Net interest margin: calculate NIM on a consistent basis

Worked example: $40 million of net interest income

Assume a hypothetical bank earns $60 million of interest income and incurs $20 million of interest expense over a full year. With $1 billion of average earning assets, NIM is ($60m − $20m) / $1,000m = 4.0%. These invented amounts illustrate the arithmetic; they are not a benchmark for a particular bank or reporting period.

Annualize quarterly income without annualizing the balance twice

If a quarter has $15 million of interest income, $5 million of interest expense and $1 billion of average earning assets, the unannualized margin is 1.0%. Multiplying the quarterly income by four gives an approximate annualized margin of 4.0%. A published figure may use an actual-day convention instead. Follow that convention when reproducing it; do not multiply an already annualized numerator or the average asset balance by four.

• Distinguish standalone-quarter figures from year-to-date income. For a second-quarter filing, a six-month income total is not a three-month total.
• Find the average-balance and yield table. Beginning-plus-ending balances divided by two are only an approximation when a more detailed average is unavailable.
• Do not replace earning assets with all assets: premises, goodwill and other non-earning assets do not belong in the same denominator.
• Keep the reporting currency and units consistent. If average earning assets are missing or zero, do not manufacture a valid margin.

Why NIM and net interest spread can differ

Continue the annual example and assume average interest-bearing liabilities of $800 million. The earning-asset yield is $60m / $1,000m = 6.0%; the rate paid on interest-bearing liabilities is $20m / $800m = 2.5%. Net interest spread is 3.5 percentage points, while NIM is 4.0%. The denominators differ. Non-interest-bearing deposits and equity can help fund earning assets, but they are not evidence that operating a bank is cost-free.

Compare the same accounting basis and reporting period

There is no universal good-NIM threshold. Compare a bank with its own history and peers with similar lending and funding models. Higher loan yields may come with higher credit losses; a fee-oriented bank may earn income that NIM does not capture. Read provisions, noninterest income, expenses and capital measures alongside the margin.

• Check tax-equivalent treatment. For example, LCNB’s 2025 financial highlights define NIM using taxable-equivalent net interest income. Do not mix that numerator with an unadjusted peer figure without explaining the difference.
• Preserve the period and population when citing an industry average. The FDIC Quarterly Banking Profile provides dated industry data rather than a permanent target range.
• The FRED USNIM series is discontinued and ends at Q3 2020. It is a historical source, not a current U.S. banking benchmark; FRED links to the FDIC Quarterly Banking Profile release for alternative data.

A rate change does not determine the next margin by itself

The OCC describes interest-rate exposure as arising from assets and liabilities with different rates and maturities. A rise in market rates can help net interest income if asset yields reprice faster than funding costs, but deposit competition, borrower prepayments, hedges and changes in balance-sheet size can alter the result. An inverted yield curve alone does not establish that every bank’s next reported NIM will fall.

• Read the bank’s disclosed sensitivity scenario and its time horizon, rather than treating a rate-rise forecast as an earnings forecast.
• Separate changes in yield, funding rate and balance mix. A rising NIM can coincide with falling net interest income if average earning assets shrink enough.