Compound interest formula
A = P × (1 + r/n)n × t, where P is the starting amount, r the annual rate, n the number of times interest is compounded per year and t the number of years.
With compounding you earn interest on your interest, so growth speeds up over time. That's why starting early matters more than investing large amounts later.
Rule of 72
A quick way to estimate how long money takes to double: divide 72 by the annual interest rate. At 8%, money doubles in about 72 ÷ 8 = 9 years.
Simple vs compound interest
Simple interest is calculated only on the original amount. Compound interest is calculated on the growing balance. The “with simple interest” figure above shows the difference for the same inputs.