Written for Mexico. This analysis applies to Mexican federal tax law — ISR (income tax), IVA (VAT) and SAT rules — and cites Mexican statutes. Amounts are in Mexican pesos (MXN).
It's one of the most baffling surprises of the year: you're a salaried employee, "they already withhold everything," and yet your annual return comes out owing. It's not a punishment or a portal error — it's arithmetic, and it almost always has one of three explanations. The tool to understand it (and sometimes reverse it) is free and lives on the portal of the SAT (Mexico's tax administration): the payroll viewer.
Why a salaried employee can end up owing on the annual return
Your employer's monthly withholding (art. 96 of the LISR, the income tax law) is an advance on the annual tax, calculated with the progressive schedule of art. 152. It works perfectly when you have a single stable job all year. It breaks when: you had more than one employer (over the year or at the same time); you received extraordinary income that was under-withheld (a finiquito (final pay), a large bonus); or the employer misapplied the subsidy or an exemption. In all those cases, the sum of what was withheld falls below the real tax for the year — and that difference is your balance due. It didn't appear out of nowhere: it was latent all year.
Two employers in the year: the most common cause
This one deserves its own section because it's number one. When you work for two employers — whether at the same time or because you switched jobs mid-year — each one withholds as if theirs were your only income, starting from the low brackets of the schedule. But ISR is annual and cumulative: when you add the two salaries, your total income lands in a higher bracket, with a higher marginal rate. The combined withholding of the two employers falls short of that real tax, and the difference is charged on the annual return. That's why the law (art. 98, fracc. III LISR) requires you to file if you had two or more employers — it isn't optional. The payroll viewer shows you the two (or three) sets of pay stubs and what each one withheld, so you can see the gap with numbers.
The viewer: where you see what was actually withheld from you
Before accepting the annual return the SAT pre-fills for you, open the payroll voucher viewer with your RFC (federal taxpayer ID) and password. Verify that all your employers for the year are there, that the total ISR withheld is what you expect, and that the exempt earnings are right. If the balance due is real, you still have a legitimate lever: the personal deductions of art. 151 LISR — medical, professional and hospital fees, real interest on your mortgage, tuition (by decree), contributions to your personal retirement plan, major-medical insurance premiums. Entered correctly, they lower your base and can turn a balance due into zero, or a zero into a refund.
When you're required to file (and when it's in your interest)
As a salaried employee you must file an annual return if: you earned salary income over MXN $400,000 in the year; you had two or more employers (simultaneous or not); you also earned other income (professional fees, rental, interest); or you stopped providing services before December 31 and the annual calculation wasn't done for you. Outside those cases, filing is optional — but if you have personal deductions, it almost always pays, because that's how you recover the ISR that was over-withheld from you: in that case the result isn't a balance due, it's your refund. Checking the viewer first guarantees that refund is calculated on correct data.
Frequently asked questions
Am I required to file if I'm only a salaried employee?
Yes, in several cases: if you earned more than MXN $400,000 in the year, if you had two or more employers, if you also had other income, or if you stopped working before year-end without your employer running the annual calculation (art. 98 LISR). Beyond that it's optional — but if you have personal deductions, filing usually means a refund in your favor.
Why does having two jobs make me pay more?
Because each employer withholds separately, from the low brackets of the schedule, as if you were their only worker. The real ISR is on the sum of both salaries, which lands in a higher bracket. The combined withholding falls short of that annual tax, and the difference is your balance due. You're not overpaying: you're paying what wasn't withheld during the year.
Do personal deductions lower the balance due?
Yes, and they're your best legitimate lever. Medical, hospital and dental expenses, real interest on your mortgage, tuition, major-medical insurance premiums and contributions to your personal retirement plan are subtracted from your base (art. 151 LISR, with caps). Entered correctly, they can turn a balance due into zero or into a refund — as long as the payments are backed by a CFDI (digital tax invoice) and made through banking channels.
Does the viewer tell me how much I'll pay?
Not directly: the viewer shows you the inputs — salaries, ISR withheld, exemptions per employer — not the final calculation. The result, owing or in your favor, comes out when you assemble the annual return, adding up all your income and subtracting deductions. But without checking the viewer first, that calculation can start from wrong data; that's why it's step zero.
Your annual return comes out owing and you're not sure it's right?
Sometimes the balance due is real and is only optimized with well-assembled deductions; sometimes it's a stamping error by an employer that you have no reason to pay. Telling one from the other — and leaving your return at its optimal legal point — is a short review. Bring us your numbers and the viewer, and we'll resolve it.