Compound Interest Calculator
See exactly how your money grows over 20 years — year by year
Compound interest is how wealth is actually built. Every dollar you invest earns returns, and those returns earn returns — exponentially, not linearly. This calculator shows the real numbers: enter your starting amount, any monthly contribution, and your expected annual return rate, and see exactly what your balance looks like each year for the next 20 years. The bar that turns orange is the year your money doubles.
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Start the CourseFrequently Asked Questions
What is compound interest?
Compound interest is interest earned on both your original principal and the interest you have already accumulated. Unlike simple interest, which only applies to the principal, compound interest grows exponentially the longer you leave money invested. Albert Einstein reportedly called it the eighth wonder of the world.
How do I use this compound interest calculator?
Enter your starting amount, any monthly contributions you plan to make, and your expected annual return rate. The calculator projects your balance year by year over 20 years and highlights the year your money doubles in orange. The default 7% rate reflects the long-run average annual return of a diversified US stock index fund.
What is the Rule of 72?
The Rule of 72 is a shortcut to estimate how long it takes to double your money. Divide 72 by your annual return rate. At 7% annual return, money doubles roughly every 10.3 years (72 / 7 = 10.3). This calculator shows the exact year your balance crosses the doubling threshold.
Why does starting early matter so much?
The earlier you start, the more compounding cycles your money goes through. An extra 10 years at the start of an investment period can produce more wealth than doubling your monthly contribution later. This is why time in the market matters more than timing the market.
How much should I contribute monthly?
Even small amounts make a large difference over time. At 7% annual return, contributing an extra $200 per month adds over $52,000 to your balance after 20 years. Use the monthly contribution field to see exactly what your specific number means for your outcome.