The short answer
Risk of ruin is the probability that a string of losses wipes out your account before your edge can pay off. The key insight: even a genuinely profitable method can blow up if you risk too much per trade, because losing streaks are inevitable. This is why the small, fixed risk-per-trade rule exists — sizing, not setups, is what decides survival. Position sizing is the most important skill in trading, and the least glamorous.
What risk of ruin is
Risk of ruin is the probability that you lose so much that your account cannot recover — practically, that you blow up — before your edge has time to work. It depends on three things: your edge (expectancy), your risk per trade, and the reality of losing streaks.
The uncomfortable truth it reveals: a positive expectancy is not enough. If you size too large, a normal run of losses can end you before the average ever plays out.
Losing streaks are inevitable
Even a method that wins most of the time will string losses together. With a 50% win rate, runs of five, six, seven losses in a row happen routinely over a long enough sample — that is just how randomness works, not a sign the method is broken.
The question is not whether you will hit a losing streak, but whether your account survives it. And that is decided entirely by how much you risked on each trade.
Why the small risk-per-trade rule exists
This is the maths behind the classic rule of risking a small, fixed percentage — often around 1% — of your account per trade. Risk 1% and a horrific ten-loss streak costs you roughly 10% — painful but survivable, and easily recovered. Risk 10% per trade and that same streak wipes you out.
The size is an output of your risk rule and your stop distance, never an input you pick because you "feel confident." A position-size calculator turns your risk cap and stop into the correct size automatically.
Position sizing is survival
Here is the hierarchy that most traders get backwards. Finding setups is the fun part and the part everyone obsesses over. But position sizing is what keeps you in the game long enough for any edge to matter — and it is the least glamorous skill in trading.
You cannot control whether the next trade wins. You can completely control how much it costs you if it loses. Get the sizing right and even a rough patch is survivable; get it wrong and even a good method can end your account. That is why, on this site, risk comes before reward, always. This is education, not advice.
Frequently Asked Questions
What is risk of ruin?
The probability that a string of losses wipes out your account before your edge can pay off. It depends on your expectancy, your risk per trade, and the inevitability of losing streaks.
Can a profitable strategy still blow up an account?
Yes. Even a method with positive expectancy can blow up if you risk too much per trade, because losing streaks are inevitable. A normal run of losses can end an over-sized account before the average ever plays out.
Why risk only 1% per trade?
Because it keeps losing streaks survivable. Risking 1%, a ten-loss streak costs about 10% — painful but recoverable. Risking 10% per trade, the same streak wipes you out. The small, fixed risk rule is what controls risk of ruin.
Are losing streaks normal?
Completely. Even a method that wins most of the time will string losses together — runs of five, six or seven losses happen routinely over a long sample. A losing streak is not proof a method is broken.
What is the most important skill in trading?
Position sizing. You cannot control whether a trade wins, but you can control how much it costs if it loses. Sizing is what keeps you in the game long enough for any edge to matter — and it is the least glamorous skill.