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Correlation between currency pairs changes over time. The window used for calculation affects how quickly the number reacts.

Comparison:

• 20-day correlation: reacts quickly to new relationships, but is noisier.
• 60-day correlation: more stable, but slower to show changes.

Uses:

• Risk management: 20-day shows current exposure better.
• Strategy development: 60-day or longer shows structural relationships.
• Comparing both: if they differ a lot, the relationship is changing.

Example: EURUSD and GBPUSD often show high positive correlation, but it drops during UK-specific events.

In practice, many traders use the 20-day value to size exposure and the 60-day value to decide which pairs to trade together. When the shorter window turns sharply lower, it often signals that a currency-specific story has started.

Which correlation window do you use, and why?

I run both windows and act on the gap between them rather than picking one. When the 20 day sits more than 0.25 under the 60 day, I treat EURUSD and GBPUSD as separate trades again.

Using daily returns rather than price levels matters as much as the window. Price based correlation can read 0.9 in a month when daily moves barely agree.
The illustration uses synthetic daily returns with a UK specific episode: the 20 day falls from about 0.87 to 0.14 while the 60 day only bottoms at 0.61, and the gap flag stays on for 11 sessions. The 60 day keeps sliding after the story ends, purely because old days are still inside its window.

That lag cuts both ways. By day 157 the short window was back above 0.8, yet the long one stayed under 0.75 until day 173, so sizing off 60 days meant treating the pairs as diversified for over two weeks too long.

For risk I use whichever window reads higher, not the 20 day alone as the post suggests. Correlations snap back toward 1 in a broad dollar move, which is exactly when a low reading turns two positions into one.