Net interest margin is a measure of the difference between the interest

Interest

Interest is a fee paid by a borrower of assets to the owner as a form of compensation for the use of the assets. It is most commonly the price paid for the use of borrowed money, or money earned by deposited funds....

 income generated by banks or other financial institutions and the amount of interest paid out to their lenders(for example, deposits), relative to the amount of their (interest-earning) assets. It is similar to the gross margin of non-financial companies.

It is usually expressed as a percentage of what the financial institution earns on loans in a time period and other assets minus the interest paid on borrowed funds divided by the average amount of the assets on which it earned income in that time period (the average earning assets).

Net interest margin is similar in concept to net interest spread

Net interest spread

Net interest spread refers to the difference in borrowing and lending rates of financial institutions in nominal terms. It is considered analogous to the gross margin of non-financial companies....

, but the net interest spread is the nominal average difference between the borrowing and the lending rates, without compensating for the fact that the earning assets and the borrowed funds may be different instruments and differ in volume. The net interest margin can therefore be higher (or occasionally lower) than the net interest spread.

Calculation

NIM is calculated as a percentage of interest bearing assets. For example, a bank's average loans to customers was $100.00 in a year while it earned interest income of $6.00 and paid interest of $3.00. The NIM then is computed as ($6.00 – $3.00) / $100.00 = 3%. Net interest income equals the interest earned minus the interest paid out to customers.