Start from your reference pay: the higher of your average monthly gross over the last 12 months and over the last 3. Multiply it by a quarter for each year of service up to 10, and by a third for each year after that. A part year counts by complete months, and service runs to the end of your notice, whether you work it or not.
Your convention collective often gives more, particularly for cadres. The employer owes whichever is higher. Anything negotiated on top is the supra-légale part: it can still be tax-free, but it pays CSG and CRDS and pushes back the start of your unemployment benefit by up to 150 days (75 for an economic dismissal).
Under the 2026 rules, the indemnity is free of income tax up to the highest of the legal or conventional amount, twice last year's gross pay and half the indemnity, capped at €288,360 for the last two. It's free of social contributions up to €96,120. The 2027 budget bills would replace all of that with one ceiling of one PASS.