The Core Difference
Simple interest calculates returns only on the original principal. Compound interest calculates returns on both the principal and accumulated interest. This seemingly small difference creates dramatically different outcomes over time.
Simple Interest Formula
The formula for simple interest is: I = P × r × t, where P is principal, r is the annual rate, and t is time in years. The total amount is A = P + I = P(1 + rt).
Example: $10,000 at 7% simple interest for 20 years = $10,000 × 0.07 × 20 = $14,000 in interest, for a total of $24,000.
Compound Interest Formula
The formula for compound interest is: A = P(1 + r/n)^(nt). The same $10,000 at 7% compounded monthly for 20 years grows to $40,064 — more than 67% more than simple interest.
When Is Simple Interest Used?
Simple interest is used for short-term loans, car loans (in some cases), and certain bonds. It is also used as a quick approximation when the compounding effect is small. Most savings accounts, mortgages, and investment accounts use compound interest.
The Practical Takeaway
When you are saving or investing, compound interest works in your favor — always seek accounts that compound frequently. When you are borrowing, compound interest works against you — understand how often your debt compounds and prioritize paying it off quickly.