For loans, bonds, or deposits that don't compound.
Simple interest is calculated only on the original principal — it never "stacks." $10,000 at 5% simple interest earns $500 every year, forever. Compound interest earns interest on interest, growing exponentially.
I = P × r × t — where I = interest, P = principal, r = rate (decimal), t = years.
Looking for compound growth? Try our compound interest calculator.