Methodology
How assumptions become retirement estimates.
Last updated: October 2026
No financial advice
This page explains a model for estimate work. Results are educational and should not be interpreted as personalized financial guidance.
This is an educational planning model. It is designed to compare scenarios clearly, not to predict a single guaranteed date.
What the model calculates
The calculator uses your current savings, monthly contributions, target retirement age, inflation, return assumptions, withdrawal rate, and optional Social Security bridge as inputs.
The FIRE number is a retirement-year-dollar target: today’s modeled spending need is inflated to the target retirement age, then divided by the safe withdrawal rate. The projected portfolio is also expressed at retirement age, so the funding gap compares two numbers in the same unit.
The projection continues from retirement through a fixed life expectancy age of 95, so longer or shorter retirements change the number of withdrawal years in both the deterministic path and Monte Carlo runs.
Each retirement year withdraws your inflation-adjusted spending need, not a percentage of the portfolio. The safe withdrawal rate sets the size of the FIRE target and the safe withdrawal amount, but it does not change the Monte Carlo success rate for the same inputs.
Social Security bridge (modeled)
When enabled, the model subtracts a flat educational Social Security amount of $21,600 per year starting at age 67. This is not a personalized SSA estimate. Before age 67, retirement withdrawals still use full spending; after 67, withdrawals use spending net of that bridge amount, never above the original spending target.
If you retire before 67, the FIRE target stays conservative by using full pre-Social-Security spending (inflated to retirement age) rather than assuming benefits begin at retirement.
Monte Carlo and volatility
In addition to a deterministic projection, the model runs 1,000 independent annual return paths drawn from a normal distribution using your expected return and volatility. There is no mean reversion or historical sequence replay.
A path “survives” if the portfolio stays non-negative through every retirement year until life expectancy age 95. The success rate is the share of surviving paths. It is a probability estimate under the model, not a guarantee.
What is simplified
The model omits taxes, fees, employer plan rules, account location, pensions, healthcare premiums, Social Security claiming strategy detail, and market regime shifts. Contributions are modeled as end-of-year deposits during accumulation.
For personal decisions, compare your own constraints and consult a qualified professional.
Interpretation and safety notes
Use the result trio together: FIRE number, projected portfolio, and success rate. Any single value on its own can mislead.
No result is a promise. This is a planning aid for estimating ranges.