Choosing a position by the face value it controls rather than by the loss it can cause.
Notional sizing means deciding "I want $50,000 of exposure" and buying whatever quantity delivers it. It is natural for cash equities, where notional and capital are the same, and dangerous for derivatives, where they are not.
One ES futures contract at 5,000 controls $250,000 of index (notional-value = price x $50 multiplier) on maybe $15,000 of initial-margin. A trader who thinks of it as "a $15,000 position" has mis-stated their exposure by 16x. The same gap appears in perpetual-futures and in any leveraged FX position.
Use notional to understand what you actually control, and use dollar-risk to decide size. The two answers should be checked against each other: if a 1% adverse move in the notional exceeds your intended loss, your stop is doing more work than it can bear.
Original diagrams for the ideas on this page. Illustrative, not real market data.
Margin and leverage. A $5,000 deposit can control a $100,000 position, which is 20:1 leverage. Because the loss is measured on the full $100,000, a 2.5% move against you halves the deposit and brings a margin call, and a 5% move uses all of it.
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