Net interest margin (NIM) is a measure of the difference between the interest 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 (or gross profit 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, 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.
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.
In particular, for a bank or a financial institution if the non-performing assets are high, their NIM will go down as the interest earning assets are that much reduced by non-performing assets.
Successful Bank Asset/Liability Management: A Guide to the Future Beyond Gap, John W. Bitner, Robert A. Goddard, 1992, p. 185.