Net Interest Margin (NIM): Definition, Banking Formula, Yield on Advances vs Cost of Funds & Valuation Impact

Last updated: July 28, 2026 | 12-minute read

Definition: Net Interest Margin (NIM) is the principal financial profitability metric for commercial banks and Non-Banking Financial Companies (NBFCs), measuring the net spread between the interest income generated on loans and advances versus the interest expense paid out to depositors and bondholders, expressed as a percentage of total average interest-earning assets.

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+---------------------------------------------------------------------------------------------------+
|                        NET INTEREST MARGIN (NIM) VALUE TRANSMISSION ENGINE                        |
+---------------------------------------------------------------------------------------------------+
                                                  │
         ┌────────────────────────────────────────┼────────────────────────────────────────┐
         ▼                                        ▼                                        ▼
+──────────────────────────+             +──────────────────────────+             +──────────────────────────+
| INTEREST EARNED (ASSETS) |             | INTEREST EXPENDED (LIAB) |             | AVERAGE EARNING ASSETS   |
| • Retail Mortgages / Auto|             | • Low-Cost CASA Deposits |             | • Performing Loan Book   |
| • Corporate Credit Loans |             | • Fixed Term Deposits    |             | • RBI Repo Reverse Funds |
| • Govt Treasury Yields   |             | • Subordinated Tier Bonds|             | • Interbank Placements   |
+──────────────────────────+             +──────────────────────────+             +──────────────────────────+
         │                                        │                                        │
         └────────────────────────────────────────┼────────────────────────────────────────┘
                                                  ▼
+---------------------------------------------------------------------------------------------------+
| FORMULA: $\text{NIM} = \frac{\text{Interest Income} - \text{Interest Expended}}{\text{Average Earning Assets}} \times 100$ |
+---------------------------------------------------------------------------------------------------+

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📐 1. Mathematical Formulation & Structural Drivers

Net Interest Margin serves as the fundamental gross margin equivalent for the banking industry:

$$\text{NIM} = \left( \frac{\text{Net Interest Income (NII)}}{\text{Average Total Interest-Earning Assets}} \right) \times 100$$

Where: $$\text{Net Interest Income (NII)} = \text{Total Interest Earned} - \text{Total Interest Expended}$$

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|                           THE TWO ARMS OF NET INTEREST MARGIN (NIM)                               |
+---------------------------------------------------------------------------------------------------+
 [Yield on Advances & Investments (%)] ◄── High-Margin Retail Lending (Credit Cards, Personal Loans)
                   │
                   ▼ (Minus Spread)
 [Cost of Deposits & Funds (%)]        ◄── Low-Cost CASA Deposits (Savings 3.0%, Current 0.0%)
                   │
                   ▼
 [Net Interest Margin (NIM) = Spread % Realized on Average Balance Sheet]
+---------------------------------------------------------------------------------------------------+

The Primary Determinants of High NIM in Indian Banking:

1. Current Account & Savings Account (CASA) Ratio: Banks with high CASA ratios (e.g., Kotak, ICICI, HDFC at 38%–45%) procure raw capital at blended costs below 4.0%, allowing them to earn superior NIM spreads.
2. Loan Mix (Retail vs Corporate): High-yield unsecured retail loans (Credit Cards at 36%–42% APR, Personal Loans at 11%–14%) expand NIM, while AAA-rated corporate loans (at 7.5%–8.5%) depress NIM but offer lower credit default risk.
3. Monetary Policy Transmission: When the RBI cuts repo rates, floating loans reprice downwards immediately, while fixed deposits reprice with a lag, temporarily compressing bank NIMs.

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📊 2. Comparative Benchmarking: Indian Banking Sector NIMs

+---------------------------------------------------------------------------------------------------+
|                         INDIAN BANKING SECTOR NIM & PROFITABILITY BENCHMARKS                      |
+---------------------------------------------------------------------------------------------------+
| Bank / Financial Institution | Domestic NIM (%) | CASA Deposit Ratio (%) | Primary Asset Driver   |
+------------------------------+------------------+------------------------+------------------------+
| HDFC Bank                    | 3.45%–3.65%      | 38.5%                  | Mortgages & Auto Retail|
| ICICI Bank                   | 4.25%–4.40%      | 41.2%                  | Digital Consumer Retail|
| Kotak Mahindra Bank          | 4.90%–5.10%      | 47.5%                  | High-Yield Retail/SME  |
| State Bank of India (SBI)    | 3.15%–3.30%      | 41.0%                  | Corporate & Infrastructure|
| Bajaj Finance (NBFC)         | 9.50%–10.20%     | N/A (Wholesale Borrow) | Consumer Durables & EMI|
+---------------------------------------------------------------------------------------------------+

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🔍 3. Strategic Importance in Equity Valuation

For bank equity analysts, NIM trends are the leading indicator of Return on Assets (RoA) and Return on Equity (RoE):

• NIM Expansion: Signals pricing power, strong deposit franchise, and room for earnings upgrades $\to$ drives Price-to-Book (P/B) multiple re-ratings.
• NIM Compression: Signals aggressive price competition for deposits, rising cost of funds, or asset quality slippage $\to$ triggers stock valuation derating.

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📌 The Bottom Line & Actionable Rules

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|                              TOPIC SLUG ALIGNED ACTIONABLE TAKEAWAYS                              |
+---------------------------------------------------------------------------------------------------+
| Topic Slug                           | Core Actionable Analytical Takeaway                        |
+--------------------------------------+------------------------------------------------------------+
| net-interest-margin-nim              | NIM is the single best barometer of banking pricing power. |
| banking-profitability-metrics        | Look for private banks maintaining stable NIMs above 4.0%. |
| yield-on-advances-vs-cost-of-funds   | Track the spread between loan yields and term deposit rates|
| casa-deposit-ratio-leverage          | A high CASA ratio is the ultimate moat against rate hikes. |
| bank-equity-valuation-models         | NIM expansion directly drives Price-to-Book multiple upside|
+---------------------------------------------------------------------------------------------------+

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