Direct Answer
Risk of ruin (RoR) is the probability that a bettor with a defined edge and unit size will go bankrupt before their edge realizes. Even profitable strategies have non-trivial RoR when bet sizes are too large.
Key Takeaways
- Even profitable strategies can ruin you at the wrong size.
- Unit size is the dominant input to RoR.
- Fractional Kelly and 1–2% sizing minimize RoR.
What drives RoR
Three inputs: edge size, variance, and unit size relative to bankroll. Larger units mean exponentially higher RoR. This is why fractional Kelly and 1–2% sizing dominate professional practice.
Worked intuition
A 53% bettor against –110 with 5% unit sizing has a small but real RoR — meaningful losing streaks can cascade. The same bettor at 1% sizing has near-zero RoR. The math heavily rewards smaller units.
Frequently asked questions
Can I have zero RoR?+
Only with arbitrage or with bet size approaching zero. All real-world strategies carry some RoR; the goal is to make it negligible.
Educational only. Not wagering, financial, or legal advice. See our editorial policy.