How does CPF validation work?

A CPF is 11 digits: 9 base digits plus 2 check digits. Each check digit is computed with a weighted MOD-11 sum over the preceding digits. The first check digit weights d0…d8 by 10 down to 2; the second weights d0…d9 by 11 down to 2. For each: take the sum modulo 11 — if the remainder is 0 or 1 the check digit is 0, otherwise it is 11 minus the remainder. The CPF is valid only when both recomputed digits match the entered ones, and the number is not an all-identical sequence.

Why does the CPF have two separate check digits instead of one?

The Receita Federal designed the two-digit scheme to dramatically reduce the probability that a random typo produces a valid-looking number. A single check digit catches roughly 90 % of single-character errors; a second independent digit cuts the residual false-positive rate to about 1 in 100, making the CPF robust enough for high-stakes financial and legal documents.

Why are CPFs like 111.111.111-11 rejected even though the maths works?

The MOD-11 formula happens to produce consistent check digits for any all-identical 9-digit base (a consequence of the arithmetic properties of 11). The Receita Federal therefore explicitly blacklists all eleven such sequences — 000.000.000-00 through 999.999.999-99 — so they can never be assigned to a real person. This tool rejects them before running the algorithm.

What does the fiscal region digit tell me?

The 9th digit (d8, zero-indexed) encodes which Receita Federal regional office issued the CPF: 1=SP, 2=MG, 3=RJ/ES, 4=RS (modern), 5=BA/SE, 6=PR/SC, 7=Amazônia, 8=Centro-Oeste/DF, 9=Nordeste, 0=RS (legacy). This is the region of original registration, not the holder current address — people move, but their CPF regional digit never changes.

Does a structurally valid CPF confirm the number belongs to a real person?

No. The MOD-11 check only confirms the 11-digit sequence is internally consistent. It cannot tell you whether the CPF is registered with the Receita Federal, whether the holder is alive, or whether the number has been cancelled or suspended. Only the Receita Federal public consultation service can confirm registration status.

Is it safe to enter a real CPF into this tool?

The validation runs entirely in your browser — no data is ever sent to a server, stored, or logged. That said, we recommend using the obviously-fake example number provided (529.982.247-25) for testing and demos, and avoiding pasting real CPFs into any online tool as a general hygiene practice.

Brazil CPF Validator

The CPF (Cadastro de Pessoas Físicas — Individual Taxpayer Registry) is Brazil’s national identification number, issued by the Receita Federal. Every Brazilian resident and many foreigners conducting financial transactions in Brazil hold one. The CPF appears on bank accounts, tax returns, property deeds, employment contracts, health insurance cards, and e-commerce checkouts — making it one of the most frequently validated strings in Brazilian software.

This tool recomputes both MOD-11 check digits from scratch, compares them to the digits you entered, shows you exactly which step failed (if any), decodes the fiscal region embedded in the 9th digit, and lets you step through the full arithmetic. Everything runs locally — your CPF never leaves your device.

Structure of a CPF

A CPF is always 11 digits, conventionally formatted as NNN.NNN.NNN-DD:

Position	Digits	Meaning
d0 – d7	First 8	Base number
d8	9th digit	Fiscal region code
d9	10th digit	First check digit
d10	11th digit	Second check digit

The dots and the dash are purely cosmetic separators. Internally the Receita Federal stores and processes the raw 11-digit string.

The MOD-11 algorithm — step by step

First check digit (d9)

Take the 9 base digits (d0 through d8).
Multiply each by a descending weight starting at 10: d0×10 + d1×9 + … + d8×2.
Compute sum mod 11.
If the remainder is 0 or 1, the check digit is 0. Otherwise the check digit is 11 − remainder.

Second check digit (d10)

Take the same 9 base digits plus the first check digit computed above (d9).
Multiply each by a descending weight starting at 11: d0×11 + d1×10 + … + d8×3 + d9×2.
Compute sum mod 11.
Same rule: remainder 0 or 1 → digit is 0; otherwise 11 − remainder.

The key design detail is that d9 itself feeds into the second sum. This creates a chain between the two check digits — if you flip a single digit anywhere in positions d0–d9, the mismatch will show up in d10 even if d9 happened not to change.

Worked example using the obviously-fake CPF 529.982.247-25

Base digits: 5 2 9 9 8 2 2 4 7

d9 calculation:

5×10 + 2×9 + 9×8 + 9×7 + 8×6 + 2×5 + 2×4 + 4×3 + 7×2
= 50 + 18 + 72 + 63 + 48 + 10 + 8 + 12 + 14
= 295
295 mod 11 = 9   →   11 − 9 = 2   ✓ matches the entered 2

d10 calculation (base digits + d9 = 2):

5×11 + 2×10 + 9×9 + 9×8 + 8×7 + 2×6 + 2×5 + 4×4 + 7×3 + 2×2
= 55 + 20 + 81 + 72 + 56 + 12 + 10 + 16 + 21 + 4
= 347
347 mod 11 = 6   →   11 − 6 = 5   ✓ matches the entered 5

Both digits match → 529.982.247-25 is a structurally valid CPF.

The fiscal region digit

The 9th digit (d8) is not random — it encodes which Receita Federal regional superintendency originally issued the CPF:

d8	Region
1	São Paulo (SP)
2	Minas Gerais (MG)
3	Rio de Janeiro / Espírito Santo (RJ / ES)
4	Rio Grande do Sul — modern series (RS)
5	Bahia / Sergipe (BA / SE)
6	Paraná / Santa Catarina (PR / SC)
7	Amazônia — AM, PA, AC, RR, AP, RO
8	Centro-Oeste / Brasília — GO, MT, MS, DF, TO
9	Nordeste — MA, PI, CE, RN, PB, PE, AL
0	Rio Grande do Sul — legacy series (RS)

This digit is fixed at registration. Brazilians who move between states keep the same CPF, so a person with d8 = 3 living in Manaus still has a Rio de Janeiro / ES CPF — the digit reflects origin, not current location.

Privacy and limitations

Privacy: the entire calculation runs in JavaScript in your browser tab. No number you enter is transmitted, logged, or stored anywhere.

Limitations: structural validity is not the same as existence in the Receita Federal database. A CPF that passes this validator may be unregistered, suspended, cancelled, or belong to a deceased person. Only the Receita Federal public consultation confirms live registration status.