Index arbitrage (index futures versus the stock basket)
Index arbitrage is trading equity index futures against the basket of stocks in the index whenever the futures price leaves a band around fair value: buying the stocks and selling the futures when the futures are rich (cash-and-carry), selling the stocks and buying the futures when they are cheap (reverse cash-and-carry). It is the mechanism that keeps ES and NQ tied to their indices.
Senzoukria · Glossary · Updated September 2026
How the trade works
An index future settles at expiry to a value computed from the index, so a position long the stocks and short the futures is worth a known amount at expiry whatever the market does. If the futures price exceeds fair value, the index plus financing minus dividends to expiry, by more than the costs, buying the basket and selling the futures locks in the excess. If the futures trade below fair value by more than the costs, the reverse trade sells or lends the stocks and buys the futures. Either position can also be unwound early if the premium returns to fair value before expiry.
Worked example
The index is at 5,000.00 and the fair value premium is 15.00 points, so fair value is 5,015.00. ES trades at 5,021.00, 6.00 points rich. One ES contract corresponds to 5,000 × $50 = $250,000 of stock. Selling one contract and buying $250,000 of the basket locks 6.00 × $50 = $300 before costs. If, hypothetically, trading the basket and the futures costs the equivalent of 4.00 points, the net is 2.00 points, $100 per contract; if costs were 6.00 points or more, there would be nothing to earn.
Why there is a band, not a line
- Executing hundreds of stocks at once costs commissions, spreads and market impact.
- The reverse trade needs stock to borrow, which is not always available or cheap.
- Dividends between now and expiry are estimates, not certainties.
- Capital and margin are tied up until the position is unwound.
- Participants with lower costs set the band's edges; the premium can drift within it without triggering anything.
What it means for a futures chart
When the premium reaches the edge of the band, arbitrage orders arrive in the futures and in the stocks at the same time. On an ES footprint they look like any other aggressive orders; nothing in the data identifies them. What the mechanism explains is why a large move in the stock market is mirrored in the futures within moments during regular hours, and why the futures keep a stable offset from the index that shrinks as expiry approaches.
In Senzoukria
The application does not compute a fair value, an arbitrage band or basket prices. It shows the executions and depth of the selected futures contract; the cash index enters only through the GEX module's options underlying, reported with its spot price at the snapshot time.
Common mistakes
- Calling any futures premium over the index an arbitrage opportunity without counting carry and costs.
- Reading arbitrage-driven flow as directional conviction.
Related
In the same section
- Inactivity rule
- Index price
- In-sample vs out-of-sample
- Initial balance
- Implied volatility
- Initial margin
- Implied orders
- Initiative activity
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Frequently asked questions
- Is index arbitrage the same as the crypto cash-and-carry trade?
- The logic is the same, holding the underlying against a short derivative, but the instruments differ. An index future has a fixed expiry at which it settles to a value computed from the index, so the locked amount is known in advance; a perpetual swap has no expiry and pays or charges funding, so its carry is uncertain.
- Can an individual trader do index arbitrage?
- In principle, but it requires executing the whole basket and the futures together at very low cost, plus financing and, for the reverse trade, stock borrowing. Those requirements make it the business of firms whose costs set the edges of the band.