Risk of ruin is the probability that a string of losses wipes out enough of your capital that you can no longer trade effectively - or at all. It is the most important number most traders never calculate. You can have a winning strategy and still go broke if your position sizing lets a normal losing streak end you.

What drives it

Three inputs: your win rate, your average win versus average loss (your risk-reward), and - most powerfully - how much you risk per trade. Risk 2% per trade and a ten-loss streak costs you roughly 18% - survivable. Risk 20% per trade and that same streak is catastrophic. Losing streaks are not rare; they are guaranteed. The math only asks whether you survive them.

Why survival beats being right

A strategy that wins 60% of the time will still, over enough trades, throw a run of six or eight losers. If your size can't absorb that, your edge never gets the chance to play out. Ruin is permanent; a drawdown is temporary - but only if you're sized to see the other side of it.

You don't have to be right to survive. You have to be sized so that being wrong doesn't end you.

Engineering it toward zero

The lever you fully control is bet size. Small, consistent risk per trade pushes risk of ruin toward zero and lets the law of large numbers work for you. This is the entire logic behind disciplined sizing - and why systematic approaches treat drawdown control as non-negotiable. Protect the downside, and the upside takes care of itself.