I grabbed this from the EU website:
http://europa.eu.int/comm/taxation_customs/vies/en/faqvies.htm#item11
Q11: Is it possible to know the algorithms used by Member States in the construction of their VAT identification numbers?
The European Commission cannot divulge these algorithms. However, the structure of VAT identification numbers is given in the table below.
VAT identification number structure
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Member State Structure Format*
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AT-Austria ATU999999991 1 block of 9 characters
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BE-Belgium BE999999999 or
BE09999999992 1 block of 9 digits or
1 block of 10 digits 3
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CY-Cyprus CY99999999L 1 block of 9 characters
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CZ-Czech Republic CZ99999999 or
CZ999999999 or
CZ9999999999
1 block of either 8, 9 or 10 digits
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DE-Germany DE999999999 1 block of 9 digits
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DK-Denmark DK99 99 99 99 4 blocks of 2 digits
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EE-Estonia EE999999999 1 block of 9 digits
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EL-Greece EL999999999 1 block of 9 digits
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ES-Spain ESX9999999X4 1 block of 9 characters
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FI-Finland FI99999999 1 block of 8 digits
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FR-France FRXX 999999999 1 block of 2 characters, 1 block of 9 digits
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GB-United Kingdom GB999 9999 99 or
GB999 9999 99 9995 or
GBGD9996 or
GBHA9997 1 block of 3 digits, 1 block of 4 digits and 1 block of 2 digits; or the above followed by a block of 3 digits; or 1 block of 5 characters
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HU-Hungary HU99999999 1 block of 8 digits
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IE-Ireland IE9S99999L 1 block of 8 characters
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IT-Italy IT99999999999 1 block of 11 digits
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LT-Lithuania LT999999999 or
LT999999999999 1 block of 9 digits, or 1 block of 12 digits
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LU-Luxembourg LU99999999 1 block of 8 digits
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LV-Latvia LV99999999999 1 block of 11 digits
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MT-Malta MT99999999 1 block of 8 digits
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NL-The Netherlands NL999999999B998 1 block of 12 characters
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PL-Poland PL9999999999 1 block of 10 digits
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PT-Portugal PT999999999 1 block of 9 digits
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SE-Sweden SE999999999999 1 block of 12 digits
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SI-Slovenia SI99999999 1 block of 8 digits
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SK-Slovakia SK9999999999 1 block of 10 digits
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Remarks:
*: Format excludes 2 letter alpha prefix
9: A digit
X: A letter or a digit
S: A letter; a digit; "+" or "*"
L: A letter
Notes:
1: The 1st position following the prefix is always "U".
2: The first digit following the prefix is always zero ('0').
3: The VAT number of a Belgian trader can appear in any of these two formats. The (new) 10-digit format is the result of adding a leading zero to the (old) 9-digit format.
4: The first and last characters may be alpha or numeric; but they may not both be numeric.
5: Identifies branch traders.
6: Identifies Government Departments.
7: Identifies Health Authorities.
8: The 10th position following the prefix is always "B".