What is index price?

The blended spot price, drawn from several outside exchanges, that a derivatives venue uses as the base for its mark price and its settlement.

Each venue publishes which constituent exchanges it samples, how it weights them, and what it does when one feed stalls or diverges. A well-built index drops an outlier venue automatically, which is what stops a wick on one thin market from settling contracts everywhere else.

Because the constituents differ, the same contract on two exchanges can settle at slightly different numbers. That is not an error, and it is the reason a trader comparing two venues has to compare their index methodologies rather than assuming a shared reference.

How it works

A derivatives venue needs a price it does not control. If a perpetual contract were marked against its own order book, a large trader could push that book and trigger liquidations at will. Sourcing the reference from outside removes the incentive.

The construction is usually a weighted blend of the spot price on several named exchanges. Weights may be fixed or set by volume. Around the blend sit protective rules that matter more than the weighting scheme.

1. Outlier exclusion. A constituent whose price deviates beyond a threshold from the median is dropped for that observation, so a single thin market cannot drag the index.
2. Staleness handling. A feed that stops updating is removed after a stated interval rather than being carried forward.
3. Minimum constituents. If too few sources remain valid, the venue may halt trading or switch to a fallback rather than publish an unreliable number.

The index then feeds two things. The mark price, which values open positions and decides liquidations, is derived from it, usually with a smoothing component so a momentary spike does not cascade. And settlement of expiring contracts is calculated from it, often as an average over a window rather than a single instant, which makes manipulation at the settlement moment expensive.

Example

Illustrative arithmetic. A venue builds its index from five spot exchanges. Four report prices between $60,010 and $60,050 while one, briefly, prints $58,900 after a large market sell into a thin book. The median of the five is about $60,030, and the outlier deviates by nearly 2 percent, past the venue's exclusion threshold. That constituent is dropped for the observation, and the index publishes roughly $60,030 rather than the $59,802 a naive average of all five would give. Positions across the venue are marked against the sound number, and nobody is liquidated by a wick on one exchange.

Why it matters when you buy

Spot buyers never touch an index directly, but the same underlying question applies: which venue's price you are getting and how deep that market is. The liquidity view shows measured spreads and depth per exchange, and the concentration view shows how much of an asset's volume sits on a single venue, which is exactly what makes a thin market easy to move.

Related terms

• mark price: the valuation derived from the index
• spot market: where the constituent prices come from
• perpetuals: the contract most often marked this way
• liquidation: the event a mark price triggers
• oracle: the equivalent problem for on-chain protocols
• settlement: the final calculation at expiry

Questions

Why is the index different from the last trade on this exchange?

Because it is deliberately sourced from other venues. A gap between the two is normal and is what the funding rate exists to close on a perpetual contract.

Can an index price be manipulated?

It is harder than moving one book, which is the point. Manipulation would require moving several constituent markets at once, past the outlier rules, and often across a settlement window.

Where do I find a venue's constituents?

Every serious derivatives exchange publishes an index methodology page naming the sources, the weights, and the exclusion rules. Treat that page as the authority rather than any third-party summary.