A Finnish company has two numbers you will meet on invoices and in contracts. They get confused constantly, though the difference is simple.
The Business ID
The business and organisation identifier, the Y-tunnus. Seven digits, a hyphen and a check digit — 1927400-1, for example. It is issued when the company is entered in the Business Information System. Every registered entity has one: limited companies, partnerships, cooperatives, associations, foundations and sole traders alike.
It never changes — not on a change of name, domicile or ownership — and it is not reissued after the company is removed from the register. That is what makes it a reliable way to identify a company years later.
The VAT number
The VAT identifier, in the form FI plus the eight digits of the Business ID with the hyphen removed. Business ID 1927400-1 becomes FI19274001. Unlike in many countries, you do not have to look it up separately — it can be derived.
An important detail: that the number can be built does not mean the company is liable for VAT. The Tax Administration enters a business in the VAT register separately, and only that entry makes the identifier valid. Having a Business ID says nothing about it.
So
Everyone has a Business ID. A VAT number can be derived from anyone's Business ID, but it is only valid for those entered in the VAT register. The fact that someone quotes a VAT number therefore does not mean they are VAT-registered — that has to be checked separately.
How to check them
Check a Business ID in the Trade Register or in the Business Information System: enter it and see whether the company that comes back is the one you expected. The check digit means a typo shows up before you even search. Check a VAT number in VIES, which answers for the whole EU and tells you whether the number is valid right now. Both are free and take seconds.