Conversion Arbitrage

Conversion arbitrage is an options strategy that combines a stock position with two options to lock in a small, theoretically riskless profit if prices are out of line with each other. It is built from three pieces: buying the underlying stock, buying a put option on that stock, and selling a call option on that stock, where the put and call share the same strike price and the same expiration date.

The reason this combination is "riskless" in theory is that a put bought and a call sold at the same strike and expiration, paired with owning the stock, creates a position whose value at expiration is fixed no matter where the stock price ends up. This fixed payoff is called a synthetic position, and the whole point of the trade is to compare the cost of building it this way against what it would cost to just get that same fixed payoff directly in the market. If the options and stock are priced so that building the synthetic version is cheaper than the alternative, the difference is a locked-in profit, minus transaction costs and financing.

The nuance that trips people up is that "riskless" refers to price risk between the stock and the options, not to all risk. The trader still faces things like the cost of borrowing money to buy the stock, dividends the stock might pay or not pay as expected, the risk that one leg of the trade gets exercised or assigned earlier than planned, and the practical difficulty of executing three separate legs at the exact prices needed to make the math work. In real markets these tiny mismatches are usually spotted and closed within moments by firms with fast systems, which is why individual traders rarely see genuine conversion arbitrage opportunities of meaningful size.

It helps to think of conversion arbitrage as the mirror image of reversal arbitrage, which flips the stock and options positions. Both strategies exist to enforce a pricing relationship between puts, calls, and the underlying stock called put-call parity; when that relationship breaks down even slightly, these trades are the mechanism that pushes prices back into line.