Principle:
When calculating withholding tax according to the monthly model, the month is considered the tax period. The monthly gross income is decisive for the calculation of the tax deduction, i.e., all taxable benefits paid to the employee subject to withholding tax in the corresponding month are generally to be added up and subjected to withholding tax as a whole.
For the withholding tax deduction, the circumstances at the time the taxable benefit becomes due are decisive. Normally, the withholding tax owed is calculated based on the gross income of a month by multiplying the gross income by the tax rate according to the applicable withholding tax tariff. The same procedure applies if the employee is paid by the hour and the wages are paid monthly.
In individual cases, a calculation of the rate-determining income may be necessary. This is the case, for example, with multiple employment relationships, with the start or end of the employment relationship within a month, or with the allocation of working days abroad for taxpayers not resident in Switzerland.
More information can be found in Circular 45: Circular 45
Content
Methods for rate determination
Entry/exit within a month
In the event of commencing or terminating employment during a month, the gross income earned in the month of entry or exit must be extrapolated to 30 calendar days for the purpose of determining the rate-determining gross income. Only regular (periodic) income components (e.g., ordinary wages or family allowances) are converted, not irregular (aperiodic) components (e.g., bonus payments, gratuities, overtime or holiday compensation).
Example: The employee started on 15.08.2026, thus within the month and not on the 1st of the month. The defined monthly salary (for the full month) is CHF 5000. The employee is taxed under code "A0N" and resides in the canton of Zurich.
The salary is then automatically calculated by specifying the entry within the month.
The amount is extrapolated to the full month, resulting in the deduction. According to the tariff list of the Canton of Zurich, with code A0N and a monthly salary of CHF 5000, the corresponding deduction is due. This happens automatically, and you do not need to take any further precautions.
Multiple part-time employments
There are three different ways to determine the rate-determining income. If a person subject to withholding tax is simultaneously engaged in several employments, the rate-determining income for each employment relationship must be determined as follows:
1. Method:
Your employee is employed by other employers. You know the total percentage of employment your employee has with other companies:
Conversion of periodic benefits to the effective total employment rate of all employments (incl. substitute income) and addition of aperiodic benefits for the current month:
Example: Your employee works for you at a 50% employment rate. This employee has another employer where they work 40%. This results in a total employment rate of 90%.
Your employee earns a gross salary of CHF 2500 for their employment with you. Thus, the gross salary of CHF 2500 / 40 * 90 = 4500 is calculated. The rate-determining income is therefore 4500 Swiss francs.
You can store this in bexio under "Employees - Withholding Tax" in the additional information. There, you can select "Secondary employment" for the type of employment. Under "Information on further employments", select Total employment quota for all additional employers. Then you can specify the employment percentage, which your employee is employed overall by other employers.
If this has been entered correctly, you will now see that the withholding tax deduction is calculated based on the rate-determining income of CHF 4500.
2. Method:
Your employee is employed by other employers. You know what the gross salary is with the other employer.
Conversion to the actual total gross income, provided that the effective income is known or disclosed by the person subject to withholding tax.
Example: Your employee earns a gross salary of CHF 2500 from you. From the second employer, your employee earns another CHF 1500.
The rate-determining income is the total gross salary of your employee, specifically CHF 2500 + 1500 = CHF 4000.
You can store this in bexio under "Employees - Withholding Tax" in the additional information. There, you can select "Secondary employment" for the type of employment. Under "Information on further employments", select Monthly gross income of all additional employers. Then you can enter the gross salary as an amount that your employee earns overall from other employers.
If this has been entered correctly, you will now see that the withholding tax deduction is calculated based on the rate-determining income of CHF 4000. (See next screenshot: Excerpt from the tariff list of the Canton of Zurich)
3. Method:
Your employee is employed by other employers. However, you do not know the extent of their employment or how much they earn there.
Conversion of periodic benefits to a total employment rate of 100 percent and addition of aperiodic benefits for the current month, if the effective total employment rate is not disclosed by the person subject to withholding tax. This also applies if the person subject to withholding tax does not provide conclusive information about any other employments:
Example: Your employee earns a gross salary of CHF 2500 in a 50% employment with you. The income and employment rate with other employers are unknown to you.
In this case, the gross salary is extrapolated to a 100% employment rate.
Specifically: The gross salary is calculated as CHF 2500 / 50 * 100 = 5000. The rate-determining income is therefore 5000 Swiss francs.
You can store this in bexio under "Employees - Withholding Tax" in the additional information. There, you can select "Secondary employment" for the type of employment. Under "Information on further employments", select Unknown.
If this has been entered correctly, you will now see that the withholding tax deduction is calculated based on the rate-determining income of CHF 5000. (See next screenshot: Excerpt from the tariff list of the Canton of Zurich)