During the year you pay the cuota of the bracket you chose based on your forecast. But the final bracket depends on your real income, and the difference is settled afterwards: the regularización.
How it works
The following year you file your renta.
The tax office tells Social Security your real income.
Social Security works out your computable net income and the bracket you should have paid.
It compares what you paid with what you owed.
If you paid too little
You are asked for the difference, with a deadline to pay it. If you earned much more than expected, it can be a large sum: with €45,000 of income and no expenses, the right cuota is €478.68 a month, against €401.47 with €30,000.
If you paid too much
Social Security refunds the excess automatically, without you having to ask.
How to avoid surprises
Check your income every quarter and change bracket if you drift from your forecast: you can do it up to six times a year.
Set aside each month what your real cuota will cost, not what you are paying.
Remember that deductible expenses lower your net income and, with it, your bracket.
With the calculator you can see which bracket matches your current income and expenses, and compare it with the one you pay.