Compare a flat-rate loan against a reducing-balance loan quoted at the same nominal interest rate — the two cost very differently.
A flat rate charges interest on the full original principal for the entire tenure, even as you repay it.
A reducing-balance rate charges interest only on the amount you still owe, which gets smaller over time.
Flat interest: Principal × Rate × Years
Reducing EMI: Standard EMI formula
This calculator puts two ways of charging interest side by side at the same quoted rate. With a flat rate, interest is charged on the full original loan for the whole tenure. With a reducing-balance rate, interest is charged each month only on the amount still owed. You enter the loan amount, the rate and the tenure in years, and it shows the EMI and total interest under each method.
Flat rates are still quoted in India for some two-wheeler and car loans, consumer durable finance and microfinance, often because the number looks small. A borrower hearing '8% flat' may compare it with a bank's '10% reducing' and think the first is cheaper. The comparison here shows how much more a flat-rate loan costs at the same headline figure, so you can judge offers on a like-for-like basis.
1. Write down the principal P, the quoted rate R% and the tenure in years T. Set months n = 12T.
2. Flat method: total interest = P × R × T ÷ 100.
3. Flat EMI = (P + flat interest) ÷ n.
4. Reducing method: monthly rate r = R ÷ 12 ÷ 100 and EMI = P × r × (1 + r)ⁿ ÷ ((1 + r)ⁿ − 1).
5. Reducing total interest = reducing EMI × n − P.
6. Compare the two EMIs and the two totals; the gap is the extra cost of the flat method at the same rate.
Under a flat rate, you pay interest on the full P every year, even in the last year when you have repaid almost all of it. Under a reducing rate, the interest base falls with every EMI. On average over the loan, a reducing balance is only a little more than half of P, so reducing-balance interest is roughly half of flat interest for the same rate. That is why a flat rate is nearly twice as expensive as the same number on a reducing basis, with the gap growing for longer tenures.
This calculator compares the two methods at the same rate. The other useful question is: what reducing rate gives the same EMI as a given flat rate? There is no neat formula, so it is found by trial, adjusting the reducing rate until its EMI matches the flat EMI. A handy rule of thumb is that the equivalent reducing rate is a little under twice the flat rate for common tenures. For a 9% flat loan over four years, the equivalent reducing rate is close to 16%.
In a flat-rate loan the EMI is the same every month, just as in a reducing loan, but the lender's split between interest and principal can differ. Some lenders use the rule of 78 or a similar method to allocate more interest to early instalments. This matters if you close the loan early, because the principal outstanding may be higher than you expect. Always ask for a repayment schedule and the reducing-balance equivalent rate, sometimes called the APR or annualised rate, before signing.
A used-car dealer offers Gopal a ₹4,00,000 loan at '9% flat' for 4 years, while his bank quotes a reducing-balance loan at the same 9%.
Answer: Flat-rate EMI ₹11,333; Reducing-balance EMI (same nominal rate) ₹9,954; Flat-rate total interest ₹1,44,000
Comparing a flat rate directly with a reducing rate as if they measured the same thing.
Assuming the equivalent reducing rate is exactly double the flat rate; it is a little less.
Not asking for a repayment schedule, and being surprised by the outstanding amount at foreclosure.
Ignoring processing fees, which raise the effective cost of both types of loan.
Checking dealer finance offers for vehicles and appliances.
Comparing a microfinance or NBFC flat-rate quote with a bank loan.
Teaching the difference between simple and reducing-balance interest in finance classes.
Negotiating with a lender by asking for the reducing-balance equivalent.
Why do some lenders advertise a flat rate?
A flat rate looks like a smaller number. But since it is charged on the full loan amount the whole time, the real cost is much higher than a reducing-balance loan at the same rate. Always ask for the reducing-balance version before comparing loans.