Compound Interest

When you build a snowman, rolling a small ball of snow starts off slowly. But as it grows, even a single rotation gathers an immense amount of snow. Compound interest works on the very same principle. If you leave your earned interest in the account instead of spending it, your next interest payment is calculated on your entire balance—the original principal plus accumulated interest.

In contrast, simple interest pays you only on the original sum you first deposited. Imagine investing 1 million won (around $750) at a 10% annual interest rate. With simple interest, you earn exactly 100,000 won (around $75) each year. After 10 years, your total interest earned is still just 1 million won.

With compound interest, however, your first-year interest of 100,000 won turns your balance into 1.1 million won. In the second year, you earn 10% on that full 1.1 million won, netting 110,000 won. As this process of interest earning its own interest repeats, your money multiplies at a speed far beyond what you might imagine.

The true power of compound interest shows only after time stacks up. During the first few years, the difference between simple and compound interest feels negligible. But after 10, 20, or 30 years, the gap between the two becomes staggering.

In finance, people often use a handy rule of thumb called the Rule of 72 to estimate how long it takes to double their money. Divide the number 72 by your annual interest rate (in percent), and you get the approximate number of years needed. At an annual compound rate of 6%, dividing 72 by 6 shows that your money doubles in about 12 years.

The most lethal weapon in compounding is not having a huge sum right now, but the patience to let time do the work. Starting to save or invest even a modest amount just one day earlier puts time on your side, dramatically amplifying the compounding effect. Keep in mind, however, that investment returns fluctuate year to year and carry risk of loss, so real-world results can vary.

To be precise, compound interest is not a guardian angel that works only for savers and investors. When applied to borrowed money or unpaid debt, it quickly transforms into an uncontrollable monster.

If you fall behind on credit card bills or neglect high-interest loans, unpaid interest gets tacked onto your balance, and new interest is charged on top of it. Before you know it, you are hit with a reverse compounding bomb as your debt snowballs out of hand.

Inflation works similarly, compounding year after year to erode the purchasing power of your cash. In the end, compound interest can be a powerful engine for building wealth, but left unchecked, it can easily destroy your financial foundation.