Mark Price and Index Price Design

If an exchange liquidated positions the instant the last traded price touched a trader's maintenance margin level, a single lowball trade of $500 on a thin order book could liquidate every leveraged long on the platform, even though nobody else was actually willing to sell at that price. Real trading engines don't do this. They compute unrealized profit and loss, and trigger liquidations, off a separate, deliberately smoothed number: the mark price.

Two different prices doing two different jobs

The last price is simply whatever the most recent trade printed on that specific exchange's order book — noisy, venue-specific, and easy to move with a small trade. The index price averages spot prices across several major exchanges, so no single venue's quirks dominate it. The mark price starts from that index and adjusts it for the current futures basis (the gap between futures and spot caused by funding), producing a fair-value estimate that a single manipulated trade can't move much. Exchanges use the mark price — not the last price — to calculate unrealized PnL and to decide when a position hits its liquidation threshold.

Worked example

A trader is long a perpetual future with a maintenance margin threshold at $59,500. On this particular exchange's order book, a small sell order momentarily prints a trade at $59,200 due to thin liquidity — the last price briefly dips below the liquidation threshold. But the index price, averaged across five major spot exchanges, is $59,900, and the mark price (index adjusted for the small positive basis) comes out to $59,950 — comfortably above the threshold. Because liquidation is evaluated against the $59,950 mark price, not the $59,200 last price, the position is not liquidated. Ten minutes later, the index itself genuinely falls to $59,300 across all five exchanges and the mark price follows it down to $59,450, now below the threshold — this time the position is liquidated, because the move was real and broad, not a single-venue blip.

What this means in practice

Index construction choices matter as much as the mark-price formula itself: which exchanges are included, how they're weighted, and how outliers are filtered all determine how resistant the index is to manipulation on any single constituent venue. A trader evaluating an exchange's derivatives product should check what index it marks against and how many venues feed it — a mark price built from one thinly-traded spot exchange offers little more protection than using last price directly.