16px to REM

rem stands for "root em": a size relative to the font size of the html element. To convert, divide pixels by the root font size.

16px is the browser default font size and the most common size for body text on the web. It is the reference point for rem: 1rem equals 16px unless the root font size is changed. Most accessibility guidance treats 16px as the minimum comfortable size for long reading. As spacing, 16px is one of the most-used values for padding inside cards and gaps between elements.

If you use 16px (1rem) for body or interface text, a line-height of about 1.5 works well, which gives 24px between baselines. Longer paragraphs benefit from slightly more (1.6 to 1.7), while buttons and labels can go tighter (1.2 to 1.3).

For spacing below a block of 16px text, a margin of 0.75em to 1em (12px to 16px) keeps the rhythm consistent with the text size. Writing these values in em ties them to the font size, so they adjust automatically if the text size changes.

Instead of a fixed 1rem, you can let the size scale smoothly between a smaller value on phones and 16px on desktops. The CSS clamp() function takes a minimum, a preferred value and a maximum. The example below grows from 13px on a 375px-wide screen to 16px on a 1280px-wide screen, and never goes outside that range. Because the minimum and maximum are in rem, the size still respects the user's font settings.

A rem is relative to the root font size, so 1rem stays equal to 16px only while the root is 16px. When someone raises the default font size in their browser, say to 20px, the same 1rem becomes 20px, and your layout scales with it. A hard-coded 16px stays at 16px and ignores that preference.

That matters for accessibility. Many people with low vision rely on larger default text, and the WCAG resize text guideline expects content to remain usable when text is scaled up to 200 percent. Using rem for type and spacing is the simplest way to support that without extra work.

Here is how 16px might appear in real CSS. The card title uses 1rem, spacing uses rem values from the same 16px base, and the border stays in px so it remains a crisp hairline. If a visitor's default font size is 20px, the title grows to 20px and the spacing grows with it, while the border stays 1px.

If you convert values often, let your tools do it. In Sass, a small function such as rem($px) that returns math.div($px, 16px) * 1rem turns rem(16px) into 1rem at compile time. In plain CSS, you can keep the pixel value readable with calc(16 / 16 * 1rem), which modern browsers resolve to 1rem.

In JavaScript, for example when generating inline styles or design tokens, divide by the root font size: 16 / 16 = 1. If you need the live root size (for users who changed their default), read parseFloat(getComputedStyle(document.documentElement).fontSize) instead of assuming 16.

Design systems rarely use raw numbers in components. Instead, 16px would live in a token such as --font-size-base for type or --space-4 for spacing, defined once as 1rem. Components then reference the token, so a later change to the scale updates every use at once.

If your team uses Figma variables or a tokens file (for example in the W3C design tokens format), store the value in px for designers and generate the rem version in the build step. That keeps both sides speaking the same language while the shipped CSS stays accessible.

When the browser computes a rem value, it multiplies it by the computed font size of the html element. That font size starts from the user's default (16px unless they changed it), then applies whatever your CSS sets on html. So 1rem means "1 times the root size", and it is resolved once per page, unlike em, which is recalculated at every level of nesting.

Because the root is shared, rem values are predictable across the whole document: a 1rem gap in a sidebar is exactly the same size as a 1rem gap in a modal, regardless of the text size inside each.