Your Portuguese payslip runs through two separate calculations. Social Security (Segurança Social) is a straightforward 11% of your full gross salary, with no cap at all, withheld directly every payslip. IRS is more involved: your gross income first gets a dedução específica subtracted (€4,587.09 for most salaried employees, or your actual Social Security contributions if that's higher), producing your rendimento coletável, which then runs through nine progressive escalões from 12.5% up to 48%.
The dedução específica crossover matters: below roughly €41,700 gross, everyone gets the same flat €4,587.09 deduction regardless of their actual Social Security paid. Above that threshold, since 11% of gross exceeds the flat amount, the deduction simply equals your real contribution, so the two figures track together from that point on.
What this calculator deliberately simplifies is the mínimo de existência, a statutory floor that fully or partially shields lower incomes from IRS. The full formula phases out gradually up to about €14,641 gross; this calculator applies the headline rule (full exemption at or below €12,880), which is accurate for every salary level this tool is aimed at.