Simple interest is interest on the original amount. Compound interest is interest on the original amount plus the interest already earned or charged. That is the whole trick — and over time, it can be powerful.
Simple interest is easier to predict
With simple interest, the math is cleaner because the interest is based on the starting principal. Some loans use simple interest, which makes the cost easier to understand.
Compound interest snowballs
With compound interest, the balance can grow faster because interest gets added to the base. That can be wonderful for savings and brutal for debt, depending which side of the equation you are on.
Time does most of the heavy lifting
Compounding needs time to show off. The longer money stays invested or owed, the more the difference between simple and compound interest can matter.
Frequency matters too
Daily, monthly, quarterly, or annual compounding can produce different results. The more often interest compounds, the more often the balance gets updated.
At first, simple and compound interest may look close. After years, compound interest can pull away because each round of interest creates a slightly larger base for the next round.
Good places to double-check
Look at the interest method, compounding frequency, rate, and timeline. For credit cards, savings accounts, CDs, and loans, the product terms tell you how interest is calculated.