Savings, rates and protection
What is a compound interest?
Quick definition: Compound interest is interest calculated on the original amount plus interest already added, so balances can grow or debts can increase faster over time.
At a glance
- It can help savings grow when interest is left in the account.
- It can also increase borrowing costs when interest is added to debt.
- The effect grows with time and frequency of compounding.
- It is different from simple interest.
Explain it simply
Compound interest means earning interest on interest. If you save 100 pounds and earn interest, the next interest calculation may be based on more than 100 pounds because the first interest has been added. Over time, this can help savings grow. The same idea can work against borrowers if interest is added to a debt and then more interest is charged on the larger amount. Time makes compounding more powerful.
Student explanation
Compound interest is an important idea in savings, loans and inflation-adjusted thinking. With simple interest, interest is calculated only on the original amount. With compound interest, each period's interest is added to the balance, and future interest is calculated on the new balance. The compounding frequency matters: daily, monthly or annual compounding can produce different results. Students should understand that compounding can be helpful for long-term saving but harmful when unpaid debt grows.
Professional explanation
Compound interest is the process by which interest accrues on both principal and previously accrued interest. It is central to effective annual rates, savings projections, loan amortisation, arrears balances and investment return calculations. The compounding outcome depends on nominal rate, compounding frequency, balance changes, fees, tax treatment and withdrawal behaviour. In customer communications, compounding should be explained plainly because headline rates can understate or obscure long-term effects. It should be distinguished from simple interest, flat rate pricing and annual percentage disclosure measures that follow prescribed assumptions.
UK example
If interest is added to a savings account each month and left there, future interest can be calculated on the larger balance.
Why it matters
Compound interest is one of the clearest reasons why time matters in both saving and borrowing.
Common misunderstanding
Compound interest is not always good; it helps savers but can make unpaid borrowing grow more quickly.