What is the formula to convert rem to em?

em = rem × (root font size ÷ parent font size). When the root and parent font sizes are equal, 1rem equals 1em.

When does 1rem equal 1em?

When the parent element font size equals the root font size. At the common browser default of 16px for both, 1rem = 1em.

Why would rem and em differ?

If a parent element sets its own font size (e.g. font-size: 20px) while the root stays at 16px, then 1rem = 0.8em inside that parent, because em inherits the parent's context.

What is the default browser root font size?

Most browsers default to 16px. A common pattern sets html { font-size: 62.5% } to make the root 10px, so 1rem = 10px.

Is any data sent to a server?

No. All conversion happens in your browser using plain arithmetic. Nothing is sent anywhere.

rem to em Converter

Convert CSS rem values to em given the root font size and parent element font size.

Rem to Em Converter

Rem value

Root font size (px)

Parent font size (px)

Result 1.0000 em

Formula: em = rem × (root px ÷ parent px)

Reverse: Em to Rem

Em value

Root font size (px)

Parent font size (px)

Result 1.0000 rem

Formula: rem = em × (parent px ÷ root px)

Reference Table

Common rem values converted to em at 16px root / 16px parent (ratio 1:1 at these defaults).

Rem	Em (16px/16px)	Pixels

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Summary

Convert CSS rem values to em given the root font size and parent element font size.

How it works

Enter the rem value you want to convert.
Set the root (html) element font size in pixels (default 16px).
Set the parent element font size in pixels (default 16px).
The converter calculates: em = rem × (root px ÷ parent px).
Use the reverse section to convert em back to rem.

Use cases

Find the em equivalent of a rem value when working inside a component with a non-default font size.
Audit third-party CSS that mixes rem and em units to ensure consistent sizing.
Migrate a rem-first stylesheet to em for a scoped component with its own font-size context.
Verify that nested em values match the intended rem-based design token.

Frequently Asked Questions

Last updated: 2026-09-19 · Reviewed by Nham Vu