Flat interest vs reducing balance: which loan costs more?

What is the difference between flat and reducing-balance interest?

Flat-rate interest is calculated using the original principal for the entire loan period, even after part of that principal has been repaid. Reducing-balance interest is calculated on the amount still outstanding; as principal falls, the interest portion of later instalments usually falls too.

This distinction matters when comparing personal, consumer-durable or other instalment loans. A lender quoting "10%" without specifying the method has not given enough information to tell you the true cost.

Worked example: ₹1,00,000 over 24 months at a stated 10%

Assume there are no fees, taxes, rate changes or payment delays, and payments are monthly.

For the flat-rate illustration:

• Total interest = ₹1,00,000 × 10% × 2 years = ₹20,000
• Total repaid = ₹1,20,000
• Equal monthly payment = ₹1,20,000 ÷ 24 = ₹5,000

For a standard monthly reducing-balance EMI with a 10% nominal annual rate and 24 equal monthly payments:

• Approximate EMI = ₹4,614.49
• Approximate total interest over 24 payments = ₹10,747.82
• Approximate total repaid = ₹1,10,747.82

The rates printed above are deliberately the same nominal 10%, but the interest methods are different. This is not a claim that every flat-rate loan will cost exactly this amount: lender-specific schedules, disbursement patterns, fees and interest calculations affect actual results.

Try a reducing-balance scenario on Janooji's EMI Calculator. It models the usual amortising-loan calculation, not a flat-interest contract.

How does the reducing-balance EMI work?

With a monthly reducing-balance loan, interest for a period is based on the principal outstanding during that period. An EMI usually contains both interest and principal. Early payments generally contain more interest; later payments usually retire more principal.

For a fixed nominal annual rate, a conventional EMI formula uses the monthly rate and total number of payments. Because the principal reduces each month, simply multiplying the original principal by the annual percentage for every year is not the same calculation.

Is a lower EMI always the cheaper loan?

No. A longer tenure often gives a lower monthly payment but higher total interest. A quoted EMI can also omit processing fees or other charges. Compare the complete cost of borrowing, not the monthly amount alone.

Ask the lender for the written repayment schedule, processing charges, foreclosure/prepayment terms, any insurance charges bundled with the loan, whether the rate can reset, and the APR or equivalent all-in cost information in the lender's current disclosures. RBI consumer guidance explains how fixed and floating loans can affect repayments and tenure.

Practical comparison checklist

Before accepting a loan offer, put two offers side by side and write down:

1. Actual amount credited to you after deductions. 2. Total amount to repay over the full tenure. 3. Interest method: flat or reducing balance. 4. Number and frequency of instalments. 5. Up-front and recurring charges, including compulsory add-ons. 6. Whether interest is fixed or floating, and how it may reset. 7. What happens if you repay early or miss an instalment.

If the lender only gives a headline interest rate, ask for the amortisation schedule and total rupee cost before signing. Never assume a flat 10% and reducing 10% are interchangeable.

Bottom line

Compare total repayment and all-in annualised cost. For the same stated nominal percentage and standard assumptions, charging interest on the full original principal generally costs more than charging on the declining outstanding balance.

Related reading: How EMI works and the EMI Calculator.