Risk of ruin: the number that decides whether your edge survives
A positive expectancy does not protect you from a losing streak. Here is the arithmetic that does, and what it implies about position size.
Most traders who blow up are not wrong about direction. They are right often enough to have an edge and still lose everything, because they sized as though a losing streak were impossible. Risk of ruin is the arithmetic that puts a number on that gap.
What it actually measures
Risk of ruin is the probability that a run of losses takes your account below the point where you can keep trading, given three inputs: your win rate, your average win-to-loss ratio, and the fraction of capital you risk per trade. It is not a forecast of any single trade. It is a statement about the distribution of sequences.
The uncomfortable part is how quickly it moves. A strategy that wins 50% of the time at 1.5R is genuinely profitable. Risk 2% per trade and ruin is remote. Risk 10% and the same edge — the identical strategy, unchanged — becomes a coin flip on whether you survive the year.
The market can stay irrational longer than you can stay solvent.attributed to John Maynard Keynes
That line is usually quoted about being wrong. It is more useful read as a statement about sizing: solvency is the constraint, and it is the one variable you fully control.
Losing streaks are longer than intuition suggests
At a 50% win rate, a run of seven consecutive losses is not a sign that something has broken. Across a few hundred trades it is close to expected. Traders who have not internalised this tend to do one of three things when the streak arrives, and every one of them is worse than the streak:
- Cut size after the losses and miss the recovery, locking the drawdown in.
- Increase size to win it back faster, which is the single fastest route to ruin.
- Abandon a working strategy at exactly the point its sample was becoming meaningful.
What to do about it
- Write down your per-trade risk as a percentage of capital, not a dollar figure. A dollar figure silently becomes a larger percentage as the account draws down.
- Find your longest losing streak in your own history. If you do not have one yet, assume it is longer than the worst you have seen.
- Set a maximum drawdown and a maximum daily loss before the day starts, and treat them as a stop on your own behaviour rather than a target.
- Review the trades you took after a loss separately from the rest. Revenge trades have their own statistics, and they are rarely flattering.
None of this requires new analysis. It requires the record. If you are not yet keeping one, start a free trial — it takes no card, and the first useful number arrives after about twenty trades.