Interest is charged on the amount of money borrowed from DEGIRO.
When you are trading on margin from DEGIRO, your Cash Balance will be negative. The negative balance is what you are charged interest on.
The unallocated interest rate is 4%, calculated annually and charged monthly.
It is possible to allocate the amount of Debit Money you intend to use under Allocation in your Settings. If you have allocated Debit Money, the interest charged will be 3%, charged on the full amount allocated regardless of how much is used. The chosen allocation will always be applicable for a full calendar month. This means that you can benefit from a lower interest rate but have less flexibility in the amount that you pay interest on.