Font Size Converter
Convert between CSS font size units — px, em, rem, pt, and % — with customizable base font size.
Browser default: 16px. Change if your :root has a different base.
em is relative to parent; rem is relative to :root
Common Font Size Reference
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Frequently Asked Questions
What is the difference between em and rem?
em is relative to the parent element’s font size. rem (root em) is relative to the HTML :root font size (typically 16px). rem is more predictable in nested structures because it doesn’t compound — 1rem always equals 16px (or whatever the root is), regardless of nesting.
Should I use px or rem for font sizes?
rem is preferred for accessibility — it respects user browser font size preferences. Users who set their browser to larger fonts get proportionally larger text. px overrides this. Use rem or em for font sizes, and px mainly for borders and spacing where exact pixel control matters.