Calculation Methodology
How we ensure every projection is mathematically sound and actuarially validated.
Monte Carlo Simulation
RetirePro runs 1,000 independent simulations for every retirement projection. Each simulation generates a unique sequence of annual market returns using the Box-Muller transform — the same normal distribution method used in professional financial planning software and academic research.
Default Assumptions
- • Mean return: 7% nominal (historically calibrated to S&P 500 long-term average)
- • Standard deviation: 12% (reflects actual historical market volatility)
- • Inflation: 2.5% default (adjustable by user)
- • Distribution: Log-normal returns via Box-Muller transform
This approach captures sequence-of-returns risk — the danger that poor early returns permanently damage a retirement portfolio. A single average-return projection misses this entirely, which is why Monte Carlo is the gold standard in retirement planning.
Reference: Cooley, P., Hubbard, C., & Walz, D. (1998). “Retirement Savings: Choosing a Withdrawal Rate That Is Sustainable.” AAII Journal. (The Trinity Study)
Actuarial Validation
Every core formula in RetirePro has been independently validated against industry-standard tools and regulatory tables. Our validation covered:
| Calculation Area | Validation Method | Result |
|---|---|---|
| Future Value / Compound Interest | Compared to Excel & HP 12C financial calculator | ✓ Exact match |
| Monte Carlo Simulations | Box-Muller distribution verification | ✓ Statistically valid |
| Required Minimum Distributions | IRS Uniform Lifetime Table (2024) | ✓ Exact match |
| Social Security Benefits | SSA PIA bend points & claiming adjustments | ✓ Exact match |
| Mortgage / Debt Payoff | Standard amortization formula | ✓ Exact match |
| 72(t) SEPP Calculations | All 3 IRS-approved methods | ✓ Exact match |
Full validation report available upon request. Contact support@retirepro.io.
Social Security Modeling
Our Social Security optimizer uses the actual SSA formula structure:
- •PIA calculation using current bend points ($1,174 and $7,078 for 2025)
- •Early claiming reduction — 6.67% per year for the first 3 years before FRA, 5% per year for additional early years
- •Delayed retirement credits — 8% per year increase for claiming after FRA up to age 70
- •Spousal benefit modeling — up to 50% of higher earner's PIA
Source: Social Security Administration, PIA Formula
Safe Withdrawal Rate Research
RetirePro's withdrawal analysis is grounded in peer-reviewed research:
- •The Trinity Study (Cooley, Hubbard, Walz, 1998) — original 4% rule research using historical data
- •William Bengen's research (1994) — “Determining Withdrawal Rates Using Historical Data” establishing the 4% safe withdrawal rate
- •Wade Pfau's updates — modern safe withdrawal rate analysis incorporating current interest rate environments
Rather than using a fixed 4% rule, RetirePro lets you model any withdrawal rate and see the probability of success across 1,000 simulated market scenarios. This is more informative than any single “safe” number.
Data Privacy by Design
All financial calculations execute entirely in your web browser. Your portfolio balances, income figures, and retirement inputs are never transmitted to our servers. Data is stored in your browser's localStorage — and optionally synced to an encrypted cloud backup if you create an account.
We chose this architecture deliberately: in the YMYL (Your Money, Your Life) space, users should not have to trust a third party with their complete financial picture.
Known Limitations
No model is perfect. Here are the known limitations of our approach:
- △Returns are modeled as normally distributed; actual market returns exhibit fat tails and are not perfectly normal
- △Tax calculations use simplified federal bracket projections; state taxes and deduction details vary
- △Healthcare costs are estimated using national averages; individual costs vary significantly
- △Social Security projections assume current benefit formulas; Congress may modify them
RetirePro is not a substitute for professional financial advice. Use it to understand your numbers, then consult a qualified financial advisor for personalized guidance.