Simple vs compound interest
Simple interest is calculated only on the original principal amount—the initial sum borrowed or deposited. Compound interest is calculated on the principal plus any accumulated interest that has been added to the balance. These represent two fundamentally different methods of interest calculation that produce increasingly different results over time. With simple interest, the interest amount remains constant each period because it's always based on the original principal. If $1,000 is borrowed at 10% simple interest, the interest is $100 each year regardless of how many years pass. The interest does not generate additional interest. With compound interest, each period's interest is added to the balance, and the next period's interest is calculated on the new, larger balance. The interest earns interest, creating an accelerating effect. The more frequently interest compounds—daily, monthly, quarterly, or annually—the more pronounced this effect becomes. For borrowers, compound interest increases the total cost of debt compared to simple interest. Credit cards typically use compound interest, which means carrying a balance becomes progressively more expensive over time. The interest charged in one month becomes part of the balance that generates interest the next month. For savers, compound interest works in their favor. Interest earned on savings generates additional interest in subsequent periods. Over long time horizons, the compound effect can produce results that seem disproportionate to the contributions made. Albert Einstein reportedly called compound interest the eighth wonder of the world, though the attribution is uncertain. The difference between simple and compound interest is small over short periods but grows substantially over longer ones. Over one year, the difference might be negligible. Over 20 or 30 years, compound interest produces dramatically different results than simple interest on the same principal at the same rate.
Why It Matters
Compound interest accelerates growth for savings and accelerates cost for debt compared to simple interest. Most credit products use compound interest, which means debt that is not paid down grows faster than a simple interest calculation would suggest. Understanding the difference between simple and compound interest helps in evaluating financial products. A savings account that compounds daily produces slightly more than one that compounds monthly at the same stated rate. A loan that compounds monthly costs more than one that uses simple interest at the same rate. These differences may seem technical but affect real costs and returns over time.
Example
Scenario 1: $1,000 at 10% simple interest for 3 years: Year 1 interest $100, Year 2 interest $100, Year 3 interest $100. Total interest: $300. Final balance: $1,300. Scenario 2: $1,000 at 10% compound interest (annually) for 3 years: Year 1 interest $100 (on $1,000), Year 2 interest $110 (on $1,100), Year 3 interest $121 (on $1,210). Total interest: $331. Final balance: $1,331. The extra $31 came from interest earning interest. Scenario 3: Over 30 years, the difference becomes dramatic. $10,000 at 7% simple interest: $31,000 total. $10,000 at 7% compound interest (annually): $76,123 total. The compound version produces more than twice as much because each year's interest joins the principal and generates its own interest.
