\mathop{\mathrm{corr}}(X,Y)=
\frac
 {\displaystyle \sum_{i=1}^n(x_i-\overline x)(y_i-\overline y)}
 {\displaystyle\bigg[\sum_{i=1}^n(x_i-\overline x)^2\sum_{i=1}^n(y_i-\overline y)^2\bigg]^{1/2}}