10px 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.

10px is the smallest size you will see in most interfaces, and it should be used sparingly. It appears in tiny labels on charts, badge counters, legal fine print and keyboard shortcut hints. At 10px, text is hard to read for many people, so keep it for supplementary information that is also available elsewhere. As a spacing value, 10px is common in older designs that were not built on a 4px or 8px grid.

If you use 10px (0.625rem) for body or interface text, a line-height of about 1.5 works well, which gives 15px 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 10px text, a margin of 0.75em to 1em (7.5px to 10px) 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 0.625rem, you can let the size scale smoothly between a smaller value on phones and 10px on desktops. The CSS clamp() function takes a minimum, a preferred value and a maximum. The example below grows from 10px on a 375px-wide screen to 10px 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 0.625rem stays equal to 10px only while the root is 16px. When someone raises the default font size in their browser, say to 20px, the same 0.625rem becomes 12.5px, and your layout scales with it. A hard-coded 10px stays at 10px 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 10px might appear in real CSS. The card title uses 0.625rem, 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 12.5px 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(10px) into 0.625rem at compile time. In plain CSS, you can keep the pixel value readable with calc(10 / 16 * 1rem), which modern browsers resolve to 0.625rem.

In JavaScript, for example when generating inline styles or design tokens, divide by the root font size: 10 / 16 = 0.625. 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, 10px would live in a token such as --font-size-sm for type or --size-10 for spacing, defined once as 0.625rem. 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 0.625rem means "0.625 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 0.625rem gap in a sidebar is exactly the same size as a 0.625rem gap in a modal, regardless of the text size inside each.