Italian Partita IVA

What it covers

Returns a valid Italian Partita IVA (business/VAT identifier) with a correct Luhn checksum. Use `format=vat` for the IT-prefixed EU VAT number. Set `invalid=true` for a Partita IVA with a deliberately wrong check digit.

How it’s calculated

The 11th digit of an Italian Partita IVA is a standard Luhn (mod-10) check digit computed over the first 10 payload digits.

Payload digits: 1 2 3 4 5 6 7 8 9 0. Process right to left, doubling the rightmost digit first.
Position 10, digit 0 (double): 0*2 = 0 -> 0
Position 9, digit 9 (keep): 9
Position 8, digit 8 (double): 8*2 = 16 -> 16-9 = 7
Position 7, digit 7 (keep): 7
Position 6, digit 6 (double): 6*2 = 12 -> 12-9 = 3
Position 5, digit 5 (keep): 5
Position 4, digit 4 (double): 4*2 = 8 -> 8
Position 3, digit 3 (keep): 3
Position 2, digit 2 (double): 2*2 = 4 -> 4
Position 1, digit 1 (keep): 1
Sum = 0+9+7+7+3+5+8+3+4+1 = 47
check = (10 - (47 mod 10)) mod 10 = (10 - 7) mod 10 = 3

Edge cases

• A payload that yields sum mod 10 == 0 produces a check digit of 0 (the outer '% 10' in (10 - (sum % 10)) % 10 maps the would-be 10 back to 0).
• Payloads are drawn freely with leading zeros allowed and significant, so an all-zero payload 0000000000 is a valid input; its Luhn sum is 0 and its check digit is 0.

Gotchas

• The doubling starts at the rightmost PAYLOAD digit because the check digit sits to its right; equivalently, in 1-indexed left-to-right terms over the 10-digit payload, the even positions (2,4,6,8,10) are the ones doubled.
• Overflow on doubling uses the subtract-9 rule (value -= 9 when value > 9), which is identical to summing the two digits of the product; the generator passes no custom modulus or complement — it relies on the shared luhnCheckDigit exactly as-is.
• The generator only computes/validates the mod-10 check digit and does not verify that the number belongs to a registered company; values are synthetic.