Compound interest is often called the most powerful force in personal finance — it's the reason small amounts saved early can grow into large sums, and the reason credit-card debt can spiral. The idea is simple: you earn (or owe) interest on your interest, not just on the original amount.
Simple vs. Compound Interest
Simple interest is calculated only on the original amount (the principal). If you put $1,000 in an account paying 5% simple interest, you earn $50 every year — always $50.
Compound interest is calculated on the principal plus the interest already earned. That same $1,000 at 5% compounded annually earns $50 the first year, but the second year you earn 5% of $1,050 = $52.50, then 5% of $1,102.50, and so on. Each year the base grows, so the growth accelerates.
Why Time Is the Secret Ingredient
Because compounding builds on itself, the longer your money grows, the more dramatic the effect. $1,000 at 7% becomes about $2,000 in 10 years, roughly $4,000 in 20 years, and about $7,600 in 30 years — without adding a single dollar. The last decade adds far more than the first.
This is why starting to save early matters so much. Someone who invests modest amounts in their teens or twenties can end up ahead of someone who invests much more but starts later, simply because their money had more time to compound.
The Rule of 72
A quick shortcut: divide 72 by the annual interest rate to estimate how many years it takes money to double. At 6%, money doubles in about 12 years (72 ÷ 6). At 9%, about 8 years. It's an approximation, but it's a fast way to see the power of a higher rate or a longer time horizon.
When Compounding Works Against You
The same math that grows your savings can grow your debt. Credit cards often charge around 20% or more, compounded — so an unpaid balance snowballs quickly, and making only the minimum payment can mean paying for years. Understanding compounding helps you see why paying off high-interest debt fast is one of the best financial moves you can make.
Key Takeaways
- Compound interest is interest earned on both the principal and previously earned interest.
- Time dramatically amplifies compounding — starting early beats starting big.
- The Rule of 72 estimates doubling time: 72 ÷ interest rate.
- Compounding grows savings but also grows debt — it cuts both ways.
Keep Studying
Related: Saving vs. Investing, How Credit Scores Work.
These guides are educational and written for the AP Business with Personal Finance course. They provide general information, not personalized financial advice.