The Fundamental Difference
Simple interest is calculated only on the principal. Compound interest is calculated on principal plus accumulated interest. On $10,000 at 7% for 10 years: simple interest earns $7,000 total ($700 per year). Compound interest earns $9,672 — nearly $2,700 more. Over 30 years, simple interest yields $21,000 while compound interest yields $76,123. The gap widens exponentially with time.
Real-World Examples
Credit cards use daily compounding, making debt spiral rapidly. A $5,000 balance at 20% APR with minimum payments takes 25+ years to pay off and costs over $10,000 in interest. Student loans typically use simple interest during school but may compound during deferment. Savings accounts compound monthly or daily — daily compounding yields slightly more. Mortgages use amortization (simple interest on declining balance), which is why early payments are mostly interest.
Why Time Is the Critical Factor
Compound interest needs time to work its magic. The first 10 years of compounding are modest. Years 20-30 are where growth explodes. Warren Buffett's net worth illustrates this: he reached $1 million at 30, $1 billion at 56, and $100 billion in his 90s. Over 90% of his wealth came after age 60. This is why starting to invest in your 20s is so powerful — you give compounding 40+ years to work.
Frequently Asked Questions
Which is better: compound or simple interest?
For investments, compound interest is far superior. For loans, simple interest is better for borrowers. Credit cards use compound interest, which is why debt grows so fast.
Does my savings account use compound interest?
Most do, compounding monthly or daily. Check your account terms. Online banks often compound daily, yielding slightly more than monthly compounding.
Why did Einstein praise compound interest?
While the quote may be apocryphal, the math is undeniable. Compound interest creates exponential growth that outpaces linear simple interest by orders of magnitude over long periods.