What Is the RFC and What Is It For?
The RFC (Registro Federal de Contribuyentes) is the alphanumeric tax ID that Mexico's tax authority, the SAT, assigns to every individual or company operating in the country. Without an RFC you can't issue or receive a CFDI invoice, you can't register as a taxpayer with the SAT, and you can't complete banking or tax procedures that require proof of your tax identity. In short, it's your unique identifier for anything that creates a tax obligation in Mexico — selling, getting paid, issuing payroll receipts — and it's the first thing any of those processes require.
The RFC is not the same thing as the Constancia de Situación Fiscal, which is the certificate where the SAT prints your RFC alongside your tax regime, registered address, and current obligations. The RFC is just the ID string; the constancia is the document that certifies it.
What Is the RFC: Structure for Individuals and Companies
The RFC has a different length depending on the type of taxpayer, and that length is the fastest way to tell whether a given code belongs to a person or a business.
Individual RFC: 13 Characters
An individual's RFC is made up of:
- 4 letters taken from the full name: 2 characters from the first surname (its first letter plus its first internal vowel), 1 from the second surname, and 1 from the given name.
- 6 digits for the birth date, in year-month-day format.
- 3 homoclave characters, assigned by the SAT to prevent duplicates.
Company RFC: 12 Characters
A company's (persona moral) RFC is made up of:
- 3 letters taken from the legal business name.
- 6 digits for the date the company was incorporated.
- 3 homoclave characters.
That one-character difference — 13 versus 12 — exists because an individual's RFC pulls an extra letter from the person's name, while a company name is usually distinctive enough with just three initials to avoid collisions.
What the Homoclave Is and How It's Calculated
The homoclave is the last three characters of the RFC, and it's the part that trips up most people the first time they deal with Mexican invoicing. You don't choose it: the SAT calculates it with an internal algorithm that combines your personal data (or the company's) with the order in which you registered, specifically so two people with similar names and birth dates don't end up with the same RFC.
That's why, if someone hands you "just the first 10 characters" of their RFC assuming that's enough, you're actually missing the homoclave — and without it, the RFC is incomplete for invoicing purposes. A CFDI gets rejected if the RFC doesn't match the SAT's record exactly, homoclave included.
How to Get an RFC Step by Step
Registering for an RFC with the SAT works one of two ways, depending on your situation:
- Online pre-registration with your CURP. The SAT's portal lets you generate a pre-registration using your CURP (Mexico's national ID number), which gives you a provisional RFC while you finish the process.
- In-person appointment at a SAT office. To fully activate the RFC — and in most cases to get your e.firma or issue your first invoice — you need to book an appointment at citas.sat.gob.mx, pick the nearest office, and show up with your documents.
- Document submission at the office. SAT staff verify your identity and your data, then hand you your active RFC along with your Constancia de Situación Fiscal.
- Confirming your tax details. You check that your address, tax regime, and email are correct, since those are the details you'll later use to issue and receive CFDI invoices.
For companies, the process typically starts from the articles of incorporation and requires a legal representative to attend the appointment with the company's documentation.
Documents You Need to Register for an RFC
Before booking your appointment, have these ready:
- A valid CURP (or birth certificate if you don't have one yet).
- A valid government-issued ID (INE, passport, or professional license).
- A recent proof of address.
- An email address and phone number, since the SAT uses these for notifications about the process.
- For companies: the articles of incorporation and the legal representative's power of attorney.
Bringing complete documentation avoids having to reschedule, which is common when a proof document is missing or the address doesn't match what's on file.
The Generic RFC and Special Cases
Not every transaction requires the buyer's RFC. For sales to the general public, the SAT allows the generic RFC XAXX010101000, meant for consumers who don't request an invoice with their full tax details. There's also XEXX010101000 for transactions with foreign residents who don't have a Mexican RFC. Outside of these two generic codes, every CFDI must carry the buyer's real, complete RFC, homoclave included.
Why the RFC Is Essential for CFDI Invoicing
The RFC is the first thing the SAT validates when it receives a CFDI: if the issuer's or the recipient's RFC doesn't exist in the taxpayer registry, or if the registered name doesn't match exactly what's in the SAT's system, the invoice gets rejected before it's ever stamped. That's exactly what we cover in our guide to what a CFDI is: stamping an invoice depends on the issuer's and recipient's data — starting with the RFC — passing the SAT's real-time validation.
That's why, for businesses invoicing at volume, keeping an accurate, up-to-date RFC catalog for clients and vendors isn't a minor administrative detail — it's what keeps an invoice from falling apart right when a client needs it for their own accounting. Our CFDI invoicing service validates that tax data before stamping, so a mistyped RFC doesn't turn into a cancelled and reissued invoice.
Common RFC Mistakes When Invoicing
The most frequent RFC mistakes businesses run into are:
- Mixing up letter case or leaving stray spaces when entering an RFC; the SAT's system is more forgiving than it used to be, but it's still good practice to enter it exactly as it appears on the constancia.
- Using an RFC that's been deregistered or suspended, something you can confirm before invoicing directly on the SAT's portal.
- Pairing the wrong tax regime with an RFC, a related issue we cover in depth in our guide to SAT tax regimes.
- Not requesting an updated Constancia de Situación Fiscal from a client, especially after they change tax regime or address; our guide on what the Constancia de Situación Fiscal is explains how to get one and what it confirms.
- Invoicing with generic buyer data when the client actually wants their full RFC on the invoice, which forces you to cancel and reissue the CFDI.
Many of these mistakes keep happening because RFC entry is still a manual step, especially when a business has to invoice dozens of different clients on the same day — something we also cover in our guide to invoicing requirements in Mexico.
Frequently Asked Questions
How many characters does an individual's RFC have?
Thirteen: four letters from the name, six digits for the birth date, and three for the homoclave. That's one character more than a company's RFC.
How many characters does a company's RFC have?
Twelve: three letters from the legal business name, six digits for the incorporation date, and three for the homoclave.
Can I get an RFC without visiting a SAT office?
You can generate an online pre-registration with your CURP, but to fully activate it and use it on your first invoices you'll almost always need to attend an in-person appointment at a SAT office.
What exactly is the RFC's homoclave?
It's the last three characters, calculated by the SAT using an algorithm that combines your personal data with your registration order, so that two taxpayers with similar data don't end up with the same RFC.
What happens if I invoice with the wrong RFC?
The SAT rejects the CFDI at the moment of stamping if the RFC doesn't exist or doesn't match the registered name, so you'll need to correct it and generate the invoice again.
If your business invoices a lot of clients and validating each RFC by hand is already costing you time and cancelled invoices, at AISDC we build CFDI invoicing solutions that validate tax data before stamping and connect to your existing sales process.