Mexican federal ISR withholding on ordinary salary runs through the SAT's tarifa mensual, 11 progressive bands for 2026 from 1.92% up to 35%. SAT publishes each band as a cuota fija (a fixed amount) plus a marginal percentage applied to whatever your income exceeds that band's límite inferior, a shortcut that's mathematically identical to summing the tax owed across every lower band, which is exactly how this calculator computes and displays it.
Because the tarifa is defined monthly, this calculator takes your annual salary, divides by 12, runs it through the monthly bands, and multiplies the result back up to an annual figure, matching how ISR is actually withheld from a real payroll cheque rather than approximating it as a single annual bracket lookup.
What this calculator deliberately leaves out is IMSS (Instituto Mexicano del Seguro Social), the mandatory social-security contribution. Unlike ISR, IMSS isn't one flat rate, it's split across several sub-items with different bases and caps tied to the UMA (Unidad de Medida y Actualización), and approximating it risks a misleading number. This tool models ISR only and says so clearly rather than guessing at IMSS.