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Compound interest means interest is calculated on the original amount plus previously earned or charged interest. In savings and investing, compounding can help money grow. In debt, compounding can make balances more expensive when interest keeps being added.
Simple interest vs. compound interest
Simple interest is calculated only on the original principal. Compound interest is calculated on principal plus accumulated interest. That difference becomes more important as time passes.
Example for savings
If you deposit $1,000 at 5% annual interest, simple interest would add $50 each year. With annual compounding, year two starts with $1,050, so the next 5% is calculated on a larger balance. The growth starts slowly, then accelerates over time.
Example for debt
Compounding can work against you with credit cards or unpaid balances. If interest is added and you do not reduce the principal, future interest may be charged on a larger balance. That is why paying only the minimum can keep debt expensive for a long time.
The three variables that matter
- Time: more time gives compounding more room to work.
- Rate: a higher rate accelerates growth or debt cost.
- Frequency: more frequent compounding can increase the effective annual cost or return.
How to use this concept
For savings, start earlier when possible and reinvest earnings. For debt, reduce the principal quickly and compare APRs carefully. For loans, remember that amortization is different from pure compounding, but the same principle applies: the rate, balance, and time determine how expensive borrowing becomes.
