About Centiradians
The centiradian, or centrad (crad), is one hundredth of a radian – the radian with the prefix centi- applied.
A full circle is 2π radians, which is about 628.3 centiradians.
In practice small angles are given in milliradians or arcminutes rather than centiradians. 1 centiradian is 10 milliradians, or about 34.4 arcminutes.
About NATO milliradians
One NATO mil is 1/6,400th of a circle, or 0.05625°, so a degree is about 17.78 NATO mils.
A NATO mil is close enough to a true milliradian (6,283 of those make a circle) that gunners treat it as 1 meter of width at 1,000 meters of range. A circle of 6,400 also divides neatly into quarters, eighths and sixteenths.