You may have been thinking of the formula for the volume of a cone or pyramid, and used $\displaystyle\frac{{1}}{{3}}\usepackage{cmbright}\Huge\$ as the constant factor instead of $\displaystyle\frac{{4}}{{3}}\usepackage{cmbright}\Huge\$ . Note that $\displaystyle{V}=\frac{{4}}{{3}}\pi{{r}}^{{3}}\usepackage{cmbright}\Huge\$ is the correct formula for the volume of a sphere.