Methodology & Data Sources
Table of Contents
1. What is Monte Carlo Simulation?
Unlike a single projection that assumes constant returns, Monte Carlo simulation runs 1,000 different scenarios with varying market returns to assess the range of possible outcomes.
Why 1,000 Scenarios?
Markets don't grow smoothly. Some years +30%, some years -20%. Your retirement success depends heavily on when those good/bad years happen (sequence of returns risk).
2. Our Calculation Methodology
We use parametric Monte Carlo simulation, which generates random returns based on historical averages. Each of the 1,000 scenarios simulates what could happen over your retirement based on different sequences of market returns.
How We Generate Returns
Current Approach: Parametric Simulation
Returns are generated using a normal distribution with historical mean and volatility:
- Stocks: 10% average return, 18% volatility (based on S&P 500 historical data)
- Bonds: 4% average return, 5% volatility (based on aggregate bond market)
- Inflation: 2.5-3% average (based on CPI historical data)
Each year's return is randomly generated using the Box-Muller transform to create a normal distribution.
Future Enhancement: Historical Data Replay
We're working on adding support for historical sequence simulation, which will replay actual market sequences from history (e.g., what if you retired in 1929, 2000, or 2008?). This will provide additional validation alongside the parametric approach.
Simulation Process (Each Scenario)
- Generate annual returns for stocks, bonds, and inflation
- Apply returns to each account type (taxable, IRA, 401k, Roth, etc.)
- Process annual withdrawals from accounts in tax-efficient order
- Calculate taxes on withdrawals, Social Security, and RMDs
- Deduct spending (adjusted for inflation)
- Add income (Social Security, pensions, part-time work)
- Track portfolio balance year by year until death or depletion
3. Data Sources & Assumptions
All default parameters are based on historical data from reputable sources:
Stock Returns (Equities)
Bond Returns (Fixed Income)
Inflation
Social Security
4. Tax Calculations
The calculator performs comprehensive tax calculations using current tax law. These are estimates and may not reflect your exact situation.
Federal Income Tax
Uses 2024 federal tax brackets with progressive taxation on taxable income (after standard deduction):
| Tax Rate | Single Filers | Married Filing Jointly |
|---|---|---|
| 10% | $0 - $11,600 | $0 - $23,200 |
| 12% | $11,601 - $47,150 | $23,201 - $94,300 |
| 22% | $47,151 - $100,525 | $94,301 - $201,050 |
| 24% | $100,526 - $191,950 | $201,051 - $383,900 |
| 32% | $191,951 - $243,725 | $383,901 - $487,450 |
| 35% | $243,726 - $609,350 | $487,451 - $731,200 |
| 37% | $609,351+ | $731,201+ |
Standard deduction: $14,600 (single) / $29,200 (married filing jointly)
Social Security Taxation
Social Security benefits are taxed based on combined income (AGI + 50% of SS benefits):
Single Filers:
- 0% taxable if combined income up to $25,000
- Up to 50% taxable if combined income $25,001-$34,000
- Up to 85% taxable if combined income over $34,000
Married Filing Jointly:
- 0% taxable if combined income up to $32,000
- Up to 50% taxable if combined income $32,001-$44,000
- Up to 85% taxable if combined income over $44,000
Required Minimum Distributions (RMDs)
RMDs from traditional IRA/401k accounts are automatically calculated:
- Start at age 73 for those who reached age 72 after December 31, 2022 (per SECURE Act 2.0)
- Age 75 starting January 1, 2033 for those born in 1960 or later
- Based on IRS Uniform Lifetime Table
- Automatically included in taxable income
- Penalties not modeled (assumes compliance)
State & Local Taxes
Optional state income tax (flat rate) can be specified. Does not model complex state tax rules, deductions, or exemptions.
FICA Taxes
Applied to part-time work income:
- Social Security: 6.2% on income up to wage base ($168,600 in 2024)
- Medicare: 1.45% on all wages, plus 0.9% on income over $200k/$250k
5. What This Calculator Can't Do
It's important to understand what this calculator does NOT account for:
- •Market crashes and black swan events: The simulation uses normal distribution which underestimates extreme events (e.g., 2008 financial crisis, COVID-19 crash).
- •Complex tax situations: Tax loss harvesting, capital gains timing, Medicare IRMAA brackets, Roth conversion strategies are simplified or not modeled.
- •Estate planning: Inheritance, trusts, estate taxes, and legacy planning are not included.
- •Long-term care costs: Nursing home, assisted living, or in-home care expenses beyond basic healthcare inflation.
- •Future tax law changes: Tax brackets, Social Security rules, and RMD age requirements may change.
- •Sequence correlation: Stock/bond returns are treated as independent; actual markets show serial correlation and regime changes.
- •Individual stock picking: Assumes diversified index-like returns, not individual security selection.
Bottom line: This calculator provides directional guidance and helps you understand trade-offs. It is not a substitute for comprehensive financial planning, especially for complex situations.
6. Academic Research & Validation
Our methodology is based on peer-reviewed academic research in retirement planning:
The Trinity Study (1998)
Phillip L. Cooley, Carl M. Hubbard, and Daniel T. Walz
Foundational research on safe withdrawal rates using historical data from 1926-1995. Established the "4% rule" and demonstrated the importance of portfolio allocation and time horizon.
Read the paperDetermining Withdrawal Rates Using Historical Data (1994)
William P. Bengen
Original research that coined the "4% rule" by analyzing rolling 30-year periods from 1926-1976. Showed worst-case scenarios and the impact of sequence of returns risk.
Read the paperFrom Assets to Income: A Goals-Based Approach (2020)
Vanguard Research
Comprehensive analysis of retirement spending strategies including dynamic withdrawal approaches and the role of annuities. Uses Monte Carlo simulation to assess success rates.
Read the researchResolving the Retirement Spending Puzzle
Michael Kitces, CFP
Ongoing research on dynamic spending rules, sequence of returns risk, and guardrails-based strategies for sustainable retirement income.
Read the research7. When to Consult a Professional
This calculator is a planning tool, not a substitute for personalized financial advice. Consider consulting a Certified Financial Planner (CFP) if you have:
- •Large taxable investment accounts (capital gains strategies)
- •Complex income sources (stock options, RSUs, K-1 partnerships)
- •Estate planning needs (trusts, inheritance, gifting)
- •Multiple states of residence (tax domicile questions)
- •Small business ownership (exit strategies, succession planning)
- •Pension maximization decisions (lump sum vs. annuity)
- •Special needs dependents or beneficiaries
- •Portfolio value > $2M (higher tax complexity)
How to find a fee-only CFP: Visit NAPFA.org or FeeOnlyNetwork.com to find fiduciary advisors who don't earn commissions on product sales.