Compound Interest Calculator
Calculate compound interest with monthly contributions. Visualize your investment growth year by year with an interactive bar chart.
About This Tool
Compound interest is the single most powerful concept in personal finance. Albert Einstein allegedly called it the "eighth wonder of the world" -- and while the attribution is debated, the math is not. When your investment earns returns, and those returns themselves earn returns, wealth grows exponentially rather than linearly. This calculator shows you exactly how that works with your numbers.
Unlike simple interest, which only earns returns on the original principal, compound interest earns returns on both the principal and the accumulated interest from previous periods. The difference is dramatic over time. A $10,000 investment at 7% simple interest earns $700 per year forever. The same investment with annual compounding earns $700 in year one, $749 in year two, $801.43 in year three, and so on -- accelerating each year.
This calculator goes beyond basic compound interest by including monthly contributions. Regular monthly investments are how most people actually build wealth -- through retirement accounts, brokerage deposits, or savings plans. The combination of compounding returns and consistent contributions creates a powerful growth engine that this tool quantifies precisely.
You can customize the compounding frequency to match your actual investment. Most savings accounts compound daily or monthly, CDs often compound monthly or quarterly, and some bonds compound semi-annually or annually. The more frequently interest compounds, the faster your money grows, though the differences become smaller as frequency increases.
The year-by-year growth chart below the calculator provides a visual representation of how your balance builds over time. The bars show both your total contributions (in gray) and your total balance (in orange), making the power of compounding visible. In the early years, contributions dominate. In later years, compound growth takes over, and the gap between contributions and total balance widens dramatically.
How to Use
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1
Enter your initial investment
The starting amount you are investing or have already invested.
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2
Set monthly contributions
How much you plan to add each month. Enter 0 if you are making a one-time investment.
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3
Enter the interest rate
The expected annual rate of return, expressed as a percentage.
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4
Choose the time period
How many years you plan to keep the investment growing.
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5
Select compounding frequency
How often interest is calculated and added: monthly, quarterly, or annually.
Where Does This Data Come From?
The compound interest formula used is: A = P(1 + r/n)^(nt) + PMT * [((1 + r/n)^(nt) - 1) / (r/n)], where P is the principal, r is the annual rate, n is the compounding frequency, t is time in years, and PMT is the periodic contribution adjusted to the compounding period. All calculations run client-side in your browser.