In the UK, traders and businesses required to be registered for VAT purposes are issued with a VAT registration number by Her Majesty's Revenue and Customs (HMRC) in order to identify their registration. This number is not random or sequential, but actually includes a check digit1 to confirm that the number is genuine and correctly constructed. VAT numbers are issued in the form 123 4567 89. This however, is not a valid VAT number, so for the purposes of this Guide Entry the number 831 4958 16 will be used.
The check digit calculation appears quite complex, but is in fact surprisingly simple. The first seven digits of the VAT number are assigned a multiplier beginning with eight for the first digit and descending to two for the seventh.
8 3 1 4 9 5 8 (The first seven digits)
8 7 6 5 4 3 2 (The multiplier)
64 21 06 20 36 15 16 (The results)
After applying the multiplier, the results are totalled.
64+21+6+20+36+15+16 = 178
From the total, 97 is subtracted as many times as is necessary, until a negative, two digit number is produced.
178-97 = 81
81-97 = -16
This negative number becomes the check digit and relates to the last two digits of a valid VAT number.