How the check works
Instagram has no public "is this username available" API, so the only reliable, rules-friendly way to confirm a handle is to open its profile. This tool does the parts it can instantly - it checks your username against Instagram's rules and generates username variations - then links you straight to instagram.com/username for the definitive answer.
When you open the link, a profile that loads means the handle is taken. A missing-profile page means it may be available, but only Instagram can confirm that the username can be registered.
Instagram username rules
A username can be up to 30 characters and use only letters, numbers, periods and underscores. It cannot begin or end with a period, or contain two periods in a row. Usernames are not case-sensitive, so Brand and brand are the same handle.