What's mark price vs last price vs index price?

X-Perps trading uses multiple price references. Understanding the difference is important because different functions may rely on different prices.

What's last price?

The last price is the price of the most recent executed trade in the market.

It's useful for:

• seeing recent trade activity

• charting and short-term market behaviour

What's index price?

The index price is a reference price derived from external spot market sources.

It's used to:

• feed the premium index component of the funding rate calculation, which is what keeps the contract anchored to the broader market

• reduce reliance on a single trade or venue

• serve as the basis for the settlement price at contract expiry (60-month settlement)

As expiry approaches (approximately 60 months from issuance), the X-Perps contract price is expected to converge toward the index price.

Which price is used for the chart, unrealised PnL, liquidation, and settlement?

• Last price — the most recent trade to execute on the order book. The default price chart is drawn on it, and it's the price to watch to follow trading activity.

• Mark price — used for liquidation checks, and for the unrealised PnL shown on an open position.

• Index price — derived from external spot sources rather than from our own order book. It's the reference used for settlement at expiry, as defined in the contract.

• Funding rate — a rate rather than a price reference, and not simply "the index price". It's calculated from the average premium index, an interest rate component, and a cap and floor. The index price feeds the premium index part of that calculation, so funding is related to the index price without being set by it. The full formula is in Funding fee mechanism.
  

Why doesn't the unrealised PnL on my position match the chart?

The default price chart is drawn on the last price, while the unrealised PnL on an open position is calculated on the mark price. When those two prices differ, the PnL shown on the position and the figure you'd work out from the chart differ with them. Neither number is wrong: they answer different questions. If you want the two to line up, calculate from the mark price rather than from the last price on the chart.

Which price triggers a take-profit or stop-loss order?

Applies to TP/SL orders placed in Exchange mode.

A TP/SL order triggers on the price type set as its trigger price. You choose that price type when you place the order — last price, mark price, or index price — and the order watches only the price type you chose, not the other two.

Because the default chart is drawn on the last price, an order can look as though it should have triggered when the price type it actually watches did not reach your level. Check the trigger price type on the order before concluding that it failed.

For how to set and change the trigger price type, see How to set take-profit and stop-loss for contract trading and How to trade X-Perps.

What's mark price?

The mark price is a risk-management price used for certain account and position calculations, including liquidation logic (subject to platform rules).

It's designed to reduce unnecessary liquidations caused by temporary price spikes in the last traded price.

Why does this matter?

On our platform, mark price is used for liquidation checks and unrealized PnL calculations. This means a position may be affected by liquidation risk based on the mark price, even if the last price briefly looks different.

When reviewing your position, make sure you understand which price is used for:

• liquidation checks — mark price

• unrealized PnL display — mark price

• settlement at expiry — typically based on the index/settlement price as defined in the contract

Because different functions rely on different price references, the same market moment can look different depending on whether you are looking at last price, index price, or mark price.

Why is the price here different from the price on another platform?

The last price on any venue comes from that venue's own order book, and order books are independent of each other. A large market order, or a quiet moment with less depth on one side of the book, can push the last price away from where other venues are trading, and it returns once the book refills. A short spike like that is normal market behaviour rather than a fault, and it's one of the reasons the mark price rather than the last price is used for liquidation checks.

Two things follow from that:

• A stop-loss set to trigger on the last price can be taken out by a spike that other platforms didn't show. Setting the trigger price type to mark price, or leaving more room between your entry and your stop, reduces how often that happens.

• If you want to know where the wider market is trading, look at the index price rather than the last price: it's derived from external spot sources rather than from a single order book.

Why did my position open at a price different from the one I expected?

This depends on your order type and the mode you traded in, rather than on a different price reference being applied.

• Market order in Exchange mode — a market order fills at the best prices available on the order book at that moment. If your order is larger than the depth at the top of the book, or the market moves while it's filling, the final price can differ from the price displayed before you placed it. That difference is slippage.

• Limit order in Exchange mode — a limit order executes at your specified price or better. It won't fill at a worse price, but it may not fill at all if the market doesn't reach your level.

• Simple mode — the entry price is shown and locked before you confirm, so it doesn't change between confirmation and execution.

Before concluding that a different price reference was applied, check the order type, the limit price if there was one, whether the order filled in more than one part, and the execution records for those fills. See How do I start trading X-Perps on OKX?, How do I trade X-Perps in Simple mode and X-Perps fees overview.

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Disclaimer: X-Perps are leveraged derivatives. Leverage can amplify gains and losses. Losses may occur quickly and these products may not be suitable for all investors.