Run a Monte Carlo retirement simulation

By default, Beyond Payday projects your retirement using one steady growth rate every year. Turning on Monte Carlo simulation — a technique that reruns your plan up to 10,000 times with randomized market returns instead of once with a fixed rate — shows you how sensitive your plan is to bad timing, not just the average case.

Before you start

• Retirement planning is a Pro feature.
• You don't need to change anything else first — Monte Carlo mode reuses whatever growth rate and other settings you've already entered.

Steps

1. Go to Retirement (opens in a new tab) and open the Simulation Mode settings card — it's the last card in the settings panel (the Plan settings tab on mobile).

2. Under How to test your plan, choose one of three modes:

   • Quick Estimate ("Fastest", tagged Deterministic) — assumes steady growth every year for a single, instant projection.
   • Stress Test ("Recommended", tagged Monte Carlo · 1,000 runs) — simulates 1,000 random market scenarios to show how likely your plan succeeds.
   • Deep Analysis ("Most Accurate", tagged Monte Carlo · 10,000 runs) — simulates 10,000 scenarios for the most precise result, a bit slower.

   Shortcut: on a Quick Estimate, the Plan Confidence card shows a Run a Stress Test button that switches you straight to Stress Test.

3. Beyond Payday recalculates in the background — a brief "Recalculating projections..." indicator appears over the chart.

4. A Plan Confidence card appears above the chart. It shows the share of those scenarios where your money lasted — your plan's success rate — in plain language, along with the typical range of outcomes.

5. A Resimulate button also appears above the chart. Click it anytime to re-roll a fresh batch of random scenarios; the confidence score updates with each re-roll.

How the projection handles it

• Each simulated run generates its own random sequence of monthly market returns, centered on your growth rate but varying year to year the way real markets do — Beyond Payday models moderate, broad-market-like ups and downs.
• The Balance chart shows the range across every run: a darker band for the middle half of futures, a lighter band for 8 in 10, and a solid Median line (at each age, half the runs have more and half less).
• The dashed line, the summary, the table, insights, and the Cash Flow, Accounts and Year-by-Year tabs follow one sample future: the run that stays closest to the median, especially at retirement. Its balance at a given age is close to, but not exactly, the median. The Stress-test projection label identifies this mode.
• The Dashboard uses steady growth in the same month-by-month engine. Its estimate can differ from the stress-test path. Choose Quick Estimate to compare the Steady-growth projection with the Dashboard.
• Runs happen in the background (a Web Worker), split across several parallel threads, so the rest of the page stays responsive even at 10,000 runs.
• Clicking Resimulate draws a brand-new batch of random scenarios. Because each run uses new market returns, the numbers can shift each time you resimulate — that's expected, not a bug.

Good to know

• Your success rate, in plain words. "Success rate" is the share of the randomized scenarios where your money lasted the whole way through. When you run a Stress Test or Deep Analysis, the Plan Confidence card above the chart shows it directly — for example, "87% of tested markets your money lasts to age 95" — plus the typical range of ending balances and a plain read on the rough-market case. It's a modeled estimate, not a guarantee.
• More runs means steadier statistics, not a different market assumption. Deep Analysis doesn't assume a better or worse market than Stress Test — it just samples more scenarios, so results are more stable from one resimulate to the next.
• Quick Estimate is instant and great for comparing what-ifs side by side. Switch to Stress Test or Deep Analysis when you want to see a fuller range of outcomes before relying on a plan.
• Guided setup covers this too. The setup wizard's Testing your plan step asks the same question. See Set up your retirement plan with guided setup.