What the 5,000 simulations are and aren't
Last updated November 3, 2026
Your results come from 5,000 simulated retirements. Each one is built from real stock and bond returns, replayed ten years at a time in the order they happened. Run the same plan twice, and you'll get the same answer both times.
Each simulated retirement takes its returns from U.S. market history, going back to 1928. Stocks use the S&P 500's total return, and bonds use the 10-year Treasury. The two are blended by your stock and bond mix for that year.
The years come in stretches of ten. Each stretch starts at a random year in the record and carries on in order. So a crash and the recovery that followed it stay together, the way they did. A stretch that reaches past 2025 wraps back to 1928. Stringing stretches together builds far more histories than the record holds on its own.
Every plan is measured against the same set of market histories. So the same numbers always produce the same 5,000 simulated retirements and the same result, today or next week, on your phone or your laptop. When you change something, the result moves only by what the change did.
Not every number along the way uses the full 5,000. Live previews, while you're still typing, use 300 simulated retirements so they keep up. The final numbers on your dashboard use all 5,000.
What this doesn't model
- History longer than ten years in a row. A stretch like the 1970s stays together within ten years, but two stretches back to back may come from different decades.
- A forecast. It's a test against a wide range of what markets have done, not a guess at what they'll do next.