CPF/CNPJ Validator

How modulo 11 catches errors

The last pair of digits in a CPF or CNPJ is not arbitrary: it is computed from all preceding digits by a weighted sum in modulo 11. Each base digit is multiplied by a weight that decreases from left to right; the products are summed, the remainder of division by 11 is taken, and the check digit is 0 when the remainder is less than 2, or 11 minus the remainder otherwise. The second digit is computed the same way, but already including the first check digit in the sum.

The purpose of this design is to detect typing errors: swapping one number for another, or transposing two neighboring digits, almost always breaks the weighted sum and makes the check digits stop matching. That is why CPF and CNPJ can flag 'this number is wrong' without querying any database.

• CPF: 11 digits (9 base + 2 check); weights 10..2, then 11..2.
• CNPJ: 14 digits (12 base + 2 check); weights 5,4,3,2,9,8,7,6,5,4,3,2, then 6,5,4,3,2,9,8,7,6,5,4,3,2.
• Digit rule: remainder = sum mod 11; digit = 0 if remainder < 2, else 11 − remainder.
• Sequences of equal digits are rejected even when they satisfy the math.

Computing the two check digits of a CPF

Example 1, base 111.444.777. For the first digit, multiply 1,1,1,4,4,4,7,7,7 by weights 10,9,8,7,6,5,4,3,2: the sum is 162. Since 162 divided by 11 leaves remainder 8 (which is ≥ 2), the first check digit is 11 − 8 = 3. For the second, repeat with the 10 digits (including the 3) and weights 11 down to 2: the sum is 204, remainder 6, so the second check digit is 11 − 6 = 5. The well-formed number is 111.444.777-35, a classic test value, and being well-formed does not make it real or registered.

Example 2, the power to catch a mistake. If someone types 111.444.777-36 by accident, the tool recomputes and expects 35: since the last entered digit (6) differs from the expected one (5), the result is 'invalid' by check digits. That is exactly what modulo 11 exists for, a single wrong digit is caught immediately.

Valid in the math ≠ existing in the registry

Validation is useful to check a CPF or CNPJ before submitting a form, strip the mask (dots, hyphens and slash) and catch typing errors early. The tool works on the raw digits, so it makes no difference whether you paste the formatted number or just the digits.

The limit is the most important thing to understand: passing the math only means the number is mathematically consistent, not that it exists, is active or belongs to anyone. A made-up number can be well-formed and still have no real record. For decisions that depend on identity, registry status, compliance or billing, the local check is only the first filter, confirmation requires an official lookup. Everything is processed in the browser: nothing is sent to the tax authority or stored.

How the check digit works once a CNPJ has letters

Brazil's tax authority (Normative Instruction RFB nº 2,229/2024 and Technical Note COCAD/SUARA/RFB nº 49/2024) starts issuing new CNPJ numbers in an alphanumeric format on July 31, 2026: the first 12 characters (8 for the root plus 4 for the order/branch) start accepting digits 0 to 9 or uppercase letters A to Z, while the last 2 characters, the check digits, stay numeric. Every CNPJ already issued remains 100 percent numeric and valid forever, the change only applies to new registrations from that date on.

The check digit math is still modulo 11 with the same weights as before (5,4,3,2,9,8,7,6,5,4,3,2 for the first digit and 6,5,4,3,2,9,8,7,6,5,4,3,2 for the second), only the way each character becomes a number before being multiplied by its weight changes: take the character's ASCII code and subtract 48. For a digit '0'-'9' that gives the digit's own value (0 to 9, same as always); for a letter 'A'-'Z' it gives a value from 17 to 42 (A=17, B=18, C=19 and so on).

Official worked example from Brazil's tax authority: for CNPJ 12.ABC.345/01DE, the base is 1,2,A,B,C,3,4,5,0,1,D,E. Converting (A=17, B=18, C=19, D=20, E=21) and applying the weights, the first digit's weighted sum is 459; 459 divided by 11 leaves remainder 8, so the first check digit is 11 minus 8, which is 3. Repeating with all 13 characters (including that 3) and the second digit's weights gives a sum of 424, remainder 6, so the second check digit is 11 minus 6, which is 5. Result: 12.ABC.345/01DE-35, exactly the example published by the tax authority. This validator recognizes the format automatically, with no need to pick CPF, numeric CNPJ or alphanumeric CNPJ by hand, and uppercase versus lowercase makes no difference, everything is normalized before the math runs.

Brazil's tax authority recommends against using the letters I, O, Q, U and F in real CNPJs, since they can look like digits or like each other. That is an issuing guideline, not a rule of the check digit math itself, so the validator still computes correctly even if one of those letters shows up, it just surfaces an informational note.

• Structure: 8 root characters + 4 order/branch characters (alphanumeric) + 2 check digits (always numeric).
• Character conversion: ASCII code minus 48 (digit '0'-'9' becomes 0-9; letter 'A'-'Z' becomes 17-42).
• Weights and remainder rule stay the same as numeric CNPJ: modulo 11, digit = 0 if remainder < 2, else 11 minus the remainder.
• An already-issued numeric CNPJ never changes, it stays valid forever.