and design-system sizing.
px ↔ em Converter — component-scoped sizing.
px ↔ vw / vh Converter — viewport-relative sizing.
rem ↔ em Converter — translate between the two relative font units.
Color Picker — pick brand colours for your design system.

Is My Data Sent to a Server?

No. All unit conversion runs entirely in your browser using simple JavaScript math. The values you enter — including any proprietary design-system tokens — never leave your device. There is no upload, no logging, and no analytics on the numbers you enter.