Two interest types, very different outcomes — learn the formulas and when each one applies
Interest is the cost of borrowing money — or the reward for saving it. Whether you're taking out a loan, opening a savings account, or investing in bonds, understanding how interest is calculated will help you make significantly better financial decisions.
There are two main types: simple interest and compound interest. They use different formulas and produce very different results over time.
Simple interest is calculated only on the original amount (called the principal). It does not grow over time — you earn (or pay) the same amount of interest every period.
Simple interest is commonly used for short-term personal loans, car loans, and some savings bonds. It's straightforward and easy to predict.
Compound interest is calculated on the principal plus any interest already earned. This means your interest earns interest — causing the total to grow exponentially over time. Albert Einstein reportedly called it "the eighth wonder of the world."
The more frequently interest is compounded, the more you earn (or owe). Here's what $10,000 at 6% annual interest looks like after 10 years at different compounding frequencies:
| Compounding | Final Amount | Interest Earned |
|---|---|---|
| Annually (n=1) | $17,908 | $7,908 |
| Quarterly (n=4) | $18,061 | $8,061 |
| Monthly (n=12) | $18,194 | $8,194 |
| Daily (n=365) | $18,221 | $8,221 |
Compound interest rewards patience. A 25-year-old who saves $10,000 at 7% compounded annually will have $149,745 at age 65. A 35-year-old making the same investment will only have $76,123. Starting 10 years earlier nearly doubles the outcome — without saving a single extra dollar.