Simple vs compound interest
Simple interest uses one clean formula: Interest = Principal × Rate × Time. It's charged only on the original principal — the interest never earns interest of its own. That makes it easy to calculate and predictable, but it also means it grows far slower than compound interest over long periods.
On $10,000 at 5% for 3 years, simple interest is a flat $1,500 ($500 per year). Compound interest on the same terms would be slightly more (~$1,576) because each year's interest joins the balance. Over 3 years the gap is small; over 30 years it becomes enormous — which is why compounding is a saver's friend and simple interest is often a borrower's.
Where simple interest is actually used
- Short-term and personal loans — many are quoted with simple interest on the principal.
- Car loans in some markets accrue simple interest daily on the outstanding balance.
- Bonds typically pay simple interest (coupons) on their face value.
- Promissory notes and informal loans between people often use simple interest for its transparency.
A borrower's advantage
With a simple-interest loan, paying early genuinely helps: because interest often accrues daily on the remaining balance, every extra payment reduces future interest immediately. Contrast this with pre-computed interest loans, where the total interest is fixed upfront and early payoff saves less. If you have a simple-interest loan, paying a little extra whenever you can is a reliable way to cut its total cost.