Here’s a few excerpts from an eye-opening article on the risk of ruin by Brett Steenbarger:
Here’s a nice calculator of risk of ruin for poker players that easily adapts to trading. Risk of ruin represents the odds of losing all of your gambling (or trading) stake…
To adapt the calculator for our needs, instead of estimating win rate and and standard deviation per hour, we will make those daily estimates. So the win rate would represent average profit per day, and the standard deviation would represent a measure of the variability of daily returns.
Let’s say that your account total (bankroll) is $100,000. You are willing to risk 2% of your money in a single day, and 2/3 of your days will fall between losing and making 1% on your capital. You have a modest positive edge that will earn you 10% on your money in a year, which would be $40 per day on average ($40 x 250 trading days = $10,000).
Your risk of ruin in that situation--the risk of losing all your capital--is only .03%….
The lesson to be learned is that risk of ruin jumps astronomically when one’s edge is eroded and when one’s variability of returns expand. This is why it’s imperative for serious traders to cut their risk (reduce the variability of returns) when they sense that they’ve lost their feel for markets. It’s also why it’s imperative for serious traders to risk manage their day to day trading, so that they cap their daily P/L swings relative to the edge they possess.
And the frustrated trader who overtrades when he loses his edge? Risk of ruin, the calculator tells us, jumps to 100%.