What is a Pairs Trade? Definition, Formula, and Example
A pairs trade is a market-neutral strategy that buys one stock and shorts another in the same sector to profit from the relative performance between them.
What is a Pairs Trade?
A pairs trade is a market-neutral strategy that simultaneously buys one stock and shorts a highly correlated stock in the same sector or industry. The trader profits from the relative price movement between the two securities, not from the direction of the overall market. The strategy isolates the spread between the two stocks, removing broad market risk.
The pairs trade assumes the two stocks historically move together. When the spread widens beyond its normal range, the trader bets it will revert to the historical average. The long position gains if the spread narrows due to the long stock rising more than the short, and the short position gains if it falls more. The net exposure to market beta is near zero, so the trade's outcome depends on the relationship between the two stocks.
How a Pairs Trade is Constructed
The pairs trade requires selecting two stocks with a high historical correlation. The standard method uses a ratio or spread calculation:
Spread = Price of Stock A - (Hedge Ratio × Price of Stock B)
The hedge ratio is typically calculated using linear regression of stock A's returns against stock B's returns. The slope of the regression line is the hedge ratio. A common alternative uses the price ratio:
Price Ratio = Price of Stock A / Price of Stock B
The trader monitors the spread or ratio for deviations from its historical mean. A common entry signal is when the spread moves more than two standard deviations from the mean. The trade is closed when the spread reverts to the mean.
The position sizing formula is:
Short Shares of B = (Hedge Ratio × Long Shares of A × Price of A) / Price of B
This ensures the dollar exposure of the short leg offsets the beta of the long leg.
Worked Example: Pairs Trade on XOM and CVX
Assume XOM trades at $110 and CVX trades at $150. Historical regression shows that a $1 move in XOM corresponds to a $0.80 move in CVX, giving a hedge ratio of 0.80.
You buy 1,000 shares of XOM for $110,000. The short leg is:
Short Shares of CVX = (0.80 × 1,000 × $110) / $150 = 586.67 shares
You short 587 shares of CVX for $88,050. Your net market exposure is roughly zero.
The historical spread between the two stocks is $110 - (0.80 × $150) = -$10. The spread widens to -$15 when XOM drops to $105 and CVX rises to $160. You enter the trade. The spread reverts to -$10 when XOM rises to $115 and CVX falls to $155.
Your long XOM gains $10,000. Your short CVX gains 587 × $5 = $2,935. Total profit: $12,935, regardless of the market's direction during the period.
When Traders Use a Pairs Trade
Traders use pairs trades when they expect relative value to converge but have no view on the broader market. The strategy works well in sectors with many liquid, highly correlated stocks, such as energy, technology, and financials. Pairs trades also serve as a hedge for concentrated sector exposure, allowing a trader to maintain sector beta while eliminating idiosyncratic risk.
The strategy is common during earnings season, when one company's results diverge from its peer's. A trader might short the company with weak guidance and buy the peer with strong guidance, betting the spread narrows as the market reprices the relative fundamentals.
Limitations and Common Misconceptions
The pairs trade assumes correlation persists. When the underlying relationship breaks, the spread can widen indefinitely, causing losses on both legs. This happened during the 2008 financial crisis when historically correlated financial stocks diverged sharply.
A common misconception is that a pairs trade is risk-free. It is not. The hedge ratio is an estimate, and the two stocks can move in ways that the regression does not capture. Dividends, corporate actions, and sector rotation can distort the spread. The short leg also carries borrow costs and the risk of a short squeeze, which can force an early exit.