The character cost of bold on X
X counts characters by weight. Plain Latin letters count as 1, and characters from most other Unicode ranges, including the bold letters this tool produces, count as 2.
So bold is expensive in a post. A post with a bold opening of 30 letters starts at around 60 of its characters. If you are close to the limit, the composer will say so: trim the bold part first, not the content.
Bold label, plain detail
𝐍𝐞𝐰: the free plan now includes scheduling
Where bold works on X
Posts, replies, quotes, DMs, your display name and your 160-character bio all accept bold Unicode, because they are plain text. Your handle does not.
Bold works best as a label: a bold "Thread", "Update" or a date at the start of a post helps it stand out in a busy timeline. It works worst on hashtags and keywords, because neither search nor hashtags recognise styled letters.
Keep the important part plain
Bold letters are separate characters, not formatting, so assistive technology and search treat them as symbols. A whole post in bold can be unreadable for someone using a screen reader, and it will not be found by people searching its words.
The practical rule: one bold word or short phrase per post, with the message itself in plain text.