How many characters can a standard post on X (formerly Twitter) contain?

How to get close to this number

The history gives you the answer. The original limit was derived from the SMS character budget minus room for a username, because the service was built to work over text messages. So if you know the SMS figure, you know the original limit — and the current one is simply double it, after a change made once the SMS constraint no longer applied.

The reason for doubling is a genuinely interesting piece of design research, and the explanation names it: people were hitting the wall constantly in some languages and almost never in others. A character limit is not language-neutral, because a character carries far more meaning in Japanese or Chinese than in English or Spanish. The same number was a tight constraint in one place and no constraint at all in another.

Note that the limit applies to a standard post; paid tiers allow much longer ones, links are counted as a fixed length regardless of their actual size, and some characters count as more than one. A "character limit" on a modern platform is a set of rules rather than a single number, which is why the figure is stamped with a year and a source.