What is the difference between rem and em?

rem is always relative to the root (html) font size; em is relative to the current element's own font size. This tool treats both the same way, off the base size you enter, since that is the common case (no nested em scaling).

Why does the base font size matter?

rem and em are both relative units - 1rem equals the base font size in pixels, so the same rem value converts to a different pixel value depending on that base.

CSS Unit Converter (px, rem, em)

Convert between CSS px, rem and em units for any base font size.

Value

Unit

Base font size (px)

px 16px

rem 1rem

em 1em

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How it works

1
Enter a value and its unit

px, rem or em.
2
Set the base font size

Defaults to the common 16px browser default.
3
See all three units at once

px, rem and em, converted instantly.

FAQ

What is the difference between rem and em?: rem is always relative to the root (html) font size; em is relative to the current element's own font size. This tool treats both the same way, off the base size you enter, since that is the common case (no nested em scaling).
Why does the base font size matter?: rem and em are both relative units - 1rem equals the base font size in pixels, so the same rem value converts to a different pixel value depending on that base.