Data & Methodology
The historical datasets#
The backtesting tools (the SWR backtester and the Monte Carlo simulator) run on long-run monthly series covering 1871 through 2025, roughly 154 years of market history. Each series is a total-return index normalized to 100 at January 1871, so a value of 8,000 means that asset grew 80x over the period, dividends and reinvestment included.
| Asset class | Period | Frequency | What it represents |
|---|---|---|---|
| US equities | 1871–2025 | Monthly | Broad US stock market, total return |
| Ex-US equities | 1871–2025 | Monthly | International developed-market stocks, total return (proxy) |
| US bonds | 1871–2025 | Monthly | Long-term US government bonds, total return |
| Gold | 1871–2025 | Monthly | Gold spot price, in nominal terms |
| Commodities | 1871–2025 | Monthly | Broad commodity basket (proxy) |
| Cash | 1871–2025 | Monthly | Short-term rates / money market, total return |
| US inflation (CPI) | 1871–2025 | Monthly | Consumer Price Index, used to convert nominal to real |
The US equity, bond and inflation series derive from the long-run dataset compiled by Robert Shiller at Yale, the same 1871-to-present data used across the academic safe-withdrawal-rate literature (Bengen's original 4% work, the Trinity Study, and the deep retirement-research blogs). The ex-US equity, gold, commodity and cash series are proxy extensions assembled over the same window. Where a series is a reconstruction or proxy rather than a direct historical record, I have flagged it as such above.
How each calculator works#
SWR Backtester#
This is not a simulation. It runs your withdrawal plan across every historical rolling window in the 1871–2025 data and reports how the plan would actually have fared. Real returns, real inflation, real sequence-of-returns risk. If a 4% withdrawal rate failed in the 1906 or 1966 retiree's window, you will see it fail here.
Monte Carlo Simulator#
Where the backtester replays history, the Monte Carlo tool generates thousands of possible futures. It draws on the historical return, volatility and correlation of the asset classes above, then runs 10,000 simulated paths by default (selectable from 1,000 up to 100,000). Defaults: a 50-year horizon and a 3% withdrawal rate, both adjustable, with an optional advisor-fee drag. The output is a success probability: the share of simulated retirements that never ran out of money.
Savings Rate Calculator#
The "Years to FIRE" estimate assumes a 7% annual return and a 4% safe withdrawal rate (the 25x-expenses rule). These are deliberately simple round assumptions for a quick estimate. Your real timeline depends on actual market returns, taxes, currency drift and life changes, which is exactly why the backtester and Monte Carlo tools exist.
Compound Interest Calculator#
Standard compound-growth formula. You set the rate, the contribution amount and the compounding frequency; it projects the balance forward. No hidden assumptions beyond the inputs you provide.
FIRE Calculator#
Built on the 25x rule: your FI number is 25 times your annual expenses, the inverse of a 4% withdrawal rate.
Emergency Fund Calculator#
Months of essential expenses times your monthly burn. No market assumptions; it is a budgeting tool, not a projection.
Portfolio Rebalancer#
Compares your current allocation against your target, shows the drift, and lists the trades that bring you back in line. UI-only, nothing is stored.
Honest limitations#
A few things I want stated plainly rather than buried:
- Past performance is not a forecast. 154 years of data is a strong guide but doesn't predict the future. The next 30 years will not be a clean replay of the last 150.
- The history is mostly US. It is the deepest, cleanest long-run dataset available, but a globally mobile investor faces currency moves, foreign withholding tax, residency rules and estate-tax exposure that no return series captures. Treat US history as a baseline, then adjust for your own situation.
- These are planning tools, not advice. They help you reason about ranges and tradeoffs. They do not know your tax residency, your broker, or your risk tolerance. Nothing here is financial advice. See the full disclaimer.
If you spot something that looks wrong in the data or the math, tell me.
Last reviewed: June 2026.