> For the complete documentation index, see [llms.txt](https://help.rails.xyz/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.rails.xyz/trading/price-indices.md). # Price Indices ## Index Price Accurate and reliable index prices are crucial to our platform. We source index price data from multiple reputable APIs to ensure robustness, reduce the risk of price manipulation, and maintain availability. These prices underpin Rails' core services such as funding and trading metrics calculations. To ensure reliability and availability, our system uses a multi-sourced cascading fallback approach for index pricing, detailed below: | Primary Source | | :---------------------------------------------------------------------------------------------------------------------------------------------------: | | [CoinDesk Aggregate Index API - CCIX](https://developers.coindesk.com/documentation/data-streamer/index_cc_v1_latest_tick_direct_trading_methodology) |
Backup Source 1Backup Source 2Backup Source 3
CoinDesk Aggregate Index API - CCCAGG

Used if the primary source becomes unavailable.		Hyperliquid Subscriptions API - Hyperliquid

Acts as a secondary fallback to maintain price availability.		Kraken's Ticker (Level 1) API

Acts as the last fallback to maintain price availability.
## Mark Price Mark price provides a fair reference for contracts as market conditions change. Rails calculates it differently for each product: | Perpetuals Mark Price | Options Mark Price | | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | |

Perpetuals Mark Price uses the median of three values:


The adjusted index is the index price plus a 150-second EMA of the gap between Rails' bid-ask midpoint and the index price. The Rails order-book median is the median of Rails' best bid, best ask, and last trade.

|

Options use the Black-76 model with zero interest rates to calculate mark price.

Rails sources implied volatility from Deribit, then prices each option from the forward price for it's expiry. The calculation also enforces put-call parity and an intrinsic-value floor.

Both products use an external index as an anchor. Options use an implied-volatility-based pricing model.

| | Find out more information in the [Perps Metrics Guide](/trading/perpetuals/perps-metrics-guide.md) | Find out more in the [Options Metrics Guide](/trading/options/options-metrics-guide.md) | ## Forward Price Accurate and reliable forward prices are crucial to [options](/trading/options.md) valuation. We source forward price data from [Deribit’s Get Book Summary by Currency Public API](https://docs.deribit.com/api-reference/market-data/public-get_book_summary_by_currency) endpoint. This data supports contract-level mark price and implied volatility calculations. For each live options contract, the underlying price provides the forward-derived price used for valuation. Options use a forward reference price in [Black-76](/trading/options/black-76-and-the-greeks.md), so this price reflects the futures curve for that expiry rather than the current spot price.