Pairs Trading Strategy: How It Works and How to Use It

Pairs trading is a strategy that takes a long position in one asset and a short position in a closely related asset at the same time. Profit and loss come from the change in the relative price between the two, not from the direction of either asset on its own. Because the long and short positions are designed to offset each other when the broader market moves, pairs trading is often described as market neutral.

This guide covers what pairs trading is, how it works, how to choose pairs, how to size a pairs trade, the costs and risks involved, and how the strategy applies to CFD trading on stocks, indices, commodities, and forex.

What Is Pairs Trading?

Pairs trading is a relative-value strategy. Rather than betting on whether a single asset will rise or fall, a pairs trader takes a view on whether two related assets will move closer together or further apart.

The strategy was developed at Morgan Stanley in the mid-1980s by a quantitative research team that included Gerry Bamberger and Nunzio Tartaglia. The team identified pairs of stocks whose prices had historically moved together and traded the temporary deviations from that historical relationship. When the price of one asset moved unusually far from the other, they took a long position in the relatively cheaper one and a short position in the relatively expensive one, expecting the relationship to normalise.

Pairs trading is a form of statistical arbitrage, a broader category of strategies that look for short-term mispricings between related instruments. Statistical arbitrage can involve large baskets of instruments, but pairs trading is the simplest two-asset version.

The term "pairs trading" can be confusing because it sounds like it refers to trading currency pairs in forex. It does not. Pairs trading is a strategy that can be applied to any market, including forex, but the "pair" in pairs trading refers to two separate instruments traded simultaneously, not to a single currency pair.

How Pairs Trading Works

The mechanics of pairs trading come down to three elements: the historical relationship between the two assets, the spread that measures their current divergence, and the rules for entering and exiting positions.

Correlation and Cointegration

The first step is identifying two assets with a meaningful historical relationship. Two statistical concepts are commonly used.

Correlation measures the degree to which two assets move together. A correlation of +1 means they move in perfect step, -1 means they move in perfect opposite directions, and 0 means there is no linear relationship. Pairs traders typically look for high positive correlation, often above 0.8 over a chosen lookback period.

Cointegration is a more rigorous concept. Two assets are cointegrated if a specific linear combination of their prices is stable over time, even if each individual price is not. Cointegration tests, such as the Engle-Granger or Johansen tests, are commonly used in quantitative research to identify pairs whose long-term relationship is statistically robust. Cointegration is generally a stronger basis for a pairs trade than correlation alone, because it implies a tendency for the relationship to revert rather than simply a tendency to move in the same direction.

The Spread and Mean Reversion

Once a pair is identified, the trader defines a spread that captures the relationship between the two prices. The spread can be the simple price difference, the ratio of the two prices, or a more complex linear combination derived from regression.

The spread is then standardised, often using a z-score, which measures how many standard deviations the current spread is away from its historical mean. A z-score of zero means the spread is at its average. A z-score of +2 means the spread is two standard deviations above average, and -2 means two below.

The trade thesis is mean reversion: the assumption that an unusually wide or narrow spread will return towards its historical average. Pairs trading does not require a view on whether the broader market will rise or fall, only that the relationship between the two specific assets will normalise.

Entry and Exit Signals

A typical pairs trading rule set looks something like this. When the z-score crosses a defined entry threshold, often +2 or -2, the trader opens a position. If the spread is unusually wide (positive z-score), the trader sells the relatively expensive asset and buys the relatively cheap one. If the spread is unusually narrow (negative z-score), the positions are reversed. The trade is closed when the z-score returns to zero, or to a defined exit threshold closer to the mean.

A stop-loss rule is also necessary. If the spread continues to widen past a further threshold, often +3 or -3, the trade is closed to limit the loss from a structural change in the relationship.

Choosing Pairs to Trade

Pairs trading can be applied to almost any asset class. The key requirement is a credible reason to expect two assets to maintain a stable relationship over time.

Stock pairs are the original application of the strategy. Pairs of stocks in the same sector, with similar business models and similar exposure to the same macro drivers, often move together. Two large US technology companies, two major automakers, or two consumer goods companies are examples of pairs that quantitative traders study. TIOmarkets offers CFDs on more than 170 stocks, including major US and European listings, which allows both legs of a stock pair to be traded on the same platform. The full list is available on the stocks page.

Index pairs are another common application. Two equity indices that track related economies, or two indices that capture different sectors of the same economy, can show stable relationships that mean-revert. TIOmarkets offers CFDs on 16 indices, listed on the indices page, covering major US, European, and Asian markets.

Commodity pairs include the classic gold-silver pair, where the ratio between the two metals has been tracked by traders for decades. Crude oil and natural gas, or different grades of crude oil such as WTI (USOIL) and Brent (UKOIL), are other examples. The full list is on the commodities page.

Forex pairs trading involves two separate currency pairs rather than a single pair. For example, a trader might go long EURUSD and short GBPUSD if they expected the euro to outperform the pound, while the USD leg cancels out on aggregate. Cross-currency relationships of this kind are common in institutional currency trading. TIOmarkets offers more than 70 currency pairs through the forex page, supporting flexible pairs construction.

Sizing a Pairs Trade: Worked Example

Position sizing for a pairs trade requires balancing the two legs so that the overall position is as close to market-neutral as possible. There are two common approaches.

Dollar-neutral sizing equalises the notional value of the long and short legs. If the long leg has a notional value of USD 10,000, the short leg also has a notional value of USD 10,000. This is the simplest approach and the most common starting point.

Beta-neutral sizing adjusts for the relative sensitivity of each asset to broader market moves. If one asset is more volatile than the other, the size of the more volatile leg is reduced proportionally. This requires a regression analysis to estimate the relative beta and is more common in quantitative implementations.

The example below uses dollar-neutral sizing on two hypothetical stock CFDs. Assume a USD-denominated account and current illustrative prices.

Stock A is trading at USD 200 per share. The standard lot size for stock CFDs at TIOmarkets is 100 shares. A position of 0.5 lots represents 50 shares, with a notional value of 50 x USD 200 = USD 10,000.

Stock B is trading at USD 150 per share. To match the notional value of the long leg, the short position is 10,000 / 150 = approximately 66.67 shares, or 0.67 lots.

Stock CFDs at TIOmarkets carry a 5% margin requirement, equivalent to leverage up to 1:20. The margin required for each leg is 5% of the notional value, so USD 500 per leg, USD 1,000 in total for the pair.

These figures are illustrative and depend on the actual prices at the time of entry, the lot sizes selected on the platform, and the applicable account currency conversion. Use the Margin Calculator to confirm the exact margin requirement for each leg, and the Lot Size Calculator to size positions to a specific risk budget. Margin requirements are subject to change depending on market conditions and applicable regulatory requirements.

Costs to Consider

A pairs trade involves two simultaneous positions, which means transaction costs are incurred twice.

The spread, which is the difference between the bid and ask price, is paid on each leg at entry and at exit. Spreads are variable and are typically higher than minimum figures shown. On wider-spread instruments such as cross-currency pairs or less liquid stocks, the cost of crossing the spread on both legs can be material relative to the expected mean-reversion profit.

Commission applies on Raw and Nano accounts at TIOmarkets. On the Raw account, USD 6 per round turn lot is charged on forex CFDs. On the Standard and VIP Black accounts, no commission is charged. The full commission on the Raw account is charged when the position is opened and covers both the open and close of the trade.

Overnight financing applies to positions held past the daily rollover. For forex positions, swaps are credited or debited based on the interest rate differential between the two currencies. For stocks, indices, and commodities, overnight financing is generally calculated on a different basis from forex triple swap. Specific rates should be checked inside the platform.

Because a pairs trade holds one long and one short position, the financing impact depends on the direction and the relative funding rates of the two assets. In some pairs the financing on one leg offsets some of the financing on the other. In others the combined cost is meaningful and should be factored into the holding-period assessment.

Orders are executed at the best available market price, which may result in positive or negative slippage. Demo accounts often execute instantly and may not fully replicate live slippage conditions.

Risks and Limitations

Pairs trading is sometimes framed as low-risk because of its market-neutral construction. In practice, the strategy has several specific risks that can produce losses larger than the spread between entry and exit z-scores would suggest.

Correlation breakdown is the central risk. The historical relationship between two assets can change permanently because of an event such as a merger or acquisition, a regulatory change, a shift in business model, or a sector-wide structural change. When the relationship breaks, the spread does not revert to its historical mean, and the trade can produce a sustained loss.

Divergence risk is the related shorter-term version. Even when the historical relationship still holds, the spread can widen well beyond the entry threshold before reverting. A pairs trader who sized the position based on a +2 z-score entry may face a drawdown to +3 or +4 before any reversion begins. A stop-loss rule is essential for managing this scenario.

Cost drag is a structural disadvantage. Because every pairs trade has two legs, transaction costs are roughly twice what they would be for a directional trade. If the expected mean-reversion edge is small, costs can consume a large portion of it.

Margin and capital allocation matter as well. Two simultaneous positions tie up margin on both legs. At TIOmarkets, the maximum number of open and pending orders per client is 200 across all accounts, and the maximum lots per trade is 20. Active pairs traders running many simultaneous spreads need to plan margin and order allocation accordingly.

Model risk applies to any rule-based approach. Choosing the wrong lookback period, the wrong entry threshold, or the wrong pair can produce backtests that look attractive but do not survive live trading. Out-of-sample testing and conservative parameter selection help reduce this risk but do not eliminate it.

Pairs Trading at TIOmarkets

Pairs trading is available across stocks, indices, commodities, and forex at TIOmarkets, all on a single platform. Both MetaTrader 4 and MetaTrader 5 support simultaneous long and short positions across the available instruments. The Standard account is created automatically on registration, with a minimum deposit of $20 or currency equivalent. The Raw and VIP Black accounts are opened separately through the client area. All accounts support hedging, which is required for pairs trading. A swap-free Islamic account is available; contact TIOmarkets for eligibility and instrument requirements. Copy trading is available on both MT4 and MT5, allowing followers to copy strategy providers in real time.

Expert Advisors can automate pairs trading logic on the desktop terminal of either platform, including signal generation, position sizing, and execution. Mobile and web platforms support order entry and monitoring but do not run EAs. MT5 includes more order types, more analytical objects, and a built-in economic calendar, which can be useful for pairs trades that are sensitive to scheduled macroeconomic events.

Spreads are variable and are typically higher than minimum figures shown. Leverage on each instrument is subject to change depending on market conditions and applicable regulatory requirements.