The mark price is an estimate of the true value of a contract (fair price) when compared to its actual trading price (last price). The mark price calculation prevents unfair liquidations that may happen when the market is highly volatile. So while the Index Price is related to the price of spot markets, the mark price represents the fair value of a perpetual futures contract. Typically, the mark price is based on the Index Price and the funding rate - and is also an essential part of the “unrealized PnL” calculation.
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