A French salarié's paycheck runs through several systems at once. 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). Assurance vieillesse (6.90% plafonnée up to one PASS, plus 0.40% déplafonnée on the full amount) and retraite complémentaire AGIRC-ARRCO (4.01% up to one PASS, 9.72% between one and eight PASS, including the CEG equalisation contribution) are withheld the same way, on every payslip, and both are also deductible from your taxable income. Income tax is different: it's calculated once a year on your net imposable (your gross minus CSG déductible and those cotisations, 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 still leaves out is a mandatory mutuelle d'entreprise contribution, a real deduction that further reduces your actual net-to-bank figure, but one whose premium and employer/employee split are set by each employer's own collective agreement rather than a single national rate, so it can't be modeled here without misleading precision.
