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Net Interest Margin: Definition, Formula & Examples

Updated: February 23, 2023

As a business owner or investor, when you have a series of investments, you need to know whether they are working effectively. 

That’s where the net interest margin comes in. But what exactly is the NIM? We’ll take a closer look at the definition and the formula in our handy guide.

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    • Net interest margin (NIM) is a financial measurement.
    • It compares the net interest income a financial firm generates with the outgoing interest it pays.
    • It is a profitability indicator that works by approximating the likelihood that an investment firm or central bank thrives over the long haul.

    What Is Net Interest Margin (NIM)?

    Net interest margin (NIM) is a profitability metric. It compares the net interest income a financial firm generates with the outgoing interest it pays. This is to holders of savings accounts and certificates of deposit (CDs).

    It is a profitability indicator that works by approximating the likeness that an investment firm within the banking industry thrives over the long haul. It also helps potential investors figure out whether or not they should invest in a financial services firm. It does this by providing strong visibility into the profitability of its interest income. This is when it is compared to its interest expenses. 

    To put it simply, if there is a positive net interest, then it is likely that the firm is profitable. This is compared to a negative net interest, which would indicate that there is an investment inefficiency. If there does happen to be a negative net interest, a firm can take corrective action. This is by applying funds to the outstanding debt. Or alternatively, you could shift those assets towards a more profitable investment. 

    When it comes to banking, a monetary policy is set by central banks which influences a bank’s net interest margin. This works for a number of different types of bank income. Whether that’s commercial banks, retail banks, or investment banks. 

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    What Is the Net Interest Margin Formula?

    Net interest can be calculated by using the following formula:

    Net Interest Margin Formula

    Interest Revenue

    Interest revenue is generated through interest payments. These are payments that the bank receives from outstanding loans. It is made up of loans and credit lines on the balance sheet assets of the financial institution.

    Interest Expense

    Interest expense is the price that the lender charges the borrower. This is during a financing transaction. It is essentially the cost of borrowing money. It is the interest that builds up on any outstanding liability that the company may have. A common example would be wholesale financing and customer deposits. 

    Average Earnings Assets

    The earnings assets of a company are investments that produce income. This is without any purposeful effort on the part of the owner. Common earning assets are stocks, notes, certificates of deposits, and bonds. 

    In order to calculate the average earning assets, you just have to take the beginning and ending asset balance and find the average. 

    An Example of Negative Net Interest Margin

    Let’s say that Bank A has a return on investment of $100,000. Bank A also has an interest expense of $200,000 and average earning assets of $1,000,000. By using this information, we can use the net interest margin formula to calculate the interest margin percentage of Bank A. 

    NIM of Bank A = (100,000 – 200,000) / 1,000,000 

    NIM = -0.1

    Net interest margin is expressed as a percentage, so Bank A’s resulting NIM would be -10%. This indicates that it has lost more money than it has earned from its investments. The money lost would have been due to interest expenses. This would mean that this business would have been better off using its investment funds to pay off its debts. This is rather than making this investment. 

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    An Example of Positive Net Interest Margin

    Now that we’ve had a look at what a negative net interest margin may look like, let’s take a look at what a positive net interest margin would be. 

    Let’s say that Bank B has investment returns of $200,000. Bank B also has an interest expense of just $100,000 and average earning assets of $1,000,000. 

    By using this information, we can use the net interest margin formula to calculate the interest margin percentage of Bank B. 

    NIM of Bank B = (200,000 – 100,000) / 1,000,000 

    NIM = 0.1

    So in this example, we can see that Bank B’s NIM is at 10%. This means they are earning more money from receiving their interest payments than they are spending on paying their interest. This means that they are using their capital efficiently. 

    Economic Interest Rates and the Net Interest Margin

    There is a direct correlation between the interest rates in the economy and the net interest margin of financial institutions. This is because the interest rates in the economy move based on the economy’s business cycle. That means that a big factor in the net interest margin is whether or not there is a greater demand for saving or borrowing. 

    High Interest Rates

    High interest rates in the economy lead to lower net interest margins for financial institutions. So when market interest rates increase, the funding costs of a bank sharply increase. This is relative to their interest income. It will also reduce the net interest income. 

    When interest rates start rising, the demand for savings accounts increases. This is relative to loans. When this happens, the net interest margin decreases. This is because the bank will end up having larger interest payments than interest receivable. 

    Low Interest Rates

    Low interest rates in the economy lead to a greater net interest margin for financial institutions. When the market interest rates fall, the cost of funding for banks sharply falls. This is relative to their interest income. 

    When the annual interest rate starts to fall, the demand for loans starts to increase. Therefore the supply of deposits decreases. This drives the volume of bigger lending rates and amounts and lower deposit volumes. This improves interest income. 


    The net interest margin is a great indicator of how effectively a business or company is using its capital. By using the NIM formula, you can figure out the percentage at which your business is gaining or losing from your investments. 

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    FAQs on NIM

    How Do I Increase My Net Interest Margin?

    For banks, NIM is increased when more consumers are borrowing than saving. For businesses, NIM can be increased by investing smartly.

    Is Higher or Lower NIM Better?

    You want your NIM to be positive. If your NIM is negative, then it indicates that you are losing money from your investments.

    Why Is Net Interest Margin Important to Banks?

    NIM is important to banks. Because the more interest income that is generated in relation to the interest expense, the more profit the bank will ultimately make.


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