Before you can create a new Microsoft 365 tenant, you have to pick a name, and that name becomes your permanent yourname.onmicrosoft.com address. The catch is that every tenant name has to be unique across all of Microsoft 365, so the one you want may already be taken by another organization. This tool checks that for you in a second. Type in a name and it tells you whether the .onmicrosoft.com domain is still available or already in use. It's handy when you're setting up a brand-new Microsoft 365 or Office 365 tenant, planning a tenant rename, or just confirming whether a company already has an Entra ID (Azure AD) tenant under a given name. There's no sign-in and no setup. Just enter a name and check.
How this works
Every Microsoft 365 tenant gets a globally unique name.onmicrosoft.com domain, so a name can exist only once across all of Microsoft 365.
When you check a name, your browser asks a public Microsoft sign-in endpoint whether that tenant exists. If Microsoft returns the tenant's configuration, the name is taken; if it reports the tenant was not found, the name is available.
No sign-in, password, or private data is involved — this only reveals whether a tenant name is registered, which is public information. The check runs entirely in your browser, directly against Microsoft.
Reads only public Microsoft sign-in metadata. Not affiliated with or endorsed by Microsoft.