Economics
Simple Interest Rate
Quick fact
Simple interest was used in ancient Babylonian loans dating back to 2000 BCE, making it one of the oldest financial calculations.
Why this is interesting
You borrow $1,000 from a friend at a 5% simple interest rate per year. After 3 years, you owe $1,150 — but why exactly $150 and not more?
Read the full explanation
Understanding Simple Interest Rate
Simple interest is calculated only on the original amount of money, called the principal. Think of it like renting money: you pay a fixed percentage of the principal for each unit of time (e.g., year). If you borrow $1,000 at 5% per year, you pay $50 in interest each year, regardless of how long you keep the money. After 1 year, total owed is $1,050; after 2 years, $1,100; after 3 years, $1,150. The interest does not grow — it's always 5% of the original $1,000. This linear growth distinguishes it from compound interest, where interest earns interest and grows exponentially.
A deeper explanation
The mechanism behind simple interest is linear proportionality: interest (I) = principal (P) × annual rate (r) × time (t). For example, if P = $1,000, r = 0.05, t = 3 years, then I = 1000 × 0.05 × 3 = $150. The total amount A = P + I = P + P×r×t = P(1 + r×t). This formula assumes that no interest is added to the principal during the loan period. Simple interest is widely used for short-term loans (e.g., car loans, personal loans), bonds with fixed coupons, and certain savings accounts. Its simplicity makes it easy to understand and compare offers, but over long periods, compound interest yields much higher returns. Understanding simple interest is the first step toward grasping the time value of money — the idea that a dollar today is worth more than a dollar tomorrow because it can earn interest.