Px to Rem Converter — plus em, pt & percent

The px to rem converter above turns any pixel value into rem, em, pt, and percent in real time, using whatever root font size you specify — no spreadsheet, no manual division, no guesswork about whether you got the decimal right at 2 a.m. while finishing a component library.

Front-end teams reach for a tool like this constantly during the switch from a pixel-based design file to a responsive, accessible codebase. Design tools such as Figma and Sketch hand off measurements in px because that's how designers think visually, but shipping px straight into your CSS quietly breaks a browser feature almost every visitor relies on at some point: the ability to bump up text size. At Arb Digital we rebuild sites for clients migrating from rigid legacy themes, and unit conversion is one of the first things we standardize before touching layout.

What This Px to Rem Converter Does

Enter a pixel amount and, optionally, the root font size your project uses (16px is the browser default, but plenty of design systems set 10px or 62.5% on html to make mental math easier). The tool immediately returns the equivalent in four units at once: rem, em, points, and percent. The bulk box below it does the same thing for an entire list — paste every spacing value from a design spec and get a ready-to-paste column of rem values back, which is the fastest way we've found to migrate a stylesheet without opening a calculator app twenty separate times.

How to Use It

1. Set your root font size. Leave it at 16px unless your project's base CSS changes it — check your html or :root selector for a font-size declaration first.
2. Type the pixel value you're converting from your design file, mockup, or an old stylesheet.
3. Read the four conversions in the result panel — rem is highlighted as the headline number since it's the recommended unit for almost all sizing.
4. For a whole list, paste comma- or line-separated px values into the bulk box and click Convert List to get every rem value at once.
5. Paste the rem value straight into your CSS in place of the px value.

The Formula / How It's Calculated

The math behind rem is deliberately simple: rem = pixels ÷ root font size. If your root font size is the browser default of 16px, then 16px becomes 1rem, 24px becomes 1.5rem, and 8px becomes 0.5rem. Change the root to 10px (a common trick so 1rem = 10px and mental math gets easier) and the same 16px instead becomes 1.6rem. Points follow a fixed physical ratio inherited from print: 1px = 0.75pt at the standard 96 DPI screen resolution, so 16px is always 12pt regardless of root font size. Percent is simply the rem value multiplied by 100, since both are expressed relative to the same root context. The CSS specification itself documents how relative length units resolve against context — see the MDN CSS values and units reference for the authoritative breakdown of every unit browsers support.

Why Rem Beats Px for Accessibility

Pixels are an absolute unit — 16px means exactly 16 device pixels regardless of anything else on the page or in the browser's settings. That sounds precise, and it is, but precision is exactly the problem. Many users — people with low vision, older users, anyone reading on a phone in bright sunlight — change their browser's default font size in accessibility settings specifically so text renders bigger everywhere, without having to zoom the whole layout. When your CSS is written entirely in px, that setting gets ignored: your text stays locked at 16 pixels no matter what the user asked for, because px never looks at the root font size at all.

Rem, by contrast, is calculated live from the root element's font size every time the page renders. Bump the browser's default text size up 25%, and every rem-based measurement on the page — headings, body copy, button padding, line height — scales proportionally, because it was never a fixed pixel count to begin with, it was always "a multiple of whatever the root currently is." This single design decision is why the Web Content Accessibility Guidelines specifically call out resizable text as a success criterion, and why WebAIM's guidance on responsive type recommends relative units for exactly this reason. Using rem for typography and spacing costs you nothing in a normal design workflow — you convert once during handoff, using a tool like the one above — and it buys every visitor with non-default settings a page that actually respects their choice.

The Em Compounding Trap

Em looks almost identical to rem at first glance — both are relative units, both are commonly set so that 16px equals 1 — but they resolve against completely different things, and that difference causes one of the most common and hardest-to-debug CSS bugs in nested components. Rem always looks at the root <html> element's font size, full stop, no matter how deeply nested the element is. Em looks at the font size of its own parent element, and if that parent's font size was itself set in em relative to its own parent, the multiplication compounds at every level of nesting.

Here's the trap in practice: say a card component sets font-size: 1.2em, and inside that card a nested badge also sets font-size: 1.2em, and inside that badge a tiny label does the same. Each em is multiplying against its immediate parent, so the final rendered size isn't 1.2× the root — it's 1.2 × 1.2 × 1.2, roughly 1.73× the root font size, purely from three nested 1.2em declarations that each looked reasonable in isolation. Designers who test a component in a flat sandbox never catch this because the compounding only appears once the component is nested inside other components that also use em, which is exactly what happens in a real design system. This is the single biggest reason engineering teams reserve em for properties that should genuinely scale with their immediate parent — like the internal padding of a button that should grow if someone bumps the button's own font size — and use rem for everything anchored to the page's overall type scale.

When to Use Pt and Percent Instead

Points (pt) are a leftover from print typesetting and rarely belong in responsive web CSS at all — the only place you'll still see them intentionally is in print stylesheets (@media print) or when generating PDF layouts, where a fixed physical size genuinely is the goal. If you're exporting a design for print, the pt column above gives you the conversion instantly rather than making you remember the 0.75 ratio. Percent shows up constantly in layout properties like width, max-width, and flexbox sizing, but for typography it behaves almost exactly like em — it inherits from its containing context, and it can compound the same way if you're not paying attention to what that context is. The safest general rule most style guides converge on: use rem for font sizes, line heights, and any spacing that should follow the user's overall zoom/font preference; use em sparingly for measurements that should intentionally scale with one specific parent; reserve px for things that genuinely should never change, like a 1px hairline border; and treat percent and pt as situational units for layout width and print output respectively.

Common Mistakes to Avoid

• Assuming the root is always 16px. Many design systems reset html to font-size: 62.5% (10px) specifically so rem math is easier — always check before converting.
• Using em for page-level typography. Nested components with em font sizes compound unpredictably; rem stays flat and predictable no matter how deep the nesting goes.
• Mixing px and rem inconsistently in the same component, which makes it resize unevenly when a user changes their browser's default font size.
• Forgetting line-height inherits differently depending on whether it's set unitless, in em, or in px — unitless is usually safest precisely because it avoids this whole category of bug.
• Hardcoding pt values in web CSS when the page will only ever be viewed on screen — pt adds no benefit over rem there and only muddies the stylesheet.

Related Free Tools From Arb Digital

If you're cleaning up units across a whole project, the DPI/PPI Calculator helps with image resolution math, the Data Storage Converter handles KB/MB/GB conversions for asset budgets, and the Bandwidth Converter and Flow Rate Converter round out our engineering-adjacent conversion tools. Pair this page with the Viewport Size Checker when testing how your rem-based layout responds across real device widths, or browse everything in our free online tools hub.

Frequently Asked Questions

This tool runs entirely in your browser using built-in JavaScript. Nothing you enter is uploaded, stored, or sent to any server.