By default, PiggySize projects your retirement using one steady growth rate every year. Turning on Monte Carlo simulation — a technique that reruns your plan thousands of 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
- 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 Config tab on mobile).
- Choose one of three modes:
- Quick Estimate ("Fastest") — "Uses your settings for a single, instant projection. Great for fast what-if comparisons."
- Stress Test (1,000 runs) ("Recommended") — "Simulates 1,000 random market scenarios to show how likely your plan succeeds."
- Deep Analysis (10,000 runs) ("Most Accurate") — "Maximum accuracy with 10,000 scenarios. Best for finalizing your plan."
- PiggySize recalculates in the background — a brief "Recalculating projections..." indicator appears over the chart.
- 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.
- 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 — PiggySize models moderate, broad-market-like ups and downs.
- The chart, table, and insights show the median result: the one run that landed right in the middle of every scenario simulated. That gives you a realistic, typical outcome rather than the best or worst case.
- 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 it's genuinely random, the numbers can shift a little 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 (10,000 runs) doesn't assume a better or worse market than Stress Test (1,000 runs) — 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.
FAQ
What's the difference between the three simulation modes?
Quick Estimate runs one instant projection at your exact growth rate. Stress Test and Deep Analysis run 1,000 or 10,000 randomized market scenarios instead, for a more realistic range of outcomes.
What does "Resimulate" do?
It re-runs Monte Carlo mode with a brand-new batch of randomized market scenarios, so the numbers may shift slightly each time.
Will Monte Carlo mode slow down the app?
It runs in the background across several parallel threads, so the page stays usable — you'll see a brief "Recalculating projections..." indicator while it works.
Does running more scenarios make the projection more pessimistic?
No. More runs give you more statistically stable results, not a different assumption about the market.