Why this does not claim to check availability
No platform offers a public way to ask whether a handle is free. Tools that claim a live check are almost always reading the HTTP status of a profile URL, which reports suspended, private, deactivated and rate-limited accounts identically to taken ones.
Rather than show a confident green tick that might be wrong, this validates what can be validated, the format rules, which are published and deterministic, and hands you the profile link to check for yourself.
The rules that catch people out
X allows only 15 characters and no full stops, which is usually the binding constraint when picking one handle for every platform. LinkedIn takes hyphens but not underscores; Facebook takes neither. Instagram rejects a handle that starts or ends with a full stop or contains two in a row.
The practical approach: pick something that satisfies the X rules and it will usually satisfy everyone else.
Claiming a handle across platforms
Register the same handle everywhere, even on platforms you do not plan to use yet. It costs nothing and prevents the situation where a client name is taken on the one network they later decide matters.
Threads is the exception worth knowing: it uses your Instagram handle and cannot differ from it.
Frequently asked questions
Can you tell me if a username is available?
Not reliably, and we would rather say so than guess. Availability has no public API, and inferring it from an HTTP status misreports suspended and private accounts as taken. The profile links here take you to the only authoritative answer.
What is the shortest username limit?
X, at 15 characters. It is also the strictest on characters - letters, numbers and underscores only. If a handle works on X it almost certainly works everywhere else.
Can my Threads handle differ from my Instagram one?
No. Threads accounts are tied to Instagram and share the handle. Changing one changes both.
What happens if the profile link 404s?
It usually means the handle is free, but not always. Deactivated and suspended handles can return a 404 while remaining unavailable to claim. The only certain test is trying to register it.