PX to REM/EM Converter
Convert a single pixel value to rem and em, or batch-convert every px value inside a block of CSS.
About the PX to REM/EM Converter
The single converter divides your pixel value by the base font size to get both rem and em, which is exact for rem since it's always relative to the root, and holds true for em only when nothing between the element and the root has changed the font size along the way. The bulk box scans pasted CSS for any number directly followed by px, including negatives and decimals, and swaps each one for its rem equivalent while leaving selectors, property names, and already-unitless zeros untouched.
The compounding problem that makes rem the safer default
rem always means "relative to the root <html> element's font size," full stop, no matter how deeply nested the element using it is. em means "relative to this element's own computed font size," which itself might already be an em value inherited from a parent that was also set in em, and so on up the tree. Nest a few levels of em-based font-sizing (a list inside a card inside a sidebar, each with its own em adjustment) and the sizes compound multiplicatively at every level, an unintended side effect where deeply nested text keeps getting smaller or larger than anyone actually intended. rem sidesteps this entirely by always calculating from the same fixed reference point, which is why most modern CSS guidance defaults to rem for anything that doesn't specifically need to scale with its own local context.
Where em is still genuinely the better choice
Despite the compounding risk, em remains useful precisely because it ties a value to the current element's own font size rather than the page's root. Sizing a button's padding, icon dimensions, and border-radius all in em means that changing just that one button's font-size (for a "large" or "small" button variant, say) automatically scales every one of those related measurements together, proportionally, without having to separately adjust each pixel value by hand. This component-relative scaling is a legitimate, intentional use of em's cascading behavior, it's only a problem when the nesting is accidental rather than deliberate.
Why the Base Font Size field isn't always 16
16px is the near-universal default root font size across browsers, which is why it's the starting value here, but plenty of real CSS resets deliberately change it. A common pattern sets html { font-size: 62.5%; }, which shrinks the effective root size to exactly 10px (62.5% of the 16px browser default), specifically so that converting between rem and px becomes simple mental math, 1.6rem is obviously 16px, 2.4rem is obviously 24px, without needing a calculator. If your project uses that convention or any other non-default root size, changing the Base Font Size field here to match is what keeps every converted value accurate to your actual codebase rather than the browser's raw default.
An honest limitation of the bulk converter: it's a text scan, not a CSS parser
The bulk conversion is a single regular expression sweeping the entire pasted text for any number immediately followed by px, it doesn't parse or understand CSS structure the way a proper tokenizer would. That means it will just as happily convert a pixel-like number sitting inside a comment, inside a URL path, or in any other context where px happens to follow a number but isn't actually meant as a CSS length. For typical pasted style rules this is rarely an issue in practice, but if your CSS block contains comments or URLs with numbers followed by "px" as plain text, double-check the output before using it as-is. It also intentionally leaves values that are already unitless, like a bare 0 or a line-height ratio, completely untouched, since those aren't pixel measurements to begin with.
Frequently Asked Questions
Why is rem generally recommended over em for most CSS sizing?
rem always calculates from the root html element's font size regardless of nesting depth, while em calculates from the current element's own font size, which can itself be an inherited em value. Nesting several em-based sizes compounds multiplicatively, producing unintended size drift, a problem rem avoids entirely.
When is em actually the better unit to use?
When you want a value to scale proportionally with a specific component's own font size, like a button's padding and icon size scaling together when you change just that button's font-size. This is a deliberate, useful application of em's relative-to-parent behavior rather than the accidental compounding that happens with unintentional nesting.
Why would I change the Base Font Size away from 16?
Some CSS resets set the root font size to 62.5% (10px) specifically to make rem-to-px mental math easier (1.6rem = 16px). If your project uses a non-default root font size, matching the Base Font Size field to it keeps the converted values accurate to your actual codebase.
Does the bulk converter understand CSS syntax, or is it just find-and-replace?
It's a regular expression sweep across the raw text, not a real CSS parser. It will convert any number followed directly by "px" anywhere in the pasted text, including inside comments or URLs, so it's worth reviewing the output if your CSS block contains px-like text outside of actual property values.
Does the converter handle negative and decimal pixel values correctly?
Yes, both the single and bulk converters handle negative values (like -5px, common in negative margins) and decimals (like 2.5px), converting each correctly to its rem or em equivalent while trimming unnecessary trailing zeros from the result.
Why does the output sometimes show a value like 1.5rem instead of 1.5000rem?
The formatting function rounds to four decimal places internally for precision, then strips trailing zeros for a cleaner, more typical-looking CSS value, matching how developers normally write rem values by hand.