Correlation is the statistical relationship between price movements of two instruments, ranging from -1 (perfect inverse) to +1 (perfect sync). EUR/USD and GBP/USD are strongly positively correlated because both pair the dollar against a major European currency. USD/CHF and EUR/USD are strongly negatively correlated because the Swiss franc trades closely with the euro but the quote is inverted.
Ignoring correlation is a common way to accidentally double risk. Opening a long EUR/USD and long GBP/USD at the same time feels like diversification but is effectively a 2× long dollar-short position — one bad dollar print moves both trades against you together. Our correlation calculator shows the current values for any pair combination.