Formula
Future value = P(1 + r)n + C × ((1 + r)n − 1) ÷ r, where P is the starting amount, C the monthly contribution, r the monthly rate (annual ÷ 12) and n the number of months.
Compounding means you earn interest on previously earned interest, so growth speeds up the longer money is left invested.
FAQs
What is the rule of 72?
A shortcut: divide 72 by the annual rate to estimate how many years it takes money to double. At 6% that is about 12 years.