Opening an account costs 50 berries. The 50 berries used to open the account do not go towards the balance of stored berries. After opening the account, up to 10,000 berries may be stored in the bank at once.
Berries stored in the bank will accrue interest. At every half-hour mark of the pause menu's timer, the bank calculates and adds interest to the account. The interest rate is 2% of the current balance per half hour. After the account reaches 500 berries for the first time, Team Snakemouth is rewarded with the Platinum Card which permanently doubles the interest rate from 2% to 4%.
Interest is capped at 75 berries per half hour, which requires at least 1875 berries in the bank, after obtaining the Platinum Card.
Deposits, withdrawals, and the time of account creation do not effect interest calculation or timing whatsoever.
- When examining the banker, Vi will note that she believes they'll run off with the money to a hot spring, referencing Millibelle, the banker from Hollow Knight. 
- Before the 1.1 patch, the Bank offered 3% and 6% interest. There was also no interest cap.