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Risk of ruin is the probability that a trading process reaches a defined failure boundary before a specified horizon or target. “Ruin” does not have to mean a zero balance. It may mean:
  • breaching a prop firm’s daily or overall loss limit;
  • falling below required margin or operating capital;
  • reaching a drawdown at which the strategy must stop;
  • losing enough capital that recovery is no longer acceptable.
A risk-of-ruin estimate is meaningless unless the boundary and assumptions are stated.

What determines the estimate

  • probability and size of wins and losses;
  • trading costs and financing;
  • position-sizing method;
  • starting capital and failure threshold;
  • number of trades or time horizon;
  • dependence between outcomes;
  • gaps, slippage, and tail losses;
  • changes in market regime or strategy edge.
Larger position size generally increases the chance of crossing a fixed boundary, but there is no universal percentage that guarantees safety.

Why simple formulas can mislead

Closed-form formulas often assume independent trades, fixed win/loss amounts, stable probabilities, and an unlimited sequence. Real strategies can have variable payoffs, clustered losses, correlated positions, regime changes, and external rules based on equity at particular times. Monte Carlo simulation can incorporate more realistic distributions and rule mechanics, but it remains dependent on the quality of the inputs.

Practical estimation process

1

Define ruin

Specify the exact balance, equity, daily-loss, or trailing-drawdown threshold.
2

Build a net outcome distribution

Include costs and live execution, not only ideal backtest trades.
3

Model position sizing

Recalculate size as the account changes and aggregate simultaneous risk.
4

Add dependence and stress

Test clustered losses, wider slippage, correlation, and strategy degradation.
5

Estimate breach frequency

Run many paths and report the percentage that cross the boundary.
6

Repeat with uncertainty

Vary the assumptions instead of presenting one precise probability as fact.

Risk of ruin vs drawdown

Drawdown measures the decline from a prior peak. Risk of ruin estimates the probability of crossing a chosen failure level. A system can have a low probability of literal bankruptcy and a high probability of violating a 10% account rule.

Monte Carlo simulation

Model distributions and rule-specific breaches.

How losing streaks behave

Understand run risk and dependence.

How much to risk

Translate acceptable failure risk into a sizing decision.

Prop risk management

Define ruin using external loss rules.
Risk-of-ruin figures are estimates, not guarantees. Small changes in assumptions can materially change the result.