Albert Einstein reportedly called compound interest the eighth wonder of the world. Whether he said it or not, the sentiment is accurate — compound interest is the most powerful force in personal finance. Understanding it can change how you think about saving and investing forever.
Compound interest means you earn interest not just on your original amount (principal), but also on the interest you've already earned. In other words, your interest earns interest. This creates an exponential growth curve rather than a straight line.
Simple interest: You earn the same amount every year. $1,000 at 10% = $100/year every year.
Compound interest: You earn more each year. $1,000 at 10% = $100 year 1, $110 year 2, $121 year 3...
A = Final amount, P = Principal, r = Annual rate (decimal), n = Years
After 10 years: $21,589
After 20 years: $46,610
After 30 years: $100,627
Your money grows 10x in 30 years — without adding a single dollar more.
A quick mental shortcut: divide 72 by your interest rate to find roughly how many years it takes to double your money.
The lesson: harness compound interest for savings, avoid it for debt. Pay off high-interest debt as fast as possible — the same force working against you is enormous.
It depends on the account. Savings accounts often compound daily or monthly. Investments typically compound annually. More frequent compounding = slightly more growth.
For long-term stock market investments, historical average returns are around 7-10% annually. For savings accounts, 4-5% is realistic in current conditions. Always use conservative estimates when planning.
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