Economics
Simple Interest
Quick fact
If you deposit $1,000 in a savings account offering 5% simple interest per year, after 3 years you'll have earned $150 in interest—regardless of how long the money stays in the account beyond that period.
Why this is interesting
Imagine you lend $100 to a friend, and they promise to pay you back with 10% interest each year. Have you ever wondered exactly how much extra money you'll get after two years?
Read the full explanation
Understanding Simple Interest
Simple interest is like renting money. When you borrow (or lend) a sum called the 'principal', you agree to pay (or receive) a fixed percentage of that principal each time period (usually per year). The interest amount is always the same each period because it's calculated only on the original principal—never on previously earned interest. For example, if you borrow $500 at 8% annual simple interest for 2 years, each year you'll owe 8% of $500, which is $40. After 2 years, total interest is $80, and you repay $580. The formula is I = P × r × t, where P is principal, r is interest rate as a decimal, and t is time in years. This linear growth means the total interest grows smoothly and predictably over time.
A deeper explanation
The mechanism behind simple interest is fundamentally linear: the interest earned or owed is directly proportional to the principal and the time elapsed. This simplicity comes from the deliberate exclusion of compounding—interest is not added to the principal to earn more interest. The principle at work is a constant rate applied to a fixed base, making it easy to calculate and understand. Simple interest is common for short-term loans (like car loans or personal loans with fixed terms) and certain bonds. It matters because it provides a transparent way to compare the cost of borrowing or the return on savings without the complexity of compounding. Understanding this linear growth is a stepping stone to grasping how compound interest accelerates growth, contrasting a straight line with an exponential curve.