Almost every piece of text you write online has a limit attached to it. Page titles get cut off in search results somewhere past 60 characters. Meta descriptions are trimmed after roughly 160. A post on X stops at 280. Ad headlines, SMS messages, product names in a marketplace listing, the bio field on a profile - all of them count, and most of them tell you only after you have gone over.
This counter updates as you type. There is no button to press. Paste or write, and the numbers move.
What each number actually measures
Characters counts everything, spaces included. This is the figure that matters for almost every platform limit, because that is what platforms count.
Characters without spaces is the number translators, typesetters and some academic submissions ask for. Rates are often quoted per thousand characters excluding spaces, so the difference between the two figures is money.
Words splits on whitespace. A hyphenated compound such as state-of-the-art counts as one word, which is the convention most word processors follow, so the number should match what you see elsewhere.
Sentences counts terminating punctuation - full stops, question marks, exclamation marks. Abbreviations such as e.g. will inflate the count slightly. It is a guide to rhythm, not a precise measure.
Paragraphs counts blocks separated by a blank line, which is how most editors and content management systems treat them.
Reading and speaking time use 225 and 130 words per minute. Reading speed varies enormously between people and subjects, but 225 is the figure most publishers settle on for general prose. The speaking estimate is the useful one if you are timing a script or a presentation.
The limit bars
Three bars track the limits people hit most often. Sixty characters is the practical ceiling for a page title before Google truncates it - the real cut-off is measured in pixels rather than characters, so treat 60 as a safe working number rather than a hard rule. One hundred and sixty covers meta descriptions. Two hundred and eighty is the post limit on X.
The bars turn amber as you approach the limit and red once you pass it, and the note underneath tells you exactly how many characters over you are. It is faster than counting, and considerably faster than publishing something and discovering the ending was cut off.
A note on emoji and non-Latin text
Counting characters is less obvious than it sounds once you leave plain English. Many emoji are built from several code points joined together, and some platforms count each of those separately while others count the visible symbol as one. Arabic, Hindi and Thai use combining marks that behave the same way. This tool counts what JavaScript counts, which matches most western platforms but can differ by one or two from a given service. If you are writing close to a hard limit in a non-Latin script, leave yourself a small margin.