The Magic of Compound Interest

When you start rolling a small snowball, it grows only a tiny bit at first. But once it's as big as a person, just one more turn picks up a huge amount of snow all at once.

Compound interest (interest that earns more interest) works the same way. Say $1,000 earns 10% interest and becomes $1,100. Next year, the 10% is applied to the full $1,100, not just the original $1,000. That gives you $110 in new interest.

Your interest goes to work and earns even more interest. At first, it looks about the same as simple interest (interest paid only on your original deposit). But as time goes by, the gap between the two grows far bigger than you'd expect.

Just like a snowball needs to keep rolling, compound interest only shows its real power if you stick with the slow early years and give it time.

The most powerful tool in compounding isn't a big pile of money. It's time. The cycle of interest earning interest needs plenty of years to repeat again and again.

Compare two people. One starts saving $100 a month at age 20. The other starts saving $200 a month at age 30. The 30-year-old puts in far more money in total. But if returns stay high enough (roughly 6 to 7% a year or more) for a long time, the one who started early can end up with more at age 60.

If you graph how wealth grows, the line hugs the floor at first. Then, at some point, it shoots up toward the sky. This is called an exponential curve. To reach that explosive stretch, you need time as your fertilizer.

That's why people say, "The key ingredient in compounding is time."

To be more precise, compounding is a kind of magic that grows your money. But it's also a sharp blade that can turn debt into a runaway snowball.

If you can't pay off high-interest debt on time, the unpaid interest gets added to the principal. Next month, interest is charged on an even bigger amount. You fall into the "reverse compounding trap", where what you owe seems to multiply on its own.

Inflation (the general rise in prices) also works by compounding, quietly eating away at the value of your money. Even if prices rise just 3% a year, after about 20 years the real buying power of cash sitting in your account drops by nearly half.

In the end, compounding is a basic law of economics. With saving and investing, you should make it work for you. With debt and inflation, you should be on guard.