Position Sizing Rules That Survive a Bad Week

Most position sizing advice assumes a calm market and a disciplined trader. That is not the environment where sizing actually matters. The rules that hold up are the ones written for the week when the ASX opens gappy, the sector you hold gets hit by an offshore headline, and your last three trades closed red. If the framework only works on quiet Tuesdays, it is not a framework.

What follows is a fixed-fractional approach adapted to ASX session volatility. The base rule is simple: risk a fixed percentage of account equity per position, sized off the distance between entry and a structural stop, not off a dollar target. On a normal session that percentage stays constant. On a volatile session, defined here as a day where the index opens more than one standard deviation from the prior close, the percentage is halved before the first order goes in. The stop distance is measured from the prior session's range, not from an intraday chart that will redraw itself by lunch.

The part most traders skip is the drawdown ladder. After two consecutive losing positions, size drops to half the base fraction. After four, it drops to a quarter. The account does not return to full size until two consecutive winners close at the reduced level. This is not a superstition about streaks. It is a mechanical brake on the moment when the trader is most likely to misjudge their own edge, and it removes the decision from the moment entirely.

The common failure mode is the recovery trade. A trader takes a loss, then increases size on the next entry to make the loss back in one move. The math behind that instinct is almost always wrong, and the emotional cost is worse. The structural fix is to make the larger size unavailable: set a hard cap on per-position risk at the broker or platform level, so the order is rejected before it reaches the market. A rule you can override in a bad mood is not a rule.

None of this predicts direction. It does not need to. The point of sizing is to keep the account solvent long enough for whatever edge you actually have to show up in the results. On the ASX, where session gaps and sector rotation can move a book faster than a daily review cycle, that solvency is the whole game.