Copy the exact rule
Record whether the field specifies characters, words, or bytes and whether spaces are included.
Leave a small margin
If the destination performs different Unicode normalization or URL shortening, an exact boundary can still fail. Leave room when the submission is consequential.
Inspect invisible whitespace
Trailing spaces and repeated line breaks may count even when they are hard to see.
Test the final destination
A general counter cannot emulate every private platform rule. Paste the final text into the real form before publication.
Run a repeatable preflight
Paste the final plain text rather than a screenshot or rich-text preview, choose whether whitespace belongs in the visible total, and enter the destination's published limit. Review the grapheme, word, line, UTF-16, and byte metrics together. If the text is over, shorten meaningful content first instead of deleting invisible characters at random. Recopy the edited version into a fresh field and count again so clipboard formatting or a stale selection does not hide the actual submission value.
Account for destination-specific transformations
Some systems replace line endings, normalize Unicode, expand templates, shorten links, reserve space for attachments, or count mentions and emoji with private rules. A counter cannot infer those transformations from the text alone. Leave a margin when failure would waste a campaign or submission window, and retain a small test containing the same kinds of emoji, accents, links, and line breaks. The result here is an independent measurement of the supplied string, not a promise that a platform will accept it.