Conversion arbitrage is an options trade that buys 100 shares of stock, buys a put and sells a call at the same strike and expiration. The long put plus short call form a synthetic short stock that offsets the shares, so the position has no directional exposure; if the call is overpriced relative to the put under put-call parity, the difference is locked in as profit at expiration. The profit is usually a few cents per share, and it can be eroded by commissions, dividends, early assignment and pin risk, which is why market makers and algorithmic firms dominate the trade.
This guide explains how conversion arbitrage works, how to spot put-call parity mispricings, the reverse conversion, the real-world risks, and how to check parity in Excel with MarketXLS (real-time options data requires the Advanced or Business plan).
What Is Conversion Arbitrage?
Conversion arbitrage (sometimes called a "conversion" or "reversal-conversion") is a three-legged options strategy that combines:
- A long position in the underlying stock (100 shares)
- A long ATM put option (same strike, same expiration)
- A short ATM call option (same strike, same expiration)
All three legs reference the same underlying security, the same strike price, and the same expiration date. The combination creates a position with a net delta of zero — meaning the position is insensitive to directional moves in the stock.
The profit comes not from predicting which way the stock will move, but from a temporary mispricing between the synthetic position (long put + short call) and the actual stock. When the call is slightly overpriced relative to the put (or the put is slightly underpriced), the difference can indicate a theoretical opportunity, but costs, execution and assignment must be considered before treating it as a profit.
The Put-Call Parity Foundation
Conversion arbitrage is rooted in put-call parity, the cornerstone equation of options pricing:
C - P = S - K × e^(-rT)
Where:
- C = Call premium
- P = Put premium
- S = Current stock price
- K = Strike price
- r = Risk-free interest rate
- T = Time to expiration (in years)
When this equation holds perfectly, there is no arbitrage opportunity. But when the left side (C - P) diverges from the right side (S - K × e^(-rT)), a conversion or reverse conversion becomes profitable.
In simple terms:
- If the call is overpriced relative to the put → execute a conversion (buy stock, buy put, sell call)
- If the put is overpriced relative to the call → execute a reverse conversion (short stock, sell put, buy call)
Why Do Mispricings Occur?
Put-call parity violations are rare in highly liquid markets, but they do occur. Common causes include:
- Supply/demand imbalances in the options market (e.g., heavy call buying before earnings)
- Dividend uncertainty that affects one side of the equation more than the other
- Interest rate assumptions embedded in option prices that differ from actual rates
- Wide bid-ask spreads in less liquid names or far-dated expirations
- Temporary dislocations during high-volatility events (earnings, Fed announcements)
For retail traders, the window is often very small — sometimes just seconds. That is why scanning tools like MarketXLS are essential for identifying these opportunities before they vanish.
How to Implement a Conversion Arbitrage Trade
Let us walk through a conversion step by step using a concrete example.
Step 1: Identify the Opportunity
Suppose AAPL is trading at $185.00. You pull up the option chain for the $185 strike expiring in 30 days and see:
| Component | Price |
|---|---|
| AAPL Stock | $185.00 |
| $185 Call (30 days) | $5.80 |
| $185 Put (30 days) | $5.20 |
Put-call parity says C - P should approximately equal S - K (for ATM options with short expirations, the interest rate component is small). Here:
- C - P = $5.80 - $5.20 = $0.60
- S - K = $185.00 - $185.00 = $0.00
The call appears overpriced by roughly $0.60 (after accounting for any carrying cost). This is a potential conversion arbitrage opportunity.
Step 2: Execute the Three Legs
| Leg | Action | Price |
|---|---|---|
| Stock | Buy 100 shares of AAPL | $185.00 per share |
| Put | Buy 1 AAPL $185 Put | $5.20 (pay $520) |
| Call | Sell 1 AAPL $185 Call | $5.80 (receive $580) |
Net credit from options: $580 - $520 = $60
This $60 is locked in at trade entry regardless of where AAPL moves.
Step 3: Analyze the Payoff at Expiration
Scenario A: AAPL finishes above $185 (say $200)
- Stock position: worth $200 × 100 = $20,000
- Put expires worthless: $0
- Call is exercised: you sell 100 shares at $185 = $18,500
- Net stock P&L: $18,500 - $18,500 = $0 (bought at $185, sold at $185)
- Total profit: $60 (the options credit)
Scenario B: AAPL finishes below $185 (say $170)
- Stock position: worth $170 × 100 = $17,000 (unrealized loss of $1,500)
- Put exercised: sell 100 shares at $185 = $18,500 (recovers the loss)
- Call expires worthless: $0
- Net stock P&L: $18,500 - $18,500 = $0
- Total profit: $60 (the options credit)
Scenario C: AAPL finishes exactly at $185
- Stock position: no gain or loss
- Both options expire worthless (or at the money)
- Total profit: $60 (the options credit)
No matter what happens to the stock, the profit is $60 minus commissions. That is the beauty of conversion arbitrage — it is direction-independent.
The Profit Formula
The profit from a conversion can be calculated as:
Profit = Call Premium - Put Premium - (Stock Price - Strike Price) - Carrying Costs
Or equivalently:
Profit = (C - P) - (S - K) - Net Cost of Carry
When this value is positive after accounting for commissions, dividends, and interest costs, you have a viable conversion arbitrage.
Reverse Conversion (Reversal): The Mirror Image
A reverse conversion (also called a "reversal") is the exact opposite trade. You execute it when the put is overpriced relative to the call.
Reverse Conversion Legs
- Short 100 shares of the underlying stock
- Short 1 ATM put (same strike, same expiration)
- Long 1 ATM call (same strike, same expiration)
Reverse Conversion Example
Using the same AAPL example, but suppose the prices are flipped:
| Component | Price |
|---|---|
| AAPL Stock | $185.00 |
| $185 Call (30 days) | $5.20 |
| $185 Put (30 days) | $5.80 |
Now P > C, and the put appears overpriced. Execute a reversal:
| Leg | Action | Price |
|---|---|---|
| Stock | Short 100 shares of AAPL | $185.00 per share (receive $18,500) |
| Put | Sell 1 AAPL $185 Put | $5.80 (receive $580) |
| Call | Buy 1 AAPL $185 Call | $5.20 (pay $520) |
Net credit from options: $580 - $520 = $60
At expiration, the short stock is covered by either the call (if stock rises) or the put assignment (if stock falls), producing a theoretical $60 profit under the example assumptions, before financing, stock-borrow and other trading costs.
Key Differences: Conversion vs. Reverse Conversion
| Feature | Conversion | Reverse Conversion |
|---|---|---|
| Stock position | Long 100 shares | Short 100 shares |
| Call position | Short (sell) | Long (buy) |
| Put position | Long (buy) | Short (sell) |
| Triggered when | Call overpriced vs. put | Put overpriced vs. call |
| Capital required | Stock purchase + put cost | Margin for short stock + call cost |
| Dividend exposure | Receive dividends | Pay dividends on short |
Understanding Synthetic Positions
Conversion arbitrage works because of the relationship between synthetic and actual positions. Understanding synthetics is critical:
The Six Synthetic Relationships
| Synthetic Position | Components |
|---|---|
| Synthetic long stock | Long call + Short put (same strike/exp) |
| Synthetic short stock | Short call + Long put (same strike/exp) |
| Synthetic long call | Long stock + Long put |
| Synthetic short call | Short stock + Short put |
| Synthetic long put | Short stock + Long call |
| Synthetic short put | Long stock + Short call |
A conversion is essentially:
- Actual long stock + Synthetic short stock (long put + short call)
If the synthetic short stock costs less than the actual long stock (or vice versa for a reversal), the difference is your profit.
Comparison: Conversion vs. Other Arbitrage Strategies
Conversion arbitrage is one of several options arbitrage strategies. Here is how it compares:
| Strategy | Legs | Risk Profile | Capital Needed | Complexity | When to Use |
|---|---|---|---|---|---|
| Conversion | Long stock + Long put + Short call | Theoretically hedged; costs and execution risks remain | High (stock purchase) | Low | Call overpriced vs. put |
| Reverse Conversion | Short stock + Short put + Long call | Theoretically hedged; costs and execution risks remain | Moderate (margin) | Low | Put overpriced vs. call |
| Box Spread | Bull call spread + Bear put spread (same strikes) | Theoretically hedged; costs and exercise risks remain | Low (options only) | Medium | Interest rate arbitrage; no stock needed |
| Jelly Roll | Synthetic long in one expiry + Synthetic short in another | Theoretically hedged; costs and execution risks remain | Moderate | High | Calendar mispricing between expirations |
Box Spread vs. Conversion
A box spread achieves a similar risk-free outcome but uses four options (no stock). It combines a bull call spread and a bear put spread at the same strikes. The advantage is that you do not need to buy or short the stock, reducing capital requirements. However, box spreads are heavily monitored by exchanges and are primarily used for interest-rate arbitrage.
Jelly Roll vs. Conversion
A jelly roll exploits mispricings between two different expiration dates. You create a synthetic long stock in one expiration and a synthetic short stock in another. The profit comes from the difference in carrying costs between the two expirations. Jelly rolls are more complex and typically used by professional traders.
Real-World Considerations and Risks
While conversion arbitrage is theoretically "risk-free," real-world execution introduces several challenges:
1. Transaction Costs
Commissions, exchange fees, and bid-ask spreads can easily eat up the small profit from a conversion. Most retail brokers charge per-contract fees that may exceed the arbitrage profit on a single conversion.
Example: If your broker charges $0.65 per contract and $0 for stock trades, a single conversion costs at least $1.30 in commissions ($0.65 × 2 contracts). If the arbitrage profit is only $0.50, the trade is a net loss.
2. Early Assignment Risk
American-style options can be exercised early. If your short call is assigned before expiration (particularly around ex-dividend dates), you may be forced to deliver shares earlier than planned, disrupting the arbitrage.
3. Dividend Risk
If the underlying stock pays a dividend during the life of the options, it affects the economics of the trade. For a conversion (long stock), you receive the dividend, which is favorable. For a reverse conversion (short stock), you must pay the dividend, which reduces profit.
4. Margin Requirements
Short option positions and short stock positions require margin. The capital tied up in margin reduces the effective return on the arbitrage. Institutional traders with portfolio margining have a significant advantage here.
5. Pin Risk at Expiration
If the stock closes very near the strike at expiration, there is uncertainty about whether the options will be exercised. This "pin risk" can leave you with an unexpected position over the weekend.
6. Execution Speed
Mispricings in liquid names may last only milliseconds. By the time a retail trader identifies the opportunity and enters orders, the prices may have already normalized. This is why algorithmic trading dominates conversion arbitrage in practice.
How to Find Conversion Arbitrage Opportunities With MarketXLS
MarketXLS pulls stock prices, option chains and quotes into Excel, so you can check put-call parity across strikes systematically. Options data is end-of-day on the Standard plan and real-time on the Advanced and Business plans; mispricings in liquid names often last only seconds, so a spreadsheet is best for learning and screening rather than execution. Here is how to set up a conversion arbitrage scanner.
Step 1: Pull the Current Stock Price
Use the Last function to get the current stock price:
=Last("AAPL")
Formula documentation: Last
This returns the last traded price for AAPL, which becomes the "S" in your put-call parity calculation.
Step 2: Load the Full Option Chain
Pull the complete option chain with strikes, expirations, and premiums:
=QM_GetOptionChain("AAPL")
Formula documentation: QM_GetOptionChain
This populates your spreadsheet with every available call and put, organized by strike and expiration. You can filter for ATM strikes where conversion arbitrage is most likely to occur.
Step 3: Get Detailed Greeks and Quotes
For deeper analysis including implied volatility, delta, gamma, and other Greeks:
=QM_GetOptionQuotesAndGreeks("AAPL")
Formula documentation: QM_GetOptionQuotesAndGreeks
This helps you verify that the options are liquid (tight bid-ask spreads) and that the implied volatility is consistent between the call and put at the same strike.
Step 4: Build Specific Option Symbols
To price individual legs of your conversion, construct the option symbol:
=OptionSymbol("AAPL", "2026-03-21", "C", 185)
Formula documentation: OptionSymbol
This returns the standardized option symbol (e.g., @AAPL 260321C00185000), which you can then use to pull its price:
=QM_Last("@AAPL 260321C00185000")
Formula documentation: QM_Last
Step 5: Calculate the Arbitrage Spread
With the stock price and both option prices in your spreadsheet, create a formula to calculate the mispricing:
= [Call Price Cell] - [Put Price Cell] - (Last("AAPL") - [Strike Price Cell])
Formula documentation: Last
If this value is significantly positive (after accounting for commissions and carry costs), you have identified a potential conversion arbitrage.
Step 6: Scan Multiple Symbols
Repeat this process for a watchlist of liquid stocks. MarketXLS allows you to pull option chains for any ticker, so you can build a dashboard that scans dozens of names simultaneously and highlights any put-call parity violations.
Pro Tip: Focus on Liquid Names
Conversion arbitrage works best in highly liquid underlyings with tight option spreads. Focus on large-cap names like AAPL, MSFT, AMZN, GOOGL, and SPY where the bid-ask spread will not consume your profit.
Step-by-Step P&L Walkthrough: MSFT Conversion
Let us work through a complete example with Microsoft (MSFT).
Assumptions:
- MSFT stock price: $420.00
- Strike price: $420
- Expiration: 30 days out
- $420 Call price: $9.50
- $420 Put price: $8.80
Trade Entry:
| Leg | Action | Cash Flow |
|---|---|---|
| Buy 100 shares MSFT | -$42,000.00 | Debit |
| Buy 1 MSFT $420 Put | -$880.00 | Debit |
| Sell 1 MSFT $420 Call | +$950.00 | Credit |
| Net options credit | +$70.00 |
At Expiration (MSFT at $450):
- Call is exercised → sell 100 shares at $420 → receive $42,000
- Put expires worthless
- Stock cost basis: $42,000
- Stock sale proceeds: $42,000
- Net P&L: +$70 (the options credit)
At Expiration (MSFT at $390):
- Call expires worthless
- Exercise put → sell 100 shares at $420 → receive $42,000
- Stock cost basis: $42,000
- Stock sale proceeds: $42,000
- Net P&L: +$70 (the options credit)
After commissions (assuming $0.65 per contract × 2 = $1.30):
- Net profit: $68.70
The illustrated return is small and depends on the stated assumptions. It is not guaranteed: financing, stock-borrow costs, execution and assignment can change the outcome.
Historical Context: Conversion Arbitrage on the Trading Floor
Conversion arbitrage has been a staple of options market making since the early days of listed options trading on the Chicago Board Options Exchange (CBOE) in 1973. In the pit-trading era, floor traders would manually calculate put-call parity and shout out bids and offers to capture mispricings. The profit margins were larger then because information traveled slowly and electronic arbitrage did not exist.
As electronic trading evolved through the 1990s and 2000s, conversion arbitrage became increasingly automated. Today, high-frequency trading firms execute conversions and reversals in microseconds, compressing the available profit to fractions of a penny per share. Despite this compression, the strategy remains relevant because it underpins the pricing of every listed option — market makers continuously enforce put-call parity through conversion and reversal trades, which is why options markets remain efficient.
For individual traders and analysts, understanding conversion arbitrage provides deep insight into why options are priced the way they are. When you see a call and put at the same strike trading at different implied volatilities, conversion arbitrage is the mechanism that eventually brings them back into alignment.
Who Uses Conversion Arbitrage?
Conversion arbitrage is primarily used by:
- Market makers who have low or zero commissions and can execute at mid-prices
- Proprietary trading firms with high-speed execution systems
- Institutional arbitrageurs with portfolio margining that minimizes capital requirements
- Hedge funds as part of broader relative-value or volatility arbitrage strategies
Retail traders can learn from conversion arbitrage to deepen their understanding of options pricing and put-call parity, even if the strategy itself is difficult to execute profitably at retail commission levels.
Frequently Asked Questions
What is conversion arbitrage in options trading?
Conversion arbitrage is a risk-neutral options strategy that exploits put-call parity mispricings. It involves simultaneously buying 100 shares of stock, buying an ATM put, and selling an ATM call at the same strike and expiration. When the call is overpriced relative to the put, the example can show a theoretical profit independent of stock direction. Realized results still depend on execution, financing, stock-borrow costs and assignment.
What is a reverse conversion (reversal) in options?
A reverse conversion options strategy is the mirror image of a conversion. It involves shorting 100 shares of stock, selling an ATM put, and buying an ATM call at the same strike and expiration. You execute a reversal when the put is overpriced relative to the call. Like a standard conversion, the theoretical payoff is independent of stock price movement under the model assumptions. Actual profit is not guaranteed after costs and execution or assignment effects.
How does put-call parity relate to conversion arbitrage?
Put-call parity (C - P = S - K × e^(-rT)) defines the theoretical relationship between call and put prices. When actual market prices violate this equation, the difference creates an arbitrage opportunity. Conversion arbitrage directly exploits this violation — if C - P is too large, you execute a conversion; if P - C is too large, you execute a reverse conversion.
Can retail traders profit from conversion arbitrage?
Conversion arbitrage is challenging for retail traders because the profit margins are extremely thin (often just cents per share), commissions can exceed the profit, and mispricings disappear within seconds in liquid markets. However, retail traders can use MarketXLS to study put-call parity relationships and occasionally find opportunities in less liquid options where spreads are wider.
What is the difference between a conversion and a box spread?
A conversion uses stock plus two options (long put, short call) at one strike and one expiration. A box spread uses four options (bull call spread + bear put spread) at two strikes within the same expiration and does not require a stock position. Both are theoretical arbitrage structures whose realized outcomes depend on execution, financing and contract exercise terms. A box spread avoids buying stock, but its cash and margin requirements still depend on the contracts and broker.
How do dividends affect conversion arbitrage?
Dividends are a critical factor. In a conversion (long stock), you receive the dividend, which is favorable and increases your profit. In a reverse conversion (short stock), you must pay the dividend to the lender, which reduces your profit. Unexpected dividend changes can turn a seemingly profitable arbitrage into a loss, so always verify the ex-dividend date before entering.
Start Scanning for Arbitrage With MarketXLS
Conversion arbitrage rewards traders who can identify and act on put-call parity mispricings faster than the competition. With MarketXLS, you get option chains, Greeks and pricing data in Excel (real-time on the Advanced and Business plans) to build a parity scanner without code.
Whether you are a serious arbitrageur or simply want to deepen your understanding of how options are priced, MarketXLS provides the data foundation you need.
👉 Get started with MarketXLS today
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