Compound Interest Explained: The Snowball Effect
Albert Einstein is often credited with calling compound interest the eighth wonder of the world. There is no evidence he said it, but the idea is real: money that earns interest on its interest grows faster and faster over time.
Simple versus compound interest
With simple interest, you earn interest only on your original deposit. Put in 1,000 at 5% and you earn 50 every year, forever. With compound interest, each yearβs interest is added to the balance, so next yearβs interest is calculated on a larger amount. After ten years, 1,000 at 5% compounded annually becomes about 1,629, not 1,500.
The formula
A = P (1 + r/n)nt
P is the starting amount, r the annual rate, n the number of times interest compounds each year, and t the number of years. More frequent compounding gives slightly more: 1,000 at 5% for ten years becomes 1,628.89 with annual compounding and 1,647.01 with monthly compounding. You can try your own numbers with the compound interest calculator.
Why starting early beats saving more later
Time is the most powerful ingredient. Imagine two savers who each earn 7% a year. Anna invests 200 a month from age 25 to 35 and then stops. Ben waits until 35 and invests 200 a month until 65. Anna deposits for ten years and Ben for thirty, yet at 65 Annaβs pot can be bigger because her early money had three more decades to grow. The exact result depends on the assumptions, but the pattern holds.
Compounding works against debt, too
Credit cards compound interest on unpaid balances, usually at very high rates. A balance left alone can grow quickly. Paying off high-interest debt is effectively a guaranteed return equal to the interest rate.
Do not forget inflation
If your savings earn 3% and prices rise 3%, your buying power stays flat even though the balance grows. Use the inflation calculator to see what your money might buy in the future, and the savings calculator to plan a target.
Practical takeaways
- Start now, even with small amounts.
- Add money regularly; automation helps.
- Keep fees low, because they compound against you too.
- Be realistic about returns: investments rise and fall, and past performance does not guarantee future results.