Albert Einstein probably never called compound interest the eighth wonder of the world, but the math is remarkable. Earnings generate their own earnings, and time does the heavy lifting.
How compounding works
With simple interest you earn only on your original deposit. With compound interest you earn on the deposit plus prior interest. At a 7% annual return, $10,000 grows to about $19,700 in 10 years, $38,700 in 20 and $76,100 in 30, without adding a cent.
Time beats amount
Consider two savers. One invests $300 monthly from age 25 to 35, then stops. Another starts at 35 and invests $300 monthly until 65. At 7%, the early saver often ends up with more despite contributing a third as much, because those first dollars had decades to grow.
Practical takeaways
Start now even with small amounts, automate contributions and increase them with each raise. Keep fees low since they compound against you. The Rule of 72, dividing 72 by the annual return, estimates how many years it takes to double your money.
Try the numbers
Use our Compound Interest Calculator to see this in practice. With its default example (Initial deposit: $10,000; Monthly contribution: $500; Annual return: 7%), it shows future value: $462,290. Adjust the inputs to match your situation.
Key takeaways
- Time in the market matters more than the amount you start with.
- Automate contributions and raise them over time.
- The Rule of 72 gives a quick doubling estimate.
This article is for general education and is not personalized financial, tax or legal advice. Rules and limits change; confirm current figures with official sources such as IRS.gov, SSA.gov and StudentAid.gov, or consult a qualified professional. © 2026 FinanzCalc.