Markets don't return the average every year. This runs 1,000 random scenarios to estimate the chance your money lasts.
Estimates the probability your retirement savings survive by running 1,000 randomized market scenarios against inflation-adjusted withdrawals.
Each of the 1,000 trials draws a random return for every year from a normal distribution defined by your average return and volatility, then subtracts a withdrawal that grows with inflation. A trial counts as a success if the balance stays above zero for the full retirement horizon. The reported probability is simply the share of trials that finished with money remaining, which gives a fuller picture than a single fixed-return projection.
A success rate near 90% or higher is often considered comfortable, while results below 70% suggest trimming spending, working longer, or adjusting your asset mix. Remember that a normal distribution understates rare crashes and ignores sequence-of-returns nuances beyond what the random draws capture. This is an educational model, not financial advice, and real markets can behave outside the assumptions you enter.
Many planners target 85% to 95%, which balances confidence against overly cautious underspending. Lower figures signal higher risk of running short.
The returns are drawn randomly, so small run-to-run variation is normal even with the same inputs. Averaging across 1,000 trials keeps the overall probability stable.
No, withdrawals are treated as gross amounts and no taxes, fees, or Social Security income are modeled. Adjust your withdrawal figure to approximate those effects.
See the exact formula and a worked example on our methodology page.