Why convert px to rem?
A user-set browser font size (Settings → Appearance → Font size) only changes rem values, not
px. Sites built entirely in rem respect users' accessibility preferences;
sites built in px ignore them. For body copy, line heights, spacing, and breakpoints,
rem is the right unit. px stays useful for borders, fixed icon sizes, and
anything where you genuinely want a fixed visual size regardless of user preferences.
The 62.5% trick
Many designers set html { font-size: 62.5%; } so that 1rem = 10px. This makes mental
math trivial — 1.6rem = 16px, 2.4rem = 24px. Use the 10 preset to see this scale.
Caveat: changing the html font size also changes how rem is interpreted by user-agent stylesheets and inherited components, which can subtly break form controls and third-party widgets. Use this trick with awareness.
rem vs em
- rem — relative to the root (html) element. Predictable: 1rem is always the same physical size on a given page.
- em — relative to the parent element's font size. Compounds when nested.
1eminside2eminside the root = 32px. Useful for component-relative sizing (e.g., padding scales with the component's font), but easy to lose track of.
Use rem for layout. Use em for typography spacing inside a component.
Bulk conversion
Paste a CSS block in the bulk-mode textarea. Every NNpx value (where NN is any number) gets
replaced with the rem equivalent. The regex (-?\d*\.?\d+)\s*px\b is intentionally precise to
avoid mangling values inside selectors or in 0px hacks.
FAQ
What about media query px values?
Bulk mode converts all px values, including those inside @media queries. Some teams
keep media queries in px (because they correspond to physical device widths) and only convert layout px to
rem. Manually edit the bulk output if that's your style.
Why does converting 17px give 1.0625rem?
17 ÷ 16 = 1.0625. Tools rounding to 2 decimals would show 1.06rem (which is actually 16.96px). This tool keeps 4 decimals to avoid drift.
Related tools
- Color Picker — Pick colors, convert hex/RGB/HSL/OKLCH, and check WCAG contrast.
- Aspect Ratio Calculator — Calculate aspect ratios (16:9, 4:3, 21:9, etc.). Given W:H and one dimension, get the other. Responsive padding-top % for CSS aspect-ratio containers.
- Flexbox Generator — Visual CSS Flexbox playground. Live preview + copyable CSS. Control direction, wrap, justify, align, gap. Example child layouts.
- CSS Grid Generator — Visual CSS Grid layout builder. Configure columns, rows, gaps, and named areas. Live preview with copyable CSS output.
Pillar
Part of Frontend & Design.
Written by Mian Ali Khalid. Last updated 2026-04-25.