Edit any field and the others update automatically, based on the base font size. rem is always relative to the root element's font size; em here is treated the same way for simplicity (in real CSS, em can compound based on nested font sizes).
All tools/Web & CSS
CSS Unit Converter
Convert between px, rem, em, and pt.
Rem and em use the base font size above (browser default is 16px). Points assume 96 CSS px per inch.