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Loan Interest Calculation – Reducing Balance vs Flat Interest Rate

BY Team Loanstreet

Updated 20 Nov 2019

When it comes to comparing loans, the stated loan interest rate does not represent everything, and can in fact be deceiving. Aside from loan interest rates, how loan interest is calculated also plays a huge role in your cost of financing.

What's covered in this article?

There are 2 common methods to calculate loan interest in today’s banking system:

  1. Reducing balance method
  2. Flat interest rate method (a.k.a Simple Interest)

Reducing Balance Method

This method is mainly used to calculate the interest payable for housing/mortgage/property loans which you can find at our home loan comparison tool and other interest payable such as overdraft (OD) facilities, and credit cards. You only pay interest on the remaining loan balance. A reducing balance interest calculation formula can be represented like this:

Interest Payable per Installment = Interest Rate per Installment * Remaining Loan Amount

The interest rates quoted for such loans are the Effective Interest Rate, which is the same as the interest rates used for Fixed Deposits (FD) and Savings Accounts.

Flat Interest Rate Method a.k.a Simple Interest

This method is mainly used to calculate the interest payable for personal loans and car / hire purchase loans. You pay interest on the entire loan balance throughout the duration of the loan. Flat interest rate calculation formula can be represented like this:

Interest Payable per Installment = (Original Loan Amount * No. of Years * Interest Rate p.a. ) / Number of Installments

This is less desirable for the borrower, because even as you pay down the loan, the interest payable does not decrease. If you have difficulty understanding why, consider what would happen if savings accounts used flat interest rate method. 

If you put RM1,200 at 10% p.a. in a savings account and withdraw RM100 every month, the bank would still be paying you RM10 in interest every month even though your principal balance would have reduced to only RM100 by Month 12. How cool is that?

As a rule, flat interest rates range from 1.7-1.9x more when converted into the Effective Interest Rate equivalent. That is the true measure of your cost of financing. You can calculate it yourself, or you can use this Flat to Effective Interest Rate Converter that we've provided for your convenience. Once you've calculated that, you might be interested in reading our article on Top Tips On How To Pay Back Your Personal Loan to learn the various ways in which you can repay your debt!


When considering a loan product, it is always important to know if it is using the Reducing Balance Method of interest calculation or Flat Interest Rate method. And the best way to compare the true cost of the loan is to convert everything into the Effective Interest Rate equivalent. 

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About the Author

Team Loanstreet

Run by a professional human-sized team, get resourceful tips & guides from our very own library of financial articles that can help improve your financial lifestyle & make a well-informed money decision. We strive to provide you with the best service in helping you to get the most out of that DUIT!


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