Convert pixels to rem and back at any root font size, with a reference table for the sizes you actually use.
Common sizes at a 16px root
Why bother: a reader who has raised their browser’s default text size scales anything sized in rem, and nothing sized in px. That is the accessibility reason to convert, not style preference.
Count words, characters, sentences and paragraphs, and see how long the text takes to read or say out loud.
Compare two versions of a text and see exactly what was added and removed — line by line, or word by word for prose.
Every standard paper size in millimetres, inches and pixels — ISO A, B and C, US Letter and Legal, ANSI, architectural and JIS B.
Convert between Markdown, HTML and plain text — headings, bold/italic, links, lists, images, blockquotes and code all carry across.