How to calculate net interest margin (NIM)

Net interest margin is net interest income divided by average earning assets, annualized. A bank earning $40 million of net interest income on $1 billion of average earning assets has a 4.00% NIM. The inputs are where banks differ: which assets count as earning, how averages are taken and whether income is tax-equivalent.

NIM is the first number most boards, analysts and ALCO members check. A few basis points of difference between two reports can start a long meeting.

The gray area

NIM isn't defined by accounting standards, so banks make judgment calls:

1. Tax-equivalent or not? Tax-exempt municipal income is often grossed up to a fully tax-equivalent (FTE) basis so it compares with taxable income.
2. Which assets earn? Do nonaccrual loans stay in average earning assets?
3. Which balances for securities? Amortized cost, or fair value including unrealized gains and losses?
4. How are averages taken? Daily averages, or the average of month-end balances?
5. Where do loan fees go? Deferred origination fees and costs are amortized into interest income as a yield adjustment. Are other fees included?
6. How is it annualized? By actual days in the period, or by multiplying a quarter by four?

Core principles

1. Write the rules down once and apply them to every period and every report.
2. Label the variants. Reported NIM and FTE NIM are both valid. Mixing them in one deck isn't.
3. Use daily averages where the data allows. Month-end averages can be skewed by large balances that arrive or leave mid-month.
4. Keep nonaccrual treatment consistent with how the bank reports credit quality.

Calculating NIM in one system

1. Pull interest income and interest expense for the period from the GL.
2. Add the FTE adjustment if you report on that basis: tax-exempt income divided by (1 minus the tax rate), minus the original tax-exempt income.
3. Pull average earning assets from the core and investment systems: loans, securities, interest-bearing balances at other banks and fed funds sold.
4. Divide net interest income by average earning assets and annualize by days in the period.

Calculating NIM across systems

In practice, interest income comes from the GL, loan balances from the core, securities from an investment accounting system and forecasts from the asset-liability model. Each can hold its own version of earning assets. The ALCO model's NIM often differs from the reported NIM because it uses different averaging or excludes nonaccrual loans. Pick which system is the source for each input, apply the same rules to all of them, and document where the ALCO figure is meant to differ.

How Preql helps

Preql maps how your GL, core and other systems define each NIM input. Finance approves one definition for each, with an owner, version history and lineage back to source, so the board and ALCO can see exactly which NIM they're looking at.

FAQ

Should NIM be reported on a tax-equivalent basis?

Many banks report both. Label each clearly and never mix them in one comparison.

Why does our ALCO model show a different NIM?

Usually because it averages balances differently or treats nonaccrual loans differently. Document the difference so it stops surprising people.

Related: Finance KPI dictionary template

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