The question is not just what you trade, but how much. Position sizing is the most underestimated factor in trading — and simultaneously the most important for long-term survival.

The 1% Rule Foundation

The golden rule: Never risk more than 1-2% of your total portfolio per trade. With a 50,000-euro portfolio, this means a maximum of 500-1,000 euros risk per trade. This is not the position size — it is the maximum loss if the stop-loss triggers.

The Formula

The calculation is simple:

Position Size = Risk Budget / (Entry Price - Stop-Loss) x Entry Price

Example:

Portfolio: 50,000 euros
Risk per trade: 1% = 500 euros
Entry price: 100 euros
Stop-loss: 95 euros (5% below entry)
Risk per share: 5 euros
Maximum quantity: 500 / 5 = 100 shares
Position size: 100 x 100 euros = 10,000 euros (20% of portfolio)

Why Position Sizing Matters More Than Hit Rate

A system with 40% hit rate can be highly profitable — if winning trades are on average 3x larger than losing trades. Conversely, a system with 80% hit rate can lose money if a single losing trade wipes out all gains.

Expected value matters: (Hit rate x Average gain) - ((1 - Hit rate) x Average loss)

The Kelly Formula

The Kelly formula calculates the mathematically optimal position size:

Kelly% = Hit rate - (1 - Hit rate) / (Avg gain / Avg loss)

With a 60% hit rate and gain/loss ratio of 1.5:1, Kelly% = 0.60 - (0.40 / 1.5) = 0.33 = 33% of capital per trade.

In practice, traders use half-Kelly or quarter-Kelly (8-16%), because the full Kelly formula is too aggressive when estimates are wrong.

Scaling by Confidence

In AI trading, you can link position size to the confidence score:

90%+ confidence: Full position size (2% risk)
75-89% confidence: 75% of position size (1.5% risk)
60-74% confidence: 50% of position size (1% risk)

The Cardinal Sin: Averaging Down

When a trade moves against you and you increase the position (averaging down), you break every position sizing rule. You double the risk in a trade that is already showing your analysis was wrong. Professional traders add to winners, not losers.

Bottom Line: Position sizing is not sexy, but it is the difference between long-term success and ruin. Calculate every position size before the trade, not during it.