Deposits provide funding that banks can use to support loans, securities, payments and liquidity. Measuring their cost requires more than reading the rate on one savings account because different products, balances and operating needs behave differently over time.
Interest expense is the starting point
A bank records the interest it pays on savings, money market and time-deposit balances. Dividing that expense by an appropriate average balance produces an average interest cost for the period, which helps explain how existing deposit funding affected earnings.
The calculation depends on the population and time period being measured. A product-level rate, the cost of total interest-bearing deposits and the cost of all deposits answer different questions and should not be used interchangeably.
Deposit mix changes the blended result
Transaction accounts, savings products, certificates of deposit and large commercial balances can have different rates, terms and sensitivities to market conditions. A shift toward higher-rate products can raise the bank's blended cost even if no individual product rate changes.
Noninterest-bearing deposits do not create direct interest expense, but they are not necessarily free. The bank still operates accounts, processes transactions, provides service and maintains the technology and controls that support the relationship.
Average and marginal cost serve different decisions
Average cost describes the funding already on the balance sheet over a period. Marginal cost estimates what the next dollar of funding would cost through a new or repriced deposit, a borrowing or another available source.
That distinction matters when a bank prices a loan or deposit campaign. A low historical average can hide the higher current cost of attracting or retaining incremental balances, while a short-term promotional rate may not represent the relationship's longer-run economics.
Behavior and operating costs complete the view
Banks study how balances respond to rate changes, how concentrated they are and how long they are likely to remain. A deposit with a slightly higher rate may still provide useful value if its behavior is dependable, while a low-rate balance can create liquidity risk if it is highly concentrated or quick to leave.
An all-in view can also include acquisition, servicing, insurance, network, branch, technology and liquidity costs. The purpose is not to assign one universal number, but to make the assumptions behind product and funding decisions visible.
Cost is considered with liquidity and customer value
Management compares deposit cost with other funding options, asset yields, liquidity needs, interest-rate risk and the services attached to the relationship. The cheapest apparent source is not automatically the safest, most stable or most appropriate one.
Reporting therefore tracks both the measured result and the drivers behind it, including rate changes, product mix, concentrations and deposit movement. That context helps leaders distinguish a deliberate funding choice from a cost increase caused by unexpected behavior.
Read the primary material
Banking Explained prioritizes regulators, official publications and first-party announcements.