Perps Mark Price vs Index Price vs Last Price

Why there are three prices

(The Perps guide introduces all three prices.) If a position were valued at the last trade, one small trade in a thin book could move your profit and loss, your margin and even trigger a liquidation. Polymarket avoids that by keeping two kinds of price apart.

Execution prices come from fills in the order book. Calculated prices are Polymarket's own estimates, built for reference and for risk checks. The split matters because one small or isolated trade should not be able to change an account's risk state or trigger liquidation by itself.

The last price does not value your position directly, and Polymarket's pages word this in two ways. The Perps FAQ says the last trade price is not used for margining. The mark price page lists the last trade as one of the three inputs of candidate two, median(BestBid, BestAsk, LastTrade), so a recent trade can influence the mark through that candidate. This guide follows the mark price page, because it gives the formula.

The index price

The index price is the exchange's view of what the underlying asset is worth. It is built from outside price feeds such as Pyth, Chainlink Data Streams and Hyperliquid. The exchange combines the feeds as a weighted average after stale prices and outliers are dropped, which keeps one stale or odd feed from moving it. It is published every 200 milliseconds for each market.

Which feeds are used depends on the market session, as described below.

The mark price

The mark price is the median of three candidates, and it is updated every 200 milliseconds. Each candidate looks at a different source of truth.

• Candidate one follows the order book, but slowly. It is Index + EMA(Mid - Index), where the mid price sits halfway between the best bid and the best ask, and EMA is an exponential moving average, a smoothing that gives recent values more weight. The smoothing uses a window of 150 seconds, so it resists short bursts and manipulation of the book. It lags in fast markets.
• Candidate two follows what is trading locally. It is median(BestBid, BestAsk, LastTrade). A last trade is only used if it is recent, so an old print cannot anchor the price. It reacts quickly and can be pushed around when liquidity is thin.
• Candidate three is independent of the local book. It is built from separate outside mark feeds: the exchange drops stale samples, filters outliers and averages what remains. It depends on those feeds being available.

The median is what makes this robust. Polymarket's wording is that at least two of the three candidates have to line up before the mark moves. In plain arithmetic, the median of three values always sits within the range spanned by any two of them, so one candidate that runs far away cannot drag the mark beyond the other two. The result is snapped to the nearest tick size before it is published.

When data is missing the mark does not stop. Each input falls back to the index price, and in the worst case, with no book, no recent trades and no outside mark feeds, all three candidates equal the index and the mark simply tracks the index.

What the mark price decides for you

Almost everything about the safety of a position is measured at the mark price.

• Profit and loss. Unrealized profit and loss is the position size times the gap between the mark and your entry price, in your favor or against you depending on the side.
• Margin checks. Your equity, and so your margin state, moves with the mark.
• Liquidation. Liquidation starts when equity, valued at the mark, falls below the maintenance margin. See the liquidation guide.
• Funding. Here Polymarket's pages differ. The funding page builds the premium from the order book and the index price: IPD = max(bid_impact - Index, 0) - max(Index - ask_impact, 0). The mark price page lists funding premium computation among the uses of the mark price, the Perps FAQ lists funding too, and a Sep 14, 2026 changelog entry says mark, funding, and margin all follow the index. This guide describes funding as measured against the index, the way the formula reads. See the funding rate guide.

Your fills happen at traded prices, while your risk is judged at the mark. You can try your own numbers in the liquidation calculator.

Market sessions

Perps trade around the clock, apart from short maintenance windows, but many underlying markets do not. Sessions are the exchange's way of describing the state of the underlying market and its price feeds.

A session changes one thing: which set of outside feeds builds the index and the external mark candidate. Regular hours can use feeds from a primary venue, for instance, while overnight hours can use feeds from after-hours venues. Sessions do not change funding, margin, leverage tiers, matching and liquidation triggers, so those run in the same way at every hour.

Polymarket's pages disagree on which session categories exist. The market sessions page lists Regular, Overnight, Weekend, Disrupted, Halted. A Sep 21, 2026 changelog entry says that page was corrected to list Regular, Premarket, Postmarket, Overnight, Halted, Unknown, Weekend and Closed, and that Disrupted never existed in the engine. The page still showed the old list on Oct 19, 2026.

Because the changelog entry is the newer word, this guide does not describe Disrupted as a real state. Regular, Overnight, Weekend and Halted appear on both lists. Halted means a trading halt or corporate-action freeze on the underlying; it changes how the index and the mark are sourced, and the perp keeps matching orders.

PolyTerminal is an independent interface for Polymarket and is not operated or endorsed by Polymarket. Orders placed through PolyTerminal carry PolyTerminal’s Polymarket builder code, and PolyTerminal may charge a builder fee on top of Polymarket’s own fees. This page is educational and is not investment advice. Rules were compared with the official documentation on Oct 19, 2026; check the source before you act on a number.