Notional value is the total value of a leveraged position, calculated as the position size multiplied by the current market price — it represents the full market exposure you control, not the margin you've actually put up. A standard lot of EUR/USD, for instance, has a notional value of roughly $100,000 regardless of how much margin was required to open it.
Understanding notional value (rather than just margin used) is essential for accurately assessing how much real market risk a position carries.
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