How can my account be overdrawn when I just made a deposit?
Many transactions are processed overnight. These transactions may not be reflected in an available balance.
Thus it's important to keep a current and accurate check/transaction register and balance it to your monthly statement. A bank's online, telephone, or ATM balances are for information purposes only—they do not replace your check/transaction register.
On checking accounts, banks generally post deposits before withdrawals. However, there are no laws requiring national banks to do this. In addition, banks may establish a cutoff time for deposits made at a branch or through an ATM. Deposits made after that time may be treated as having been made on the following business day.
For example, a deposit made after the Friday afternoon cutoff time would be treated as if it were made on the following Monday. So any items with next-day availability would then be available the next day (Tuesday).