Compound Interest Calculator
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Total Future Value
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Total invested
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Interest earned
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Growth Over Time
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My Compound Growth Projection — Financial Depth
financialdepth.com
Quick Summary (TL;DR)
- Compound interest pays you on your principal and on prior interest — growth accelerates over time.
- Example: $10k + $500/mo at 7% for 30 yrs → ~$870k, from just $190k contributed.
- Time matters more than timing — starting earlier beats saving more later.
- Formula:
FV = P(1+i)n + PMT·[((1+i)n−1)÷i]
How Compound Interest Builds Wealth
Compound interest is the quiet engine behind every FIRE journey. Unlike simple interest, which pays only on your original principal, compounding pays you on your principal and on the interest you've already earned. Each period's gains join the base that earns the next period's return, so growth curves upward instead of climbing in a straight line — and the effect gets dramatic over decades.
This calculator projects future value with the formula FV = P(1+i)n + PMT · [ ((1+i)n − 1) / i ], where P is your initial investment, PMT is the monthly contribution, i is the monthly return (annual rate ÷ 12), and n is the number of months. Contributions are added monthly and compounded from the moment they land.
Worked example: Start with $10,000, add $500/month, and assume a 7% annual return for 30 years. You'd contribute $190,000 of your own money, but end with roughly $870,000 — meaning compounding did more of the heavy lifting than your deposits did. Stretch the timeline to 40 years and the gap widens even further. Time, not timing, is the FIRE investor's biggest advantage.
Frequently Asked Questions
What is compound interest?+
It's the interest you earn on both your original principal and on the interest already accumulated. Because each period's gains are added to the base that earns the next period's return, growth accelerates over time — the engine behind long-term investing and FIRE.
How is growth with monthly contributions calculated?+
Future value equals the initial amount compounded over the period, plus the future value of your stream of monthly contributions. We compound monthly using FV = P(1+i)ⁿ + PMT[((1+i)ⁿ − 1) / i], where i is the monthly rate and n is the number of months.
What return rate should I assume for FIRE planning?+
Many in the FIRE community model a long-run nominal return near 7% for a diversified stock portfolio, or roughly 4–5% after inflation. Use a conservative figure for planning, since actual returns vary year to year.