Answer: Presumably you are asking for the measure of a right angle in radians. That's pi/2, about 1.57079632.
Half of a pie has pi radians. A whole pie has two!! radians.
That's also the value you see on the Android calculator for cos⁻¹(0) if you tap the "deg/rad" button to pick rad. The angle whose cosine is zero is a right angle. You enter cos⁻¹(0) by swiping left, tapping inv, and then cos⁻¹, then swipe right and enter zero. Tapping deg/rad switches between 90 and 1.57079632.
Explanation:
There are pi radians in 180°
(pi/180) radians per degree, 0.01745329
(180/pi) degrees per radian. 57.2957795
Would be 60 if pi were 3.