For the calculation of the withholding tax deduction, ratifications, 13th monthly salary, bonus, loyalty premiums (e.g. seniority present) and similar benefits are – at the time of the effective payment - added to the gross income of the according month.
Example canton of Zurich (2009):
An unmarried employee without children earns a monthly CHF 4,000 (gross wage) from January to November. In December, he additionally receives the 13th monthly salary of CHF 4,000. His December salary therefore sums up to 8,000. Withholding tax for January to November is – calculated for CHF 4,000 – CHF 188 per month. The December salary of CHF 8,000 is decisive for the December withholding tax deduction. It amounts to CHF 774 for December.
