Flat vs Reducing Interest Rate: What Your Loan Really Costs

Two lenders offer you ₹5 lakh for five years, and both say 10.5%. One quote is on a reducing balance and the other is flat. The flat loan costs you ₹1,17,683 more in interest, and its EMI is almost ₹2,000 a month higher.

Indian loans are repaid in monthly EMIs on a reducing balance, but some personal and car loans are quoted at a flat rate instead, which costs much more than the same reducing-balance rate. This guide shows how each method charges interest, how to convert one into the other, and how to compare loan quotes fairly.

How a reducing-balance rate works

On a reducing-balance loan, interest is charged each month only on the amount you still owe. Each EMI pays that month’s interest first, and the rest reduces the principal. As the balance falls, so does the interest, and more of each EMI goes to principal.

The EMI comes from this formula:

EMI = P × r × (1 + r)ⁿ ÷ ((1 + r)ⁿ − 1)

Here P is the loan amount, r is the monthly rate (the yearly rate ÷ 12 ÷ 100) and n is the number of monthly EMIs.

On ₹5 lakh at 10.5% over 5 years, r is 0.875% and n is 60, which gives an EMI of ₹10,746.95. The first EMI includes ₹4,375 of interest. By the 60th EMI, the interest is just ₹93.

How a flat rate works

A flat rate charges interest on the full original loan amount for the whole tenure, however much you have already repaid. The interest is worked out up front, added to the principal, and the total is split into equal EMIs:

Flat-rate EMI = (P + P × r × t) ÷ months

Here r is the yearly flat rate as a decimal and t is the tenure in years.

On the same ₹5 lakh at 10.5% flat over 5 years, the interest is ₹5,00,000 × 0.105 × 5 = ₹2,62,500. Add the principal and divide by 60 months: the EMI is ₹12,708.33.

The arithmetic is simple. The catch is that you keep paying interest on money you have already paid back.

Worked example: ₹5 lakh at 10.5% for 5 years

The flat loan costs ₹1,961 more every month and ₹1,17,683 more over the five years. The EMI calculator shows this for any loan: enter the amount, rate and tenure, and look for the “Same rate quoted flat” figure.

Converting flat rates to reducing-balance rates

The table shows the reducing-balance rate that gives the same EMI as each flat rate. We worked these out by solving the EMI formula for the rate whose EMI matches the flat-rate EMI. They hold for any loan amount, because both EMIs rise in proportion to the principal.

Across these rates and tenures, the true rate is roughly 1.7 to 1.8 times the flat rate. You can check any row yourself: put the reducing-balance rate into the EMI calculator and you get the same EMI as the flat quote.

Why the gap grows with tenure

The longer the loan, the more extra interest a flat rate charges. Here is the same ₹5 lakh at 10.5% over different tenures:

A flat rate adds the same ₹52,500 of interest for every year of the tenure, because it is always charged on the original ₹5 lakh. A reducing-balance loan charges less each year as you repay. In the fifth year of the 5-year loan, the balance starts at ₹1,21,919 and that year’s interest is ₹7,045. The flat loan still charges ₹52,500 for the year, as if you owed the full amount.

So each extra year of tenure adds far more interest at a flat rate than on a reducing balance. Car loans commonly run up to 7 years (84 months), which is where the gap is widest in rupees.

One subtlety: the equivalent rate in the conversion table eases slightly as the tenure gets longer (18.7% at 3 years, 18.0% at 5 years for a 10.5% flat rate), because a longer reducing-balance loan keeps its balance higher for longer. But you pay that inflated rate for more years, so the extra cost in rupees still climbs.

How to compare loan quotes

1. Ask whether the rate is flat or reducing. Get the answer in writing, or ask for the EMI and check it yourself. If the EMI is higher than the EMI calculator gives for the quoted rate, the rate is probably flat.
2. Convert any flat rate to its reducing-balance equivalent using the table above, so every quote is on the same basis.
3. Add the processing fee and its GST. GST at 18% applies to loan processing fees. Compare effective annual rates that include them, not headline rates.
4. Compare the total cost over the tenure: interest plus the fee and GST.

Here is how that plays out on two quotes for ₹5 lakh over 5 years, with the processing fee and GST deducted from the amount you receive:

Offer A looks cheaper on both the rate and the fee. In fact Offer B costs ₹58,552 less, and its EMI is ₹1,074 lower every month.

To test your own quotes, enter the reducing-balance rate and the fee in the personal loan EMI calculator, which works out the effective annual rate with GST (it calls it the effective APR). To put two offers side by side, use the loan comparison calculator, entering each flat rate as its reducing equivalent. Buying a car? The car loan EMI calculator also shows what your rate would cost if it were quoted flat.

The short version

• A reducing-balance rate charges interest only on what you still owe. A flat rate charges it on the full loan for the whole tenure.
• For 3- and 5-year loans at flat rates of 7% to 12%, the true rate is roughly 1.7 to 1.8 times the quoted flat rate.
• The longer the tenure, the more a flat rate costs you in rupees.
• Always compare reducing-balance rates, with the processing fee and 18% GST included.

Unfamiliar term? Look it up in the glossary. This guide is general information, not personal financial advice. Rules and products vary by country and lender.