A French salarié's paycheck runs through two entirely separate systems. CSG and CRDS are withheld directly on every payslip at 9.70% combined, calculated on 98.25% of your gross salary (the 1.75% abattement represents notional frais professionnels). Income tax is different: it's calculated once a year on your net imposable (roughly your gross minus the 6.80% CSG déductible, then reduced by a 10% abattement forfaitaire), run through the five-band barème progressif, then collected monthly via prélèvement à la source at a rate the DGFiP works out for you.
The décote is the mechanism that smooths the transition into paying tax at all: if your barème-computed tax (impôt brut) is below €1,982 for a single filer, the décote shaves it down, sometimes to zero, rather than letting a small crossing of the first taxable band suddenly cost you the full bracket amount.
What this calculator deliberately leaves out is the rest of a real payslip: cotisations for assurance vieillesse, retraite complémentaire AGIRC-ARRCO, and often a mandatory mutuelle d'entreprise contribution. Those are real deductions that further reduce your actual net-to-bank figure, but they sit outside the salarié income tax + CSG/CRDS scope this calculator models.