Definition
Net interest margin is the difference between the interest a bank receives and the interest it pays. It measures the core business of a commercial bank: gathering funds and lending them at a higher price than they cost.
It is one part of net banking income, not all of it. Fees, gains on the trading portfolio and income from the investment portfolio come on top.
The formula
Net interest margin = interest and similar income − interest expense and similar charges.
Two ratios make it comparable from one bank to another:
- the margin over net banking income, which shows how much of revenue comes from interest;
- the margin over average total assets, annualised, which shows what each dinar of balance sheet earns.
Where to read it in Tunisian financial statements
Bank financial statements follow the Tunisian accounting standards NCT 21 to 25. The income statement carries both lines in the same place at every bank:
- PR1, «interest and similar income», within banking operating income;
- CH1, «interest expense and similar charges», within banking operating expenses.
Three reading traps
First trap: interest on classified loans is not all in PR1. Circular 91-24 of the Central Bank of Tunisia allows interest on loans classified B2, B3 and B4 to be booked as income only when collected. The rest is recorded as reserved interest. A low margin can therefore reflect a riskier loan book as much as tighter pricing.
Second trap: income from investment securities, Treasury bills included, is not in PR1. It forms a separate line, PR4. A bank that holds a large book of government securities shows a thinner net interest margin without earning less. Section 1 of the Lectures note measures that weight bank by bank.
Third trap: over a half-year, the margin over assets must be annualised. It is multiplied by 2 here, as an indication.
The 10 listed banks in the first half of 2026
Banks are shown in alphabetical order, amounts in millions of dinars:
| Bank | Net interest margin (TND m) | Margin / net banking income | Margin / average assets, annualised |
|---|---|---|---|
| Amen Bank | 67.9 | 18.7% | 1.05% |
| ATB | 87.6 | 46.8% | 2.19% |
| Attijari Bank | 125.0 | 33.4% | 1.80% |
| BH Bank | 106.3 | 30.7% | 1.39% |
| BIAT | 258.4 | 32.2% | 1.89% |
| BNA | 65.2 | 11.6% | 0.52% |
| BT | 166.7 | 57.1% | 3.74% |
| STB | 9.2 | 2.9% | 0.12% |
| UBCI | 94.7 | 48.6% | 3.19% |
| UIB | 146.7 | 53.6% | 3.36% |
The margin ranges from 2.9% to 57.1% of net banking income depending on the bank. Across the 10 banks combined, it accounts for 30.4%. Over average assets, it ranges from 0.12% to 3.74% a year.
The three banks where the margin weighs least in net banking income are also the three where the securities portfolio weighs most in income. The Lectures note reads that link bank by bank and groups the banks by model.
Key points
- Net interest margin = interest received − interest paid, lines PR1 and CH1.
- It does not tell the whole revenue story: the securities portfolio is counted separately.
- A thin margin does not, on its own, signal a less profitable bank.
Sources: interim individual financial statements at 30 June 2026 published with the Conseil du marché financier (CMF); BCT circular 91-24, as restated in the accounting policies published by the banks. The brief's data (CSV).