How Simple Interest Works
Simple interest is the easiest kind of interest to understand: it is charged only on the original amount you borrow or invest, never on interest that has already piled up. Banks use it for short-term loans, car financing, and some savings products โ anywhere the math needs to stay transparent.
The Simple Interest Formula
- I = P x R x T โ Interest equals Principal times Rate times Time.
- P is the starting amount (for example, $5,000).
- R is the annual rate as a decimal (6% becomes 0.06).
- T is the time in years.
- Total amount = P + I.
A Quick Example
Borrow $5,000 at 6% annual simple interest for 4 years: I = 5000 x 0.06 x 4 = $1,200. You pay back $6,200 total. The calculator above runs the same steps instantly and shows every line of working.
Simple vs. Compound Interest
With simple interest your balance grows in a straight line; with compound interest it curves upward because each period's interest starts earning its own interest. Over long periods the difference is enormous โ which is why long-term investing favors compounding. Compare both with our compound interest calculator, or handle the arithmetic with the four-function calculator.