Why higher interest rates can compress a bank’s margin
Higher rates raise bank income only to the extent that earning assets reprice ahead of funding. Deposit movement and refinancing can reverse an initial margin benefit.
Net interest margin is net interest income divided by average interest-earning assets. A rise in market interest rates increases that margin only when the resulting asset income grows faster than the relevant funding expense, after accounting for balance changes.
Fixed asset yields can meet resetting funding costs
Take a bank with $100 million of average earning assets yielding 5% and $90 million of interest-bearing funding costing 2%. Annual interest income is $5 million and interest expense $1.8 million. Net interest income is $3.2 million, giving a 3.2% net interest margin.
Suppose the assets remain fixed at 5% while the funding cost resets to 4%. Interest expense becomes $3.6 million. Net interest income falls to $1.4 million and margin to 1.4%, with balances held constant.
No borrower default is required for this compression. The bank continues receiving the contracted asset income while paying more to finance it.
Faster asset repricing can produce the opposite result
Now give the same bank floating-rate assets whose yield rises from 5% to 7% when funding resets from 2% to 4%. Annual interest income becomes $7 million and expense $3.6 million. Net interest income rises to $3.4 million.
The two banks experienced the same market-rate change. Their margin outcomes differ because their asset cash flows respond differently.
The comparison also depends on timing. Floating-rate loans can reset before term deposits mature, producing an initial benefit. That benefit can shrink when the deposits renew at higher prices. A quarterly margin records one interval in that sequence rather than the permanent effect of the rate change.
Funding cost changes through quantities as well as quoted rates
A bank need not raise the rate on every existing account to experience higher funding costs. Customers can move balances from low-rate transaction accounts to higher-rate deposits, withdraw them, or transfer them to another provider.
Replacing those balances with more expensive deposits or wholesale borrowing raises the weighted funding cost. The relevant measure is therefore the cost and quantity of each funding source, not one advertised deposit rate.
Service costs and liquidity needs also affect the economic value of deposits, even though they do not all appear in the net interest margin calculation.
Margin, valuation, and profit remain distinct
Higher rates can reduce the market value of fixed-rate assets while also changing interest income and expense. A valuation loss and a margin squeeze are different effects, with different accounting and cash consequences.
Credit provisions, operating costs, fees, and taxes further separate net interest margin from final profit. To explain a bank’s rate sensitivity, identify repricing dates, deposit behavior, replacement funding, and hedges. The direction of the policy-rate move alone does not determine the result.
Questions
- Do banks always benefit from higher interest rates?
- No. Funding costs can rise before asset income, particularly when fixed-rate assets are financed with liabilities that reprice sooner.
- Can funding costs rise without increasing every deposit rate?
- Yes. Customers can shift to more expensive account types or withdraw balances that the bank replaces with higher-cost funding.
- Is net interest margin the bank’s profit margin?
- No. Net interest margin excludes many operating costs, credit provisions, fee revenues, and taxes that affect overall profit.