Do you often look at the numbers in your bank passbook without really understanding where the interest comes from? Understanding “interest calculation formulas“ not only helps you check whether the interest paid by the bank is correct, it is also the foundation of financial planning. Many people only have a vague idea of how to calculate principal and interest, and therefore miss the opportunity to grow their assets. This article will guide you step by step from the most basic concepts to the simple and compound interest methods actually used by banks, helping you master the secrets of deposit interest calculation formulas so that every dollar you have can be used to its fullest potential and build a solid financial foundation for yourself.
Before diving into the more complex methods used in bank interest calculation, you must first understand the three fundamental pillars that make up interest: principal, interest rate, and time. These three elements are closely interconnected and together determine how much return your deposit will ultimately generate.
The principal is the initial amount you deposit in the bank, and it is the foundation of all interest calculations. Whether it is 10,000 dollars or one million dollars, this is the starting point of your wealth growth. The higher the principal, the more interest you can earn under the same interest rate and time conditions. In other words, saving your first pot of gold is the crucial first step to amplifying your future interest earnings.
The interest rate is the return the bank pays you for your deposit, usually expressed as a percentage (%) and representing how much your money can grow each year. For example, an annual interest rate of 2% means that for every 100 dollars you deposit, you will earn 2 dollars of interest after one year. Interest rates fluctuate due to various factors such as central bank monetary policy and market conditions. Therefore, choosing a bank that offers a better rate is a required skill for smart savers. To learn more basic financial concepts, you can refer to our beginner finance guide.
Time is the catalyst that unleashes the power of interest. The longer the interest calculation period, the more significant the accumulation effect becomes. This is why many financial experts always stress that “starting to invest early is better”. Even if the principal is small, as long as the time is long enough, the “compound interest” effect mentioned later can turn a small amount of money into substantial wealth.
After understanding the three key elements, we now move into the core topic: how interest is calculated. The two most common methods are “simple interest” and “compound interest”. Although they may seem similar, the difference in long-term returns can be tremendous. Understanding which method a bank uses is crucial for planning your savings.
Simple interest calculates interest in a very direct manner. Its main characteristic is that “interest does not generate additional interest”. No matter how long the deposit period is, each period’s interest is calculated based solely on the “original principal”, and the principal itself never changes.
For example, if you deposit 100,000 dollars at an annual rate of 2% using simple interest, the total interest after three years will be: Interest = 100,000 × 2% × 3 = 6,000 dollars.
Compound interest is praised by Einstein as “the eighth wonder of the world”, and its power lies in “interest on interest”. Each period’s interest is added back to the principal, becoming the new base for calculating future interest. As time increases, your assets grow like a snowball and expand exponentially. This is the interest calculation method used by the vast majority of bank deposits, loans, and investment products today.
Using the same example, if you deposit 100,000 dollars at an annual rate of 2% with “interest compounded once a year”, the total amount after three years will be:
Year 1 total amount: 100,000 × (1 + 2%) = 102,000
Year 2 total amount: 102,000 × (1 + 2%) = 104,040
Year 3 total amount: 104,040 × (1 + 2%) = 106,120.8
The total interest is 6,120.8 dollars, which is 120.8 dollars more than simple interest.
To help you better understand the power of compound interest, here is a chart comparing the gap between simple interest and compound interest over different time periods:
| Year | Simple interest total amount |
Compound interest total amount |
Return difference |
| Year 1 | 102,000 | 102,000 | 0 |
| Year 5 | 110,000 | 110,408 | 408 |
| Year 10 | 120,000 | 121,899 | 1,899 |
| Year 20 | 140,000 | 148,595 | 8,595 |
| Year 30 | 160,000 | 181,136 | 21,136 |
*Assumptions: principal 100,000 dollars, annual interest rate 2%
As shown in the table above, the longer the time period, the greater the growth generated by compound interest compared with simple interest. This is the true meaning of “time is your friend”. 📈
After learning the theory, let’s now look at how banks actually apply these deposit interest calculation formulas. Generally speaking, bank deposits are divided into “savings deposits” and “fixed deposits”, and the interest calculation methods differ slightly between the two.
Savings deposits are characterized by high liquidity, allowing you to deposit and withdraw funds at any time, but the interest rate is relatively low. Banks in Taiwan generally use a “daily interest calculation with interest paid every six months” model.
The advantage of this mechanism is that even if you deposit money for just one day, you can earn one day of interest. To maximize returns on savings deposits, you can look for various high-interest digital savings accounts available on the market.
A fixed deposit is an agreement between you and the bank to keep a certain amount untouched for a specified period (such as one year or two years) in exchange for a higher fixed rate than savings deposits. The interest for a fixed deposit is usually determined when you make the deposit and is mainly calculated using compound interest.
There are two interest payment methods for fixed deposits: “monthly interest payout” and “lump sum at maturity”.
Let’s use a practical example to understand the difference between various deposit methods. Suppose you have 100,000 dollars and plan to save it for one year.
From this simple principal and interest calculation example, you can see that even with the same amount of money, choosing different deposit methods can result in more than double the difference in returns. If you want to learn more about the actual interest calculation methods used by banks, you may refer to explanations provided by reputable financial institutions such as the fixed deposit interest calculation page from E.SUN Bank.
This depends on the type of deposit. Savings deposit interest is usually paid on June 21 and December 21 each year for the first and second half of the year. For fixed deposits, you may choose to receive the principal and interest together at maturity (principal and interest rollover) or receive interest monthly.
Yes, they do. According to Taiwan tax regulations, interest received from banks is classified as “interest income”. Each person has a “special deduction for savings and investment” of 270,000 dollars per year. If the total interest income from all financial institutions does not exceed this amount, tax is not required. If it exceeds the limit, the bank will withhold 10% tax in advance. In addition, if any single interest payment exceeds 2,000 dollars, an additional 2.11% National Health Insurance supplementary premium must be withheld.
The annual interest rate (Annual Percentage Rate, APR) is the interest rate for a full year of a deposit or loan. The monthly interest rate is the corresponding rate for each month. The conversion is simple:
When comparing different financial products, always confirm whether the rate shown is annual or monthly to avoid misunderstanding.
When the interest rate and principal are the same, a higher compounding frequency results in higher final returns. For example, “monthly compounding” generates more interest than “annual compounding” because interest is added to the principal more quickly. However, in practice, bank fixed deposit plans typically compound monthly or annually, so simply choose the option that best suits your needs.
In summary, whether it is the basic calculation of principal and interest or the more complex compound interest used by banks, understanding the underlying “interest calculation formulas” is an essential financial skill. Mastering the differences between simple and compound interest and understanding how savings and fixed deposits calculate interest can help you manage your assets more wisely. It is our hope that this guide helps you build clear concepts so that you can make more confident decisions when choosing deposit plans in the future. Open your online banking now and try calculating the interest on your deposits!
