Engineering Financial Independence: Compound SIP Meets the Trinity Study
Achieving FIRE (Financial Independence, Retire Early) is a deterministic engineering problem governed by four variables: your current investable principal, monthly systematic investment plan (SIP) contribution, inflation-adjusted real rate of return, and Safe Withdrawal Rate (SWR).
Core Mathematical Equations Behind the Simulator
- 1. Target FIRE Portfolio Formula:
FIRE Target = Annual Retirement Spending / Safe Withdrawal Rate (SWR). At a 4.0% SWR, you need 25× annual spending; at 3.5% SWR, you need 28.57× annual spending. - 2. Fisher Real Return Equation:
Real Return = (1 + Nominal Return) / (1 + Inflation Rate) - 1. Evaluating your portfolio in today's purchasing power prevents the "inflation illusion" over 20–30 year horizons. - 3. Coast FIRE Principal Threshold:
Coast FIRE Today = Standard FIRE Target / (1 + Real Return)^(60 - Current Age). Once your portfolio crosses this threshold, you only need to earn enough to cover current living expenses while compound interest handles your retirement.
Frequently Asked Questions (FAQ)
Q1 What is the 4% Safe Withdrawal Rate (Trinity Study) rule in FIRE?
The 4% Rule comes from the landmark Trinity Study, which showed that withdrawing 4% of your initial portfolio in year one of retirement—and adjusting that dollar amount for inflation each subsequent year—historically sustained a diversified stock/bond portfolio for 30+ years in over 95% of market periods. Mathematically, a 4% Safe Withdrawal Rate (SWR) means your Standard FIRE Number equals 25x your annual expenses (or 300x your monthly expenses). For longer 40–50 year early retirements, many planners prefer a conservative 3.5% SWR (28.6x annual expenses).
Q2 Why does this calculator emphasize "Real Purchasing Power" alongside Nominal Net Worth?
Inflation silently erodes future nominal currency values. At 2.5% annual inflation, ¥3,000,000 in 25 years only buys what ¥1,618,000 buys today. Our engine discounts future nominal wealth by the cumulative inflation factor (1 + i)^t so that all FIRE milestones (Lean, Standard, Fat, and Coast FIRE) are evaluated strictly in today’s real purchasing power.
Q3 What is the difference between Lean FIRE, Standard FIRE, Fat FIRE, and Coast FIRE?
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Lean FIRE (70% of target spending): Covers essential living costs for minimalist freedom
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Standard FIRE (100%)： Fully funds your target retirement lifestyle
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Fat FIRE (150%)： Provides an abundant buffer for luxury travel and hobbies
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Coast FIRE： The exact portfolio amount you need TODAY so that even if you never save another cent (¥0/month contribution), compound real returns alone will grow your existing nest egg to your Standard FIRE goal by age
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