Risk of Ruin in Trading: How Position Size Decides Survival
Last updated: November 2026 · 3 min read · Educational content, not financial advice
Key takeaways
- Position size has an exponential effect on risk of ruin; the edge has a much smaller one.
- With a thin edge (40% win rate at 1:2), 1% risk still touches a 10% loss in about one run in seven over 200 trades; with a strong edge it is about 0.2%.
- Compare settings with the Monte Carlo tab, and leave room for your real edge being weaker than measured.
Risk of ruin is the probability that you lose so much that you can no longer continue, either because the account is gone or because you hit a hard limit such as a funded-account maximum loss. It combines everything: your edge, your risk per trade and the number of trades you take.
The classic formula
The simplest model is the gambler’s ruin. Suppose every trade pays even money (1:1), you win with probability p above 50%, you always stake one unit, and your account holds N units. The probability that you eventually lose everything is:
With a 55% win rate, here is how the number of units in your account (which is just the inverse of your risk per trade) changes the result:
| Risk per trade | Units in account (N) | Chance of eventual total ruin |
|---|---|---|
| 10% | 10 | 13.4% |
| 5% | 20 | 1.8% |
| 2% | 50 | 0.004% |
| 1% | 100 | practically 0% |
The same edge goes from a coin-flip-like danger to effectively zero just by changing the stake size. This is the main lesson of risk of ruin: position size has an exponential effect, while the edge has a much smaller one.
Real limits are smaller than “total ruin”
You rarely need to lose the whole account to be knocked out. A funded-account challenge may end at a 10% loss, and many traders stop trading after a 20–30% drawdown. So the more useful question is: what is the chance of touching a specific loss limit? This has no neat formula once win rates and ratios vary, so the figures below come from simulation.
Simulated odds of hitting a 10% loss
Assumptions: a fixed risk per trade measured as a percentage of the starting balance, independent trades, 200 trades, a 1:2 risk-to-reward ratio, and a static loss limit of 10% below the starting balance. Each result is from 100,000 simulated runs, so treat the figures as accurate to within roughly half a percentage point.
| Risk per trade | Strategy A: 55% win rate (+0.65R) | Strategy B: 40% win rate (+0.20R) |
|---|---|---|
| 0.5% | under 0.1% | 1.9% |
| 1% | 0.2% | 13.9% |
| 2% | 4.4% | 37.3% |
| 3% | 8.0% | 45.9% |
| 5% | 28.5% | 67.6% |
Strategy A has a strong edge, so even 2% risk touches the limit only a few percent of the time. Strategy B has a thin edge, and at 1% risk it already touches a 10% loss in roughly one run in seven. At 3% it is close to a coin flip, and at 5% it happens in about two runs out of three, even though the expectancy is positive.
What this means in practice
- A positive expectancy is not protection. Strategy B makes money on average and still fails a 10% limit often at normal-looking risk levels.
- Halving the risk helps far more than it seems. For Strategy B, going from 2% to 1% cuts the chance of hitting the limit from about 37% to about 14%.
- The stronger your measured edge is, the more you can afford. But your measured edge is an estimate, so build in room for it being weaker than it looks.
Ways risk of ruin is reduced
- Smaller risk per trade (the most powerful lever).
- A higher real expectancy after costs.
- Daily or weekly loss caps that stop trading before a bad day becomes a bad month.
- Not taking several highly correlated positions at once, which effectively multiplies the risk of one trade.
Limits of these numbers
The formula and the simulation assume independent trades with a fixed win rate and ratio. Real results include slippage, correlated trades and changing conditions, all of which make ruin more likely than a clean model suggests. Use the numbers to compare settings against each other, not as a guarantee.
Try the numbers yourself
Simulate a weaker edge (40% win rate, 1:2) over 200 trades.
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This guide is for education only. It is not financial advice or a trade recommendation, and trading involves substantial risk of loss. See the Terms of Service.