The formula
net = gross − social contribution − tax withheld − other deductions
Where it comes from
A payslip subtracts two very different things from the gross. The social contribution is a fixed percentage funding pensions and unemployment cover; the withholding is an advance on income tax, settled when you file. That is why a high withholding is not money lost, while a high contribution genuinely is a cost.
How to work it out by hand
- Start from the gross annual salary
- Work out the social contribution as a percentage of the gross
- Work out the tax withheld on that same gross
- Subtract both and split the result across your instalments
What is worth knowing
The percentages are fields and not constants because they change from country to country, and within a country they change with the income band, the type of contract and family circumstances. Copy them from your last payslip: yours are there exactly, and with them the simulation is precise. The difference between withholding and contribution matters when negotiating: if a rise pushes the withholding up more, you are not earning less, that excess comes back when you file.