Correlation Coefficient Calculator

What r actually tells you

Pearson's r runs from −1 to +1 and measures how tightly two variables move together in a straight line. Near +1 they rise together; near −1 one falls as the other rises; near 0 there is no linear pattern — which is not the same as no relationship at all.

Why r² matters more than r

Square r and you get the share of variance one variable explains in the other. An r of 0.7 sounds strong, but r² = 0.49 means barely half the variation is accounted for. r² is the number to quote when someone asks “how much does X explain Y?”

The two traps

First, correlation is not causation — a lurking third variable explains most surprising correlations. Second, r is fragile: one outlier can create or destroy it, and a strong curve can hide behind an r near zero. That is why this tool always draws the scatter, not just the number.