Fixed fractional
Size = (account equity × risk %) ÷ (entry price − stop price). It keeps every trade equally important, and it automatically scales down after losses and up after gains.
Volatility-adjusted
Using ATR instead of a fixed stop distance gives calmer instruments larger sizes and jumpier ones smaller sizes, so each position contributes similar risk to the portfolio.
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Start paper trading free →What to avoid
Sizing by conviction, averaging down into losers, and doubling up after a losing streak are the three behaviours automation is best at preventing — provided you never encode them as rules.