Deposit calculator
How high may your rental deposit be? § 551 BGB caps it at three months' net cold rent.
How the deposit limit works
Under § 551 (1) BGB the security deposit may not exceed three times the monthly rent — the net cold rent (Nettokaltmiete), without operating-cost prepayments. Under § 551 (2) you may pay it in three equal monthly instalments; the first is due at the start of the tenancy.
A clause demanding more than three months' rent, or the full amount up front, is void to that extent.
Check your whole rental contract
The deposit is one clause. Upload your contract for a full clause check.