How Does Compound Interest Work?
Compound interest is interest on your interest — the force behind long-term wealth. Here's how it works and why starting early matters more than almost anything else.
Compound interest is often called the eighth wonder of the world, and for good reason: it's the single most important force in long-term wealth building. Yet most people underestimate just how powerful it becomes over time. Understanding it properly can change the way you save, invest, and think about money for the rest of your life.
Simple vs. compound interest
The difference comes down to what the interest is calculated on. Simple interest is paid only on your original principal. If you deposit $1,000 at 5% simple interest, you earn $50 every year — forever. After 30 years you'd have $2,500.
Compound interest is paid on your principal and on the interest you've already earned. That second year, your 5% is calculated on $1,050, not $1,000. The year after, on $1,102.50. Each year the base grows, so the interest grows too. After 30 years that same $1,000 becomes about $4,322 — almost double the simple-interest result, with no extra effort.
Why time is the secret ingredient
Compounding starts slowly and then accelerates dramatically. The growth curve is gentle at first, then steepens as your returns begin generating returns of their own. This is why starting early beats saving more later.
Consider two savers. One invests $200 a month from age 25 to 35 (ten years, $24,000 total) and then stops. The other waits and invests $200 a month from age 35 to 65 (thirty years, $72,000 total). Assuming a 7% return, the early saver — who put in a third of the money — often ends up with more at retirement, simply because their money had more time to compound.
The Rule of 72
Want a quick estimate of how fast your money doubles? Divide 72 by your annual return rate. At 6%, money doubles in about 12 years; at 9%, in about 8 years. It's a handy mental shortcut for seeing the power of a higher rate or a longer horizon.
Make it work for you
Three levers drive compounding: the rate of return, the time invested, and any regular contributions you add along the way. You can't always control the rate, but you can control how early you start and how consistently you contribute. Automating monthly deposits and leaving them to grow is, for most people, the most reliable path to building wealth.
Use the compound interest calculator to see exactly how your own savings could grow over time — and how much difference starting a few years earlier really makes.