FIRE Monte Carlo · Assumptions visible
Will the money last? Run it 5,000 times.
A retirement simulator that shows its working. Every figure is in today’s dollars. Every assumption behind the numbers is published on this page — and editable.
Set your inputs, watch 5,000 possible futures, and read the odds your plan is funded to the end. The random seed is shown, so any result here is one you can reproduce exactly.
The model · free & in your browser
Your plan, stress-tested.
Nothing is hidden and nothing leaves your browser. Change any input or any assumption and the 5,000 paths re-run instantly — because a result you can inspect is worth more than one you have to trust.
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Portfolio balance over time
Percentile fan across all paths, in today’s dollars. Median in institution blue; the 10th–90th and 25th–75th bands in proof red.
| Percentile | At retirement | At horizon |
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When plans run out
For paths that failed, the age at which the balance hit zero. A tall bar early is sequence-of-returns risk made visible.
The working, in prose
Method & limitations.
A model is only as honest as the assumptions it admits to. Here are the ones this tool makes — and, just as importantly, the ones it deliberately leaves out.
01Real terms throughout
Every figure — balances, contributions, spending, returns — is in today’s dollars. Inflation is already netted out of the return assumptions, so a dollar in year 30 buys what a dollar buys now. There is no separate inflation input to tune.
02IID normal returns
Each year’s equity and bond returns are drawn independently from normal distributions and combined by the stated correlation. This ignores mean reversion and regime clustering — real markets trend, crash, and recover in ways independent normal draws never will. Fat tails and prolonged bad decades are under-represented.
03No tax or superannuation rules
There is no modelling of tax, or of Australian superannuation rules — preservation ages, contribution caps, transfer balance caps, the Age Pension. The portfolio is treated as a single unified pool of investable assets. Your real position will be shaped by all of these.
04Longevity is fixed, not stochastic
The planning horizon is a fixed age you choose, not a modelled lifespan. The tool answers “is the plan funded to this age?” — not “will you outlive the plan?” Mortality is not simulated.
05Sequence-of-returns risk is emergent
Nothing special models the danger of poor early-retirement returns — it simply falls out of simulating each path year by year. A weak first decade of withdrawals can sink a plan that the same returns in a different order would leave standing. That is why the depletion chart clusters where it does.
This tool is general information and an educational model — not financial advice.
GridGo · the full model
This is the free model. The full one is a spreadsheet you own.
The full spreadsheet version — with superannuation, taxes, the Age Pension, and editable everything — is the GridGo Retirement & FIRE Planner. Same philosophy: no black boxes, every formula visible, the assumptions yours to change.
Get the GridGo planner →