Free Monte Carlo Simulation Calculator for Retirement Portfolios
A Monte Carlo simulation calculator tests a retirement plan against many randomized market paths instead of one fixed average return. This free version runs 100 correlated simulations in your browser: enter your age, savings, contributions, and retirement spending, pick an allocation and a published forward-looking forecast, and it returns your plan's success rate plus 10th, 50th, and 90th percentile ending balances. The full QuantCalc planner extends the same engine to 10,000 paths with taxes, RMDs, IRMAA, and ACA modeling — also free.
What This Monte Carlo Calculator Does
A single average-return projection hides the most important risk in retirement planning: the order of returns. Two retirees with identical average returns can have completely different outcomes depending on whether the bad years arrive early or late. Monte Carlo simulation addresses this by generating many possible market futures and replaying your plan through each one.
Each simulated path on this page works like this:
- Accumulation: from your current age to retirement age, your balance grows with simulated market returns plus your monthly contributions.
- Withdrawal: from retirement age to your plan-to age, the portfolio pays your inflation-adjusted monthly spending while continuing to experience market returns.
- Scoring: a path succeeds if the balance stays above zero through the plan-to age. The success rate is the share of paths that succeed.
Returns for US stocks, international stocks, bonds, real estate, and cash are drawn as correlated log-normal variables, so asset classes move together the way they do in real markets rather than independently. The expected returns, volatilities, and correlations come from the forecast source you select.
How to Read the Results
The headline number is the success rate. The three percentile figures describe the spread of inflation-adjusted ending balances across paths: the 10th percentile is what a poor sequence of markets leaves you with, the median is the middle outcome, and the 90th percentile reflects strong markets. A wide gap between the 10th and 90th percentiles means your plan's outcome depends heavily on market luck — which is exactly what this kind of simulation is designed to reveal.
Methodology
This page uses the same simulation engine as the full QuantCalc planner, limited to 100 paths per run. Asset returns are generated as correlated log-normal draws: a Cholesky decomposition of the correlation matrix transforms independent random draws into realistically co-moving asset returns, which are then combined according to your allocation. All sources simulate on a consistent geometric (compound annual) return basis, and results are reported in inflation-adjusted dollars.
The full planner extends this engine to 10,000 paths per run and layers on federal and 51-jurisdiction state income tax, Required Minimum Distributions, IRMAA Medicare surcharge tiers, the ACA subsidy cliff, stochastic inflation, and quasi-Monte Carlo (Sobol) sampling.
Frequently Asked Questions
What does a Monte Carlo retirement simulation calculate?
Instead of assuming one fixed average return, it replays your plan across many randomized market paths — each with a different sequence of good and bad years drawn from the expected return, volatility, and correlation of your asset classes. The output is a success rate plus a range of possible ending balances.
What does the success rate mean?
It is the percentage of simulated paths in which your portfolio never runs out of money before your plan-to age. A 90% success rate means the portfolio sustained your retirement spending in 90 of 100 simulated paths.
How many simulations does this free calculator run?
100 per click, using the same engine as the full planner. The full QuantCalc planner runs 10,000 paths per simulation and adds taxes, RMDs, IRMAA, and ACA modeling — also free to run.
Where do the return assumptions come from?
From the forecast source you select — forward-looking capital market expectations derived from publicly available research published by major asset managers. Details, formulas, and conventions are documented on the methodology page.