Albert Einstein reportedly called compound interest 'the eighth wonder of the world,' adding: 'He who understands it, earns it; he who doesn't, pays it.' Whether he actually said this or not, the principle behind the quote is undeniably one of the most important concepts in personal finance.
Simple Interest vs. Compound Interest
With simple interest, you only earn returns on your original principal. If you invest $1,000 at 10% simple interest, you earn $100 every year, forever. After 10 years, you have $2,000. Compound interest, however, is radically different. You earn interest not just on your original $1,000, but also on the interest that has already been added. After year one, you have $1,100. In year two, you earn 10% on $1,100, giving you $1,210. This snowball effect accelerates dramatically over time.
The Magic of Time
The single most important variable in compound interest is time. A 25-year-old who invests $200 per month at an average 8% annual return will have approximately $702,000 by age 65. A 35-year-old investing the exact same $200 per month at the same rate will only have around $298,000. That extra decade of compounding nearly triples the final amount—without investing a single extra dollar.
How Compounding Frequency Matters
Interest can be compounded annually, semi-annually, quarterly, monthly, or even daily. The more frequently interest compounds, the faster your money grows. A savings account that compounds monthly will yield slightly more than one that compounds annually at the same nominal rate. This is why it is crucial to ask your bank or investment platform about their compounding frequency.
Calculate Your Own Wealth Trajectory
Our Compound Interest Calculator allows you to plug in your starting principal, monthly contribution, expected annual rate of return, and compounding frequency to see exactly how your wealth will grow over 5, 10, 20, or even 40 years. It is the single most motivating financial tool you will ever use—and it runs entirely in your browser with zero data collection.