Two loans advertised at the same rate can cost wildly different amounts, because "12% interest" means two different things depending on how it's calculated. Knowing which one you're being quoted can save you from a loan that's nearly twice as expensive as it sounds.

Two ways to calculate the same number

Reducing balance interest is charged only on what you still owe. As you pay down principal each month, the interest portion shrinks — this is how home loans, most personal loans, and car loans from banks work.

Flat rate interest is charged on the original loan amount for the entire tenure, even though your outstanding balance keeps dropping. It's common with consumer durable loans, some personal loans from NBFCs, and gold loans.

Converting flat to its real equivalent

Because a flat rate ignores the shrinking balance, its true reducing-balance equivalent is roughly 1.8–2x higher. A loan advertised at 12% flat over a typical 3–5 year tenure behaves like a 21–24% reducing balance loan — closer to credit card rates than a bank loan.

The rough conversion: reducing rate ≈ flat rate × 2 × n / (n + 1), where n is the number of installments. The longer the tenure, the closer the multiplier gets to 2x.

Where this trap shows up most

Consumer electronics "no-cost EMI" offers and some personal loan ads lean on flat rates because the headline number looks smaller. Always ask the lender directly: "Is this flat or reducing balance?" If they hesitate, that's usually your answer.

Check the real number

Before signing anything, run the actual EMI and total interest through our Loan Eligibility Calculator or Home Affordability Calculator — both use reducing balance math, so you can see exactly what a flat-rate offer would really cost once converted.