Mark Price vs Index Price vs Last Trade on Polymarket Perps: Which One Moves Your Margin?
You watch a fill print at 64.20 — and then your liquidation warning moves anyway, in the wrong direction. That’s because the price you trade at and the price you’re margined at are different numbers. Polymarket Perps runs three prices side by side: Index Price (fair value from external feeds), Mark Price (the margin/PnL/liquidation/funding price), and last trade price (just the most recent fill on the local book — used for nothing in risk math). Confusing them costs real money.
What each price does
Index Price is the protocol’s estimate of the underlying’s fair value, aggregated from external oracle feeds. It’s the anchor — the perp would converge here in a frictionless market. When the underlying market is closed or an feed degrades, sessions logic kicks in (see below).
Mark Price is what the system actually uses for margin requirements, unrealized PnL, liquidation triggers, and funding. It’s built from candidates — order book mid, recent trades, external marks — each of which degrades gracefully to the Index when its own source goes stale or fails sanity checks. If everything fails, Mark tracks Index directly.
Last trade price is exactly what it says: the most recent fill. It does not touch margining. A single aggressive market order can print a last trade far from fair value on a thin book — and it still won’t move your liquidation price by a cent.
Why liquidations use Mark, not last trade
Liquidation exists to close positions before the account goes under-collateralized. A price anyone can move by trading (last trade) is a price anyone can manufacture — a manipulator could print a fake fill to trigger cascades. Mark, anchored to Index and multi-source candidates, is expensive to distort. The same logic applies to funding: premium is sampled against the book, not against the last print — see funding rate mechanics.
What happens when feeds go down
Polymarket categorizes feed health into two states, and neither stops trading:
- Disrupted — external feeds unavailable or failing sanity checks. Mark candidates fall back to whichever feed set is still healthy, ultimately the Index.
- Halted — the underlying itself has a trading halt or corporate action. Same fallback logic.
In both states the order book, matching, margin checks, funding, and liquidations continue to run — only the sourcing of Index and Mark changes. Your position stays live; its risk math just leans harder on the Index anchor. The practical takeaway: don’t assume “market closed” means “nothing can happen to my perp” — funding and liquidations run 24/7 regardless, see perps vs prediction markets for how this differs from event settlement.
Practical rules
- Manage risk off Mark. Your equity, buffer, and liquidation distance all reference Mark — monitor that number, not the ticker.
- Treat thin-book prints as noise. On illiquid instruments, last trade can sit far from Mark for minutes.
- Expect Mark ≈ Index in calm markets and divergence during news — divergence beyond your funding assumptions is a signal to re-check the sanity of your quotes before adding size.
- Sessions change feed sourcing only — not funding, not leverage tiers, not matching.
FAQ
What is the difference between Mark Price and Index Price on Polymarket Perps? Index Price is the aggregated fair value from external oracle feeds. Mark Price is built from book/trade/external candidates and is the single price used for margin, PnL, liquidation, and funding. Last trade price is used for neither — it’s just the most recent fill.
Which price triggers liquidation on Polymarket Perps? Mark Price. Liquidation fires when your equity versus Mark-based requirements breaches maintenance margin — never on last-trade prints, which a manipulator could otherwise manufacture cheaply.
Does the last trade price affect my funding payments? No. Funding is computed from a premium sampled by walking the order book (1,000 quote-asset notional per side, every 5 seconds), averaged over the hourly charge window — see how funding is calculated.
What happens to my perp position when the underlying market is closed? Trading, funding, margin, and liquidations continue 24/7. Only the feed set used for Index/Mark changes (sessions: Disrupted or Halted states), with graceful fallback to the Index anchor.
Why did my PnL change without any new trades printing? Mark moved. Book quotes or external feeds updated, so the price your PnL is computed on shifted even with zero fills on the local book.
Educational content, not financial advice. Not affiliated with Polymarket. Last updated October 2026.