Polish NIP

What it covers

Returns a valid Polish NIP (tax identifier) with a correct checksum. Use `format` to choose between hyphenated and digits-only rendering. Set `invalid=true` to get a NIP with a deliberately wrong check digit, for testing checksum validators.

A NIP (Numer Identyfikacji Podatkowej) is the ten-digit tax identifier the Polish Ministry of Finance issues to businesses. The first nine digits identify the taxpayer; the tenth is a mod-11 check digit.

How it’s calculated

The tenth digit is a weighted mod-11 check digit — equal to the weighted sum mod 11, with residue 10 unissued.

1×6 = 6
2×5 = 10
3×7 = 21
4×2 = 8
5×3 = 15
6×4 = 24
7×5 = 35
8×6 = 48
9×7 = 63
sum = 6+10+21+8+15+24+35+48+63 = 230
230 mod 11 = 10
residue is 10 → this prefix has no valid check digit and is rejected

Edge cases

• The first digit is drawn 1–9 — a NIP is never zero-padded, because tax-office prefixes don’t start with zero.
• Not every nine-digit prefix has a valid check digit: when the weighted sum is congruent to 10 mod 11, there is no issuable check digit and the prefix is discarded and redrawn. So the generated space is slightly smaller than all 9-digit prefixes.

Gotchas

• The NIP check digit is the weighted sum taken mod 11 directly — it is NOT “11 minus the remainder”. A validator that subtracts will reject every real NIP.
• Formatting (the dashes in 123-456-78-90) is cosmetic; the checksum runs on the ten bare digits.