Physics (10th Edition)

There are $63.7$ grads in one radian.
We know there are $2\pi$ radians in a circle, so one-quarter of a circle has $\frac{\pi}{2}$ radians. Therefore, $100$ grads $=$ $\pi/2$ radians. That means $1$ radian $=$ $100$ grads $\times\frac{1rad}{\pi/2rad}=63.7$ grads