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Free PX to REM Converter — Bidirectional, Instant

Convert PX to REM (and back) instantly. Includes vw, bulk CSS replacer, and scale export for Tailwind & CSS variables.
Quick Answer

ZerofyTools PX ↔ REM Converter is a free, instant CSS unit calculator that converts pixels to REM, REM to pixels, and both to viewport width (vw) — all in your browser. Set a custom base font size, use the quick reference grid for common sizes, bulk-convert entire CSS files with one click, and export a full spacing scale for Tailwind or CSS custom properties.

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How do I use the PX ↔ REM Converter?

Convert PX to REM (and back) instantly. Includes vw, bulk CSS replacer, and scale export for Tailwind & CSS variables. This tool runs entirely in your browser — your files and data never leave your device. No registration, no watermarks, no limits.

01
Check the Quick Reference grid at the top for common sizes — click any tile to load it and copy the rem value instantly.
02
Type a value in any field (px, rem, or vw) and the other two update automatically.
03
Click "Advanced Settings" below the converter to change the base font size (default 16px) or viewport target width (default 1440px).
04
Paste your CSS into the Bulk CSS Replacer and click "Convert All" to replace every px value with rem in one shot.
05
Copy the Auto Scale Export snippet (Tailwind or CSS Variables) for a ready-to-paste spacing scale.

What are the key features of the PX ↔ REM Converter?

  • ✓Tri-directional conversion: PX ↔ REM ↔ VW simultaneously.
  • ✓Quick Reference grid with 12 common sizes — click any to load and copy.
  • ✓Advanced Settings: custom base font size (8–64px) with presets, custom viewport width with device presets.
  • ✓Bulk CSS Replacer: paste any CSS block and convert every px value to rem in one click.
  • ✓Auto Scale Export: generates a full spacing scale in Tailwind config or CSS custom properties format.
  • ✓100% client-side — no data is sent anywhere, works offline after load.

What is the PX to REM formula?

Divide the pixel value by the root font size. Every major browser defaults to 16px, so the standard formula is:

/* formula */
rem = px ÷ base font size
/* examples at base 16px */
16px ÷ 16 = 1rem
24px ÷ 16 = 1.5rem
12px ÷ 16 = 0.75rem

The Advanced Settings panel in the converter above lets you change the base from the default 16px to any value — useful if your project sets a custom html { font-size }.

What is a REM unit and why should you use it?

REM stands for Root EM. It is a CSS unit relative to the font-size set on the html element. Since all major browsers default that root size to 16px, 1rem = 16px unless overridden.

Accessibility
REM units scale with the user's browser font-size preference. Pixel-based layouts ignore that setting, which can fail WCAG 2.1 SC 1.4.4 (Resize Text).
Responsive scaling
One :root font-size override in a media query reflows every REM value at once — no hunting through hundreds of pixel declarations.
Consistency
REM always references the same root, unlike EM which stacks relative to each parent. 1.5rem means the same thing anywhere in your stylesheet.

PX to REM reference table (base 16px)

Common pixel values and their REM equivalents at the standard 16px browser default.

Pixels (px)REMPixels (px)REMPixels (px)REM
1px0.0625rem4px0.25rem10px0.625rem
2px0.125rem6px0.375rem12px0.75rem
3px0.1875rem8px0.5rem14px0.875rem
16px1rem18px1.125rem20px1.25rem
22px1.375rem24px1.5rem28px1.75rem
32px2rem36px2.25rem40px2.5rem
44px2.75rem48px3rem56px3.5rem
64px4rem72px4.5rem80px5rem
88px5.5rem96px6rem100px6.25rem

Using a non-standard base? Use the Advanced Settings in the converter above to change the base and all values update automatically.

EM vs REM — what is the difference?

EM
  • Relative to the parent element font-size
  • Values compound — nested elements multiply
  • Useful for component-level scaling
REM (recommended)
  • Relative to the root html font-size only
  • Values never compound — predictable everywhere
  • Best choice for global spacing and typography

Why use this tool and other frequently asked questions?

How do I convert px to rem?
Divide the pixel value by the base font size. For a standard 16px base: 24px ÷ 16 = 1.5rem. Use the converter above to calculate instantly — just type the px value and the rem result updates immediately.
What is a base font size and why does it matter?
The base font size is the root font size set on the <html> element (typically 16px in browsers). REM units are always relative to this value: 1rem = 1× base. Changing it with the Advanced Settings adjusts all conversions accordingly.
Why use REM instead of pixels?
REM units respect the user's browser font-size preference and make your layout accessible. When a user increases their default font size for readability, REM-based layouts scale proportionally — pixel-based layouts stay fixed and can become inaccessible.
What is the vw unit?
1vw equals 1% of the current viewport width. For example, at a 1440px viewport, 16px = 1.11vw. Viewport-relative units are commonly used for fluid typography and responsive spacing. The Advanced Settings let you change the viewport width for accurate vw calculations.
How does the Bulk CSS Replacer work?
Paste any CSS (e.g. from an existing stylesheet) into the Bulk CSS Replacer input. It scans every px value in the text and replaces it with the equivalent rem value using your current base font size, then outputs the entire converted block ready to copy.

Related Articles

Learn more with these guides from the ZerofyTools blog.

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May 19, 2026

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PX to REM Converter — Free, Instant, Bidirectional | ZerofyTools