A VAT number is a government given number that is used to identify value added taxpayers. VAT numbers are given to each and every individual or establishment that deals with goods on which VAT is payable and is used in the filing of VAT returns to identify these establishments.
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Your VAT number should include two letter numbers that identify your member state, like GB for United Kingdom, then other alphanumeric numbers that should add up to 15 characters. Your VAT number will be calculated with standing VAT regulations.
VAT is used to refer to value added tax. It is a sales tax levied on the sale of goods and services. To know your VAT number there are websites where you fill a form with your details and then the number will be shown. You can also visit the VAT department and ask.