Risk of ruin as a math idea (not a calculator pitch)
Education only — not a recommendation to buy, sell, or hold anything.
Risk of ruin is a name for a simple idea: if you keep taking risks that are large relative to your stake, a string of losses can drive the stake to zero (or to a level where you cannot continue). This page is vocabulary and intuition — not a calculator, not a recommended risk percent, and not a trading system.
Practice only with fake balances on /mock/ and /portfolio/.
The intuition
Imagine a practice bankroll. Each idea risks some fraction of it. Even with a process that is"fair" in expectation, variance can produce long losing streaks. If each loss is huge, one streak ends the account. If each loss is smaller, the same streak hurts less.
That is the educational core of risk of ruin: size interacts with streaks.
See also position sizing concepts and what is risk.
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What this is not
| Not this | Why |
|---|---|
| A formula we tell you to plug in | Numbers without your constraints become fake precision |
| Proof you have an edge | Edge vs randomness |
| A promise that small size cannot lose | Small size still loses; it changes speed of ruin |
| Advice for leveraged / options products | Some products can lose more than the cash you planned — know the product |
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Streaks are normal
Coin-flip thinking (illustration only): long runs of heads/tails happen more often than intuition suggests. Markets are not coin flips — but the lesson transfers: do not treat a short sample as destiny, and do not size as if streaks cannot happen.
Journal process grades separately from fake P&L (trading journal).
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Leverage and "I can only lose what I put in"
For plain long stock in a cash-like practice book, losses are often bounded by the position. For other products (options, futures, margin, liquidations), losses and exit mechanics differ. Educational takeaway: know whether ruin can arrive faster than your mental model.
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Paper drill (fake money)
- Pick one simple process and a deliberately large practice size.
- Pick the same process with a much smaller size.
- Run both through a simulated losing streak (or review history).
- Journal which book can still take the next drill.
No recommended percents — only the comparison.
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Related
- Position sizing concepts
- Edge vs randomness
- Backtesting concepts
- Next: Regime change · Advice vs education
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Sentry CLEAR · Education only. No risk calculator as advice.