Estonian KMKR

What it covers

Returns a valid Estonian KMKR (käibemaksukohustuslase number, VAT number) with a correct check digit. Use `format=vat` for the `EE`-prefixed intra-EU VAT identifier. Set `invalid=true` for a KMKR with a deliberately wrong check digit.

How it’s calculated

The 9th digit of an Estonian KMKR (VAT registration number) is a weighted mod-10 check digit computed over the first eight digits with the repeating weight pattern 3, 7, 1.

Payload digits: 1,9,2,8,3,7,4,6; weights: 3,7,1,3,7,1,3,7.
Weighted products: 1×3=3, 9×7=63, 2×1=2, 8×3=24, 3×7=21, 7×1=7, 4×3=12, 6×7=42.
Sum S = 3+63+2+24+21+7+12+42 = 174.
r = 174 mod 10 = 4.
Check digit c = (10 - 4) mod 10 = 6.
Cross-check (full-number rule, weights 3,7,1,3,7,1,3,7,1): 174 + 6×1 = 180; 180 mod 10 = 0. Valid.

Edge cases

• When the weighted sum of the eight payload digits is already ≡ 0 mod 10, the check digit is 0 (the outer mod-10 collapses the would-be '10' to '0'); it never spills into two digits, so no prefix is ever redrawn.
• The common generator draws d1 in 1-9 (no leading zero); a leading-zero first digit is deliberately reserved for the separate edge-case generator.

Gotchas

• The check digit uses the repeating weight pattern 3,7,1 (NOT the ICAO/Luhn family). Only eight weights [3,7,1,3,7,1,3,7] are applied to the payload; the 9th position's implicit weight is 1 and appears only in the full-number validation view Σ wᵢ·dᵢ ≡ 0 (mod 10).
• The 'EE' VAT prefix is a formatting decoration, not checksum input — compute the check digit on the nine numeric digits only.
• Corruption for edge/invalid samples adds an offset 1..9 mod 10 to the correct check digit; because the offset is never 0 mod 10, the mutated digit is guaranteed to fail the weighted-mod-10 check.