Correlation-Adjusted Sizing

Correlation adjusted sizing means reducing position sizes when your trades are likely to move together. Five separate positions each risking 1% look like 5% total risk spread across five ideas. But if all five are technology stocks, or all long the US dollar, they can lose together on the same day, behaving like one 5% bet on a single idea. Adjusting for correlation keeps your real risk where you think it is.

How correlation hides risk#

Correlation measures how closely two assets move together, from +1 (perfectly together) through 0 (unrelated) to minus 1 (perfectly opposite). See Covariance and Correlation.

With low correlation, losses on some positions are offset by gains or smaller losses on others. With high correlation, losses stack.

Simple adjustment rules#

You do not need complex mathematics to manage correlation. Practical rules many traders use:

See Portfolio Heat and Concentration Risk.

A slightly more formal approach#

For two positions with risk amounts R₁ and R₂ and correlation ρ, combined risk (as standard deviation) is:

Combined risk = √(R₁² + R₂² + 2 × ρ × R₁ × R₂)

With ρ = 1, combined risk is R₁ + R₂ (no diversification). With ρ = 0, it is √(R₁² + R₂²), lower than the sum. With ρ = minus 1, the positions can offset each other.

For whole portfolios, this extends to a covariance matrix. See Portfolio Construction and Risk Contribution and Risk Decomposition.

Correlations change#

Correlations are not fixed. In calm markets, many assets look loosely related; in crises, correlations often jump towards 1 as everything is sold at once. A portfolio that looks diversified on normal days can behave like one large position in a crash. For that reason, many traders assume higher correlation than recent data shows when setting limits. See Correlation Management.

Common correlation traps#

• Sector clustering: many "different" stocks from the same industry.
• Index plus components: holding an index ETF and its largest members.
• Currency overlap: several pairs sharing the same base or quote currency.
• Crypto: most altcoins move closely with Bitcoin, especially in sharp sell offs.
• Hidden factors: stocks in different sectors that all depend on low interest rates or the same economic theme.

Common mistakes#

• Counting the number of positions rather than the number of independent bets.
• Using correlations from calm periods to size for stressful ones.
• Forgetting about direction: long one asset and short a correlated one can reduce risk, while long both increases it.

Frequently asked questions#

Why does correlation matter for position sizing?#

Because correlated positions tend to lose at the same time, so their combined risk is larger than each position's individual risk suggests.

How do I adjust for correlated positions?#

Group correlated trades and cap their combined risk, or treat highly correlated positions as a single position in your risk budget.

Do correlations stay the same?#

No. They change over time and often rise sharply during market stress.

Next, learn where to place the stops your sizing depends on: Stop Loss Strategies.