How to run a retirement Monte Carlo simulation
What the simulation actually does, why it beats a straight-line projection and how to interpret your probability number.
Open Retirement Monte CarloWhat This Guide Covers
This page turns the topic into a decision process: what to measure, which assumptions matter, where the answer can flip, and which calculator to use for your own numbers.
Retirement
Category
12 min
Read time
Jul 2026
Review
Decision Framework
Step 1
Separate accumulation decisions from income decisions. Saving rate, asset allocation, claiming age, withdrawal rate, and taxes each move the result in different ways.
Step 2
Run a conservative case before acting on the expected case. Retirement errors are usually caused by sequence risk, inflation, tax drag, and healthcare costs appearing together.
Step 3
Translate the result into an annual action: save more, spend less, rebalance, adjust tax location, delay claiming, or revisit the plan date.
Key Takeaways
- Define the decision this guide is solving before comparing options: What the simulation actually does, why it beats a straight-line projection and how to interpret your probability number.
- Use your own numbers instead of rules of thumb. Defaults are useful for orientation, but the answer usually changes with income, time horizon, tax rate, debt cost, or location.
- Save a calculator scenario after reading so the assumptions are visible when you come back later.
Before You Decide
- Write down the current baseline.
- Run the conservative case.
- Change one assumption at a time.
- Record the point where the answer changes.
- Check income timing, tax buckets, healthcare reserve, and survivor assumptions.