A flat interest rate charges interest on the full loan amount for the whole term, so it costs far more than the same number as a reducing-balance rate. A 10% flat rate over 3 years costs about the same as a 17.9% reducing rate; over 5 years it’s about 17.3%. To compare loans fairly, convert a flat rate to its reducing-balance equivalent (roughly the APR) before you sign.
Below: how each method works, a conversion table, a worked example, and why a popular shortcut formula overstates the true rate.
Flat Rate vs Reducing Balance: the Difference
- Flat rate: interest = loan × rate × years, worked out once at the start and spread evenly over the payments. You pay interest on money you’ve already repaid.
- Reducing balance (also called diminishing or declining balance): each month’s interest is charged only on what you still owe. As you repay, the interest part of each payment shrinks.
Car loans, consumer finance and some personal loans are often quoted at a flat rate, while mortgages and most bank loans use reducing balance. Because a flat rate looks lower on paper, always ask which method a quote uses.
Worked Example: 10,000 Over 3 Years
At 10% flat: interest = 10,000 × 10% × 3 = 3,000. Total repaid 13,000, over 36 months = 361.11 a month.
At 10% reducing balance: using the standard EMI formula, the payment is 322.67 a month and total interest is about 1,616.
Same headline “10%”, but the flat loan costs nearly twice as much interest. To find the reducing rate that gives the same 361.11 payment, you solve the EMI formula for the rate: it comes to about 17.9%. The EMI calculator has both a flat-rate and a reducing-balance mode, so you can enter a quote each way and compare the payments.
Flat Rate to Reducing Rate Conversion Table
Equivalent reducing-balance rate (per year) for monthly payments:
| Flat rate | 1 year | 2 years | 3 years | 5 years | 7 years |
|---|---|---|---|---|---|
| 6% | 10.9% | 11.1% | 11.1% | 10.9% | 10.6% |
| 8% | 14.5% | 14.7% | 14.6% | 14.1% | 13.7% |
| 10% | 18.0% | 18.2% | 17.9% | 17.3% | 16.7% |
| 12% | 21.5% | 21.6% | 21.2% | 20.3% | 19.5% |
| 15% | 26.6% | 26.6% | 26.0% | 24.7% | 23.6% |
Rule of thumb: the reducing equivalent is roughly 1.6 to 1.8 times the flat rate for typical 1–5 year loans. Our 10% over 5 years figure matches independent converters such as emi-calc.com, which gives 17.27%.
Why the Quick Formula Overstates It
A shortcut you’ll often see is reducing rate ≈ 2 × flat rate × n ÷ (n + 1), where n is the number of payments. It’s easy to use, but it overstates the true figure, and more so for longer loans:
| 10% flat for… | Exact reducing rate | Shortcut formula |
|---|---|---|
| 1 year | 18.0% | 18.5% |
| 3 years | 17.9% | 19.5% |
| 5 years | 17.3% | 19.7% |
Use the shortcut only for a rough sense of scale. For an exact figure, find the reducing rate that gives the same monthly payment, as the table above does.
How to Compare Two Loan Offers
- Ask each lender whether the rate is flat or reducing balance, and for the APR, which includes fees in many countries.
- Compare the monthly payment and the total amount repaid for the same loan amount and term.
- Add any processing fee, insurance or early-repayment charge to the total cost.
- Check what happens if you repay early: on a flat-rate loan, the interest may already be fixed, so early repayment can save less than you expect.
Frequently Asked Questions
Is 10% flat the same as 10% reducing?
No. A 10% flat rate charges interest on the original amount for the whole term, while 10% reducing charges only on the balance still owed. Over 3 years, 10% flat is equivalent to about 17.9% reducing, so the flat loan costs almost twice the interest.
How do I convert a flat rate to a reducing rate?
Work out the flat-rate monthly payment, then find the reducing-balance rate that gives the same payment over the same term. The table above does this for common rates, and an EMI calculator with both modes lets you check any quote. The shortcut formula overstates the result.
Why do lenders quote flat rates?
They’re simple to explain and calculate, and the number looks lower than the true cost. That’s why many regulators require lenders to disclose an APR or effective rate as well, so borrowers can compare offers on equal terms.
Which is better for a borrower, flat or reducing?
At the same headline rate, reducing balance is always cheaper, because you stop paying interest on the money you’ve repaid. A flat-rate offer is only competitive if its rate is much lower, roughly 55–60% of the reducing rate for typical terms.
Compare flat-rate and reducing-balance payments side by side with the free EMI calculator, and see how the maths works in how to calculate loan EMI.