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Compound Interest Calculator 2026 · Project Your Investment Growth

Compound interest is what Albert Einstein famously called the "eighth wonder of the world" — he who understands it earns it, and he who doesn't pays it. In simple terms, compound interest is interest earned on interest. Over long periods, the exponential growth generated by compounding dwarfs the original capital you invested.
For American investors, understanding compound interest is the key to building substantial wealth through vehicles like 401(k) retirement plans, Roth IRAs, and low-cost S&P 500 index funds. Historically, the US stock market has delivered an average annualized return of approximately 10% (around 7% after adjusting for inflation) over multi-decade horizons.
Use this Compound Interest Calculator to visualize your future portfolio balance. Simulate different initial deposits, monthly contributions, expected annual rates of return, and investment time horizons to discover the exponential power of starting early.
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When to Use This Calculator

Everyday financial situations where this tool gives you fast, accurate clarity:

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Retirement Portfolio Growth

Project your nest egg at retirement by contributing systematically to a 401(k) or Roth IRA each month.

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College Savings (529 Plan)

Calculate how much you will accumulate for college tuition by investing from your child birth to age 18.

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Long-Term Generational Wealth

See how modest, regular contributions outperform waiting years to invest a lump sum.

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The Cost of Delay

Measure the impact of starting to invest in your 20s versus waiting until your 30s.

lightbulbFormula & Worked Example

How It Works

The standard compound interest formula with regular monthly additions is:\n\n
\nA = P * (1 + r/n)^(n*t) + PMT * [ ((1 + r/n)^(n*t) - 1) / (r/n) ]\n
\n\nWhere:\n* P = Initial principal balance\n* r = Annual interest rate (in decimal format, e.g., 0.08 for 8%)\n* n = Compounding frequency per year (12 for monthly)\n* t = Number of years the money is invested\n* PMT = Monthly contribution amount\n* A = Total accrued amount (future value)

Step-by-Step Example

If you start with $10,000 and invest $500 every month for 20 years at an average annual return of 8%:\n\n* Your total contributions: $130,000 ($10,000 initial + $120,000 monthly)\n* Interest earned: approximately $214,570\n* Ending balance: over $344,570\n\nMore than 60% of your final balance comes directly from interest, not your own pocket.

Frequently Asked Questions

Compound interest is interest earned not just on your initial principal, but also on the accumulated interest from previous periods. As your balance grows, each compounding cycle generates larger returns, producing exponential wealth growth over time.
The Rule of 72 is a mental shortcut to estimate how many years it will take for your investment to double at a given annual interest rate: divide 72 by your expected annual return. For example, at an 8% return, your money doubles approximately every 9 years (72 / 8 = 9).
Historically, the S&P 500 index has returned an average of approximately 10% per year before inflation over the past 50 years. After adjusting for US inflation, the historical real return is roughly 7% annually. Most financial planners use 6% to 8% for long-term projections.
The more frequently interest compounds, the faster your money grows. Daily compounding yields slightly more than monthly compounding, which yields more than annual compounding. For stock portfolios with monthly contributions, monthly compounding provides a standard, accurate projection.
Simple interest is calculated only on the original principal balance. Compound interest is calculated on the initial principal plus all previously accumulated interest. Over a 30-year horizon, compound interest can generate 3 to 5 times more wealth than simple interest.
Time is the most powerful multiplier in compounding. An investor who invests $300/month from age 25 to 35 and then stops will often end up with more money at age 65 than someone who invests $300/month every single month from age 35 to 65, purely because of an extra decade of compound growth.

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