The usual measure of skewness for a dataset $x_i$ ($i=1,2,\ldots,n$) with mean $\bar{x}$ is given by:
$$\frac{ \frac{1}{n} \sum_{i = 1}^{n}{\left(x_i - \bar{x}\right)^3}}{\left( \frac{1}{n}\sum_{i=1}^{n}(x_i-\bar{x})^2 \right)^{\frac{3}{2}}}$$