# What Is an Index Option Contract? A Guide for US Traders
Author: Huzefa Kudrati
Author URL: https://tradetron.us/blog/author/huzefa-kudrati
Published: 2025-12-30
Updated: 2026-10-03
Meta Title: What Is an Index Option Contract? SPX, NDX, XSP
Meta Description: What an index option contract is, how cash settlement works with a worked SPX example, the main US index options, SPX vs SPY, taxes and automation.
URL: https://tradetron.us/blog/what-is-an-index-option-contract-a-complete-guide-for-us-traders-using-automation

An **index option contract** is an option whose underlying is a stock market index, such as the S&P 500 (SPX), Nasdaq-100 (NDX) or Russell 2000 (RUT), instead of a single stock. Because you can't take delivery of an index, these contracts settle in cash: at expiry, the in-the-money amount × the contract multiplier ($100 for SPX) is paid to the holder.

:::takeaways
- An index option gives you a view on the whole market, not one company, through a single contract.
- Major US index options (SPX, XSP, NDX, RUT) are cash-settled and European-style, so they can't be exercised early.
- One SPX contract is worth $100 per index point; XSP is one-tenth of that.
- SPX has expiries every weekday, which is what made 0DTE trading possible.
- For US taxpayers, broad-based index options get 60/40 tax treatment under Section 1256.
:::

## How an index option contract works

Every option has the same parts. For an index option they look like this:

| Part | What it means for an index option |
|---|---|
| Underlying | An index level, such as the S&P 500 at 6,000 |
| Call or put | A call gains as the index rises above the strike; a put gains as it falls below |
| Strike | The index level the option is measured against |
| Expiry | The day the option is settled; SPX has one every weekday |
| Premium | The price, quoted in index points; multiply by the multiplier for dollars |
| Multiplier | $100 per point for SPX and NDX; XSP uses one-tenth of the S&P 500 level, so it is one-tenth the size |
| Settlement | Cash: the in-the-money amount is paid, no shares change hands |
| Exercise style | European: exercised only at expiry |

### A worked example

You buy one SPX 6,000 call for 40 points, which costs 40 × $100 = $4,000. At expiry, the settlement value of SPX is 6,120. The call is 120 points in the money, so you receive 120 × $100 = $12,000 in cash. Your profit is $12,000 − $4,000 = $8,000. If SPX settles below 6,000, the call expires worthless and you lose the $4,000.

![Payoff at expiry of one SPX 6,000 call bought for $4,000: loss capped at $4,000 below 6,000, breakeven at 6,040, profit of $8,000 if SPX settles at 6,120](https://tradetron.us/blog/media/54ca45687b1c53e57904.png "A long SPX call at expiry (example figures)")

:::note AM vs PM settlement
Standard monthly SPX options (third Friday) settle on a special opening price calculated from the opening trades of the index's stocks that morning. The weekly and daily SPX contracts settle on the closing value. If you hold to expiry, check which one you hold: the opening and closing values can differ a lot.
:::

## The main US index options

| Contract | Index | Size | Style | Settlement |
|---|---|---|---|---|
| SPX | S&P 500 | $100 × index | European | Cash |
| XSP (Mini-SPX) | S&P 500 ÷ 10 | $100 × (index ÷ 10) | European | Cash |
| NDX | Nasdaq-100 | $100 × index | European | Cash |
| RUT | Russell 2000 | $100 × index | European | Cash |
| VIX | Cboe Volatility Index | $100 × index | European | Cash, priced off VIX futures |

VIX options behave differently from the rest: they are priced from VIX futures, not the spot VIX number you see quoted, and they expire on Wednesdays. Treat them as their own subject.

## Index options vs ETF options (SPX vs SPY)

Many traders choose between SPX options and options on SPY, the ETF that holds the S&P 500.

| | SPX (index option) | SPY (ETF option) |
|---|---|---|
| Settlement | Cash | 100 shares of SPY |
| Exercise | European, only at expiry | American, any time, so early assignment can happen |
| Size | About 10× SPY | Smaller |
| Dividends | None to worry about | Short calls can be assigned early before ex-dividend |
| US tax (taxable accounts) | 60% long-term, 40% short-term, regardless of holding period | Normal short- and long-term rules |

For multi-leg strategies held to expiry, SPX's cash settlement means a short leg can't leave you holding shares you didn't plan for. SPY's smaller size suits smaller accounts. XSP sits in between: cash-settled like SPX, at one-tenth the size.

## Why traders use index options

- **Directional views on the whole market** without picking stocks.
- **Hedging a portfolio:** buying SPX puts or put spreads to offset losses in a stock portfolio that moves with the index.
- **Income strategies:** selling credit spreads or iron condors on SPX, with defined risk.
- **Event and volatility trades** around Fed meetings, inflation data and earnings seasons.
- **Short-dated trading:** daily SPX expiries allow strategies that open and close on the same day. See [the best 0DTE options strategy](https://tradetron.us/blog/the-best-0dte-options-strategy-isnt-a-secret-setup-its-a-system-you-can-actually-run).

:::warning Defined risk is a choice, not a feature
Index options can be bought (risk limited to the premium) or sold. Selling a naked index call or put has a large potential loss, because one SPX point is worth $100 per contract. Most systematic traders sell index options only as part of a spread with a bought option limiting the loss.
:::

## Trading index options systematically

Index options suit rules: time windows, strike distances, stops and time exits can all be written down and tested. A systematic approach usually looks like this:

:::steps
1. **Define the view and structure:** for example, "sell a 50-point-wide SPX iron condor when the index is range-bound."
2. **Write every variable:** entry conditions, strike selection, size, profit target, stop and time exit.
3. **Build it** on a platform that handles multi-leg positions and cash-settled index options.
4. **Backtest** across calm and volatile periods.
5. **Run it simulated** in live markets, then go live with one contract.
:::

:::tradetron Automate index option strategies on Tradetron
Tradetron lets you build index option strategies without code and run them during US hours.

- **Multi-leg positions:** spreads, iron condors, straddles and strangles built as legs with strikes chosen by rule.
- **Risk rules:** per-leg SL and Target triggers, PNL-based exits, a trailing stop and time exits on the NYSE clock.
- **Test first:** backtest, then run **Live Offline**: the strategy runs and records every trade, but nothing is sent to the exchange.
- **Go live** through a connected US broker such as Tastytrade, or start from strategies on the marketplace.

[Start free](https://tradetron.us/signup?utm_source=blog&utm_medium=post) [Browse ready-made strategies](https://tradetron.us/strategies?utm_source=blog&utm_medium=post)
:::

Next, read [how US traders automate options](https://tradetron.us/blog/algo-trading-options-how-us-traders-can-automate-and-win-in-2025) or [how to automate a 0DTE SPX strategy without coding](https://tradetron.us/blog/how-to-automate-a-0dte-spx-strategy-without-coding-using-tradetron).

## Frequently asked questions

### What is an index option contract?

It is an options contract whose underlying is a stock index, such as the S&P 500, rather than a single stock. It gives the buyer the right to the difference between the index level and the strike at expiry, paid in cash.

### How are index options settled?

In cash. At expiry, if the option is in the money, the holder receives the in-the-money amount in points multiplied by the contract multiplier ($100 for SPX). No shares or index units change hands.

### What is the difference between SPX and SPY options?

SPX options are on the S&P 500 index, cash-settled, European-style and about ten times larger. SPY options are on the SPY ETF, settle in shares, are American-style and can be assigned early.

### Can index options be exercised early?

The main US index options (SPX, XSP, NDX, RUT) are European-style, so they can only be exercised at expiry. You can still sell or buy back the option at any time before then.

### How are index options taxed in the US?

For US taxpayers, broad-based index options such as SPX are Section 1256 contracts: gains and losses are treated as 60% long-term and 40% short-term, whatever the holding period, and positions are marked to market at year-end. Check with a tax advisor for your situation.

### Can I automate index option trading?

Yes. On Tradetron you can build multi-leg index option strategies with entry rules, stops, targets and time exits, backtest them, run them Live Offline, and then trade through a connected US broker.