To use the formula for radian measure, you must first have the radius and the arc length in the same units, so you must convert one of them.

Now $\displaystyle{\left(\frac{\pi}{{{6}}}\text{ft}\right)}\cdot{\left(\frac{{{12}\ \text{ in}}}{{{1}\ \text{ ft}}}\right)}={2}\pi\ \text{ in}\usepackage{cmbright}\Huge\$ , so: $\displaystyle\theta=\frac{{s}}{{r}}=\frac{{{2}\pi\ \text{ in}}}{{{12}\ \text{ in}}}=\frac{\pi}{{6}}\usepackage{cmbright}\Huge\$