The calculation is done with the modified Bavarian formula and it requires:
1. the best possible grade Nmax,
2. the lowest grade Nmin at which you would still have passed, and
3. your grade Nd, which you received.
Your grade in the German system is then calculated as
x = 1+3*(Nmax-Nd)/(Nmax-Nmin)
The conversion is carried out by the Central Admissions Office at Humboldt-Universität zu Berlin. In individual cases, the calculation may differ from the modified Bavarian formula.
Example: In the Netherlands, the best possible grade is Nmax=10 and the lowest grade with which you can pass is Nmin=6. If your grade is Nd=8, then in the German system this corresponds to 1+3*(10-8)/(10-6) = 2.5.