Every trader talks about profit targets. Fewer talk about the probability of losing so much that trading cannot continue. That probability is called risk of ruin, and understanding it changes how you think about position size, strategy edge and patience. This article explains the idea in plain terms without heavy mathematics.
What is risk of ruin?
Risk of ruin is the chance that a sequence of losses takes your account to a level where you can no longer trade it, or where you quit. "Ruin" can be defined as zero, or more usefully as a drawdown you cannot tolerate, such as 30% or 50%. It depends on three inputs: your win rate, your reward-to-risk ratio and the fraction of your account risked per trade.
Why risk per trade dominates
Consider two traders with the identical strategy, a 45% win rate and an average winner of 1.5 times the average loser. Its expectancy is positive: 0.45 x 1.5 - 0.55 x 1 = +0.125R per trade. See win rate, risk-reward and expectancy for that calculation.
Trader A risks 1% per trade. Trader B risks 10%. Even with a positive edge, trader B can face a ruinous sequence far more easily. Losing streaks of seven or eight trades are not unusual at a 45% win rate; our article on losing streaks shows the probabilities. Eight straight losses at 1% risk leaves about 92% of the account. At 10% it leaves about 43%, and the drawdown math says it then needs a gain of roughly 130% just to recover.
A simple way to estimate it: simulate
You do not need a formula to build intuition. Use a spreadsheet or small script:
- Choose a win rate, average reward-to-risk and risk per trade.
- Simulate 200 trades, drawing win or loss randomly with your win probability.
- Track the account balance, noting whether it ever falls below your "ruin" level.
- Repeat 5,000 times and count how often ruin occurs.
Run it for 1%, 2%, 5% and 10% risk. The jump in the ruin percentage between low and high risk is usually dramatic. The result is only as good as your inputs, which is the difficulty in the next section.
The input problem
Your true win rate and reward-to-risk are unknown. You only have estimates from a limited number of trades, and a trader with 50 trades has a wide margin of error. Strategy performance also changes with market regimes. Treat any ruin estimate as optimistic and build in a margin. Be sceptical of your own backtest as well: see backtesting honestly for the usual pitfalls.
What raises risk of ruin
- Higher risk per trade. The biggest lever by far.
- Correlated positions. Three trades on related pairs can behave like one large trade; see currency correlation.
- Averaging down or martingale sizing. Increasing size after losses makes a long streak catastrophic.
- High leverage and thin margin. Margin calls close positions at the worst moment; see margin calls and stop-outs.
- Negative or zero expectancy after costs. With no edge, ruin is only a question of time. Costs matter, as covered in spreads and slippage.
- Behavioural errors. Revenge trades and breaking rules are the hidden risk not included in any model.
What lowers it
- Risk a small fixed percentage; many experienced traders use between 0.5% and 2%.
- Set a daily or weekly loss limit and stop trading when it is hit.
- Reduce size during drawdowns instead of increasing it.
- Cap total open risk across all positions.
- Trade a strategy with a measured, positive expectancy.
Ruin is not only about money
A trader may quit well before the account reaches zero. A 25% drawdown that leads to panic decisions is a form of ruin if it causes the behaviour that deepens the loss. So define your personal ruin level honestly. If a 20% drawdown would make you abandon your rules, then 20% is your real limit and your risk per trade should be sized accordingly. This is why position sizing is a psychological tool as much as a mathematical one.
Key takeaways
Positive expectancy is necessary but not sufficient: you must also survive the sequence of outcomes that comes with it. Small, fixed risk per trade, controlled correlation and honest estimates push ruin probability towards negligible. Run the simulation yourself and let the numbers set your size, rather than your hopes.
A quick worked comparison
Take a strategy with a 40% win rate and an average winner of twice the average loser, an expectancy of +0.2R per trade. Over 100 trades at 1% risk, the expected gain is roughly 20% before compounding, and a drawdown of 10% to 15% would not be surprising along the way. At 5% risk, the same sequence of outcomes produces swings five times larger: a plausible 50% drawdown on the path to the same expected result. The edge is identical; only survivability differs. That is why traders who last tend to be those who keep size small enough that bad luck is an inconvenience rather than an ending. Treat these numbers as illustrations and run your own simulation with your own inputs.
Educational content only; not trading advice. All figures are illustrative.