Compound Interest & FIRE Wealth Simulator
Model how recurring contributions and compounding returns accelerate wealth accumulation toward financial independence.
Calculation Parameters
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Mathematical Formula & Notation
Standard discrete compound interest formula with recurring periodic deposits:
Real-World Wealth Scenario: $500/Month from Age 25 to 50
Suppose an investor starts at age 25 with an initial balance of $10,000 and commits to contributing $500 per month into a low-cost broad-market index fund averaging an 8.0% annualized return:
- Initial Investment (P): $10,000 * Monthly Deposits (PMT): $500 × 12 × 25 years = $150,000 in personal savings * Total Personal Capital Invested: $160,000 Over the 25-year horizon, compounding interest contributes $319,285.40 in pure investment gains. The final portfolio reaches $479,285.40. More than 66.6% of the final wealth was generated by compounding interest, demonstrating why starting early is far more impactful than trying to time the market.
Asset Class Growth Comparison: $10,000 Initial + $500/Mo over 25 Years
| Asset Vehicle | Avg Annual Return | Total Invested | Final Portfolio Value | Interest Multiplier |
|---|---|---|---|---|
| High-Yield Cash / CDs | 4.0% | $160,000 | $285,150 | 1.78x |
| Conservative Bond Mix | 5.5% | $160,000 | $355,420 | 2.22x |
| Balanced 60/40 Portfolio | 7.0% | $160,000 | $445,890 | 2.78x |
| Broad Market Index (S&P 500) | 9.5% | $160,000 | $645,820 | 4.03x |
Frequently Asked Questions
What is the Rule of 72 and how does it estimate portfolio doubling time? ▼
The Rule of 72 is a mental shortcut to calculate approximately how many years it takes for an investment to double at a fixed annual return. Divide 72 by the annual interest rate: at an 8% return, your money doubles approximately every 9 years (72 ÷ 8 = 9).
How does inflation affect the purchasing power of future compound growth? ▼
Inflation erodes the purchasing power of future dollars over time. To model real wealth rather than nominal wealth, subtract the expected annual inflation rate (historically ~2.5% to 3.0% in the US) from your expected gross return. For example, an 8% nominal stock return equates to roughly a 5% to 5.5% real return in today's purchasing power.
What is the difference between APR and APY? ▼
APR (Annual Percentage Rate) reflects the simple annualized interest rate without taking compounding into account within the year. APY (Annual Percentage Yield) reflects the true effective annual rate including the compounding effect. Because compounding earns interest on interest, APY is always slightly higher than APR for frequencies greater than once per year.
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