Compound interest pays interest on your interest, and the curve gets steeper every year. Enter a starting balance, an annual return, the number of years and any yearly additions to see the whole journey — including exactly how much of the final balance is your own money versus growth.
The formula
End balance = Principal × (1 + r)^years, plus, for a fixed annual addition A made at each year-end: A × ((1 + r)^n − 1) ÷ r. The tool computes this every year so you can see the growth rate of the growth itself.
The rule of 72
Divide 72 by your annual return to estimate the doubling time: 6% ≈ 12 years, 8% ≈ 9 years, 3% ≈ 24 years. It is a famous approximation and it is surprisingly accurate in the 4–12% range.
The cost of starting late
Invest $2,000 per year at 7%: starting at age 25 gives about $440,000 by 65; starting at 35 gives about $203,000 — ten fewer years costs more than half the outcome. Time in the market beats extra contributions later.