Pixel to Percentage Converter

What this tool does

This converter translates a CSS length between pixels and a percentage of a parent container. You enter a value, choose whether it is in px or %, and set the parent or container size in pixels — 960 by default. The tool shows the equivalent in both units, with the working written out, and recalculates as you type. Conversion runs in both directions: enter a pixel width to get the percentage, or enter a percentage to get the pixel width. Everything is computed locally in your browser.

Why you might need it

Responsive layouts are built on percentage widths, but designs are usually delivered in fixed pixels. Translating between the two is constant work: a sidebar drawn at 320px inside a 960px layout needs to become a percentage so it flexes; a column specified as 33.333% needs a pixel value to check against a mockup. You might be sizing a flexible grid, setting a max-width image, or auditing why an element overflows its container. Doing the division by hand for every element invites slips, and a converter that keeps the parent size explicit makes the relationship clear and the numbers reliable.

How to use it

1. Enter the value you want to convert.
2. Choose its unit — px or %.
3. Set the parent / container size in pixels. This is the element your value sits inside and is measured against.
4. Read the headline result and the two equivalent cards. Use the copy button to grab the converted value.

How it’s calculated

The conversion is a single proportion. The tool first normalises your input to pixels: a pixel value is already in pixels, while a percentage is turned into pixels with px = (% ÷ 100) × parent. It then expresses that pixel figure as a share of the parent with % = (px ÷ parent) × 100. So a 480px element inside a 960px container is 480 ÷ 960 × 100 = 50%, and 25% of an 800px container is (25 ÷ 100) × 800 = 200px. The parent size is the denominator in every case, which is why it must be greater than zero.

Common pitfalls

The most common confusion is forgetting what the percentage is relative to. In CSS a percentage width refers to the containing block, not the viewport and not the element’s own content — so the same percentage produces different pixel results inside different parents. Padding and borders matter too: depending on the box-sizing model, a percentage width may or may not include them, which can make a measured element appear wider than the percentage suggests. Finally, a result above 100% means the element is larger than its container; the maths is correct, but the layout will overflow.

Tips and related conversions

When you convert a fixed design to a fluid one, set the parent size to the design’s container width and read the percentage for each child — that gives a layout that scales cleanly. Some fractions are worth memorising: a half is 50%, a third is 33.333%, a quarter is 25%. If you also work in em or rem, remember those are font-relative while a percentage here is container-relative — different references for different jobs. Because this calculator is pure arithmetic running locally, you can convert an entire layout quickly, and nothing you enter ever leaves your browser.