If you know the diameter: radius = diameter ÷ 2
If you know the circumference: radius = circumference ÷ (2π)
If you know the area: radius = √(area ÷ π)
If you know the arc length and central angle: radius = arc length ÷ angle in radians
If you know the chord length and sagitta: radius = (chord² ÷ (8 × sagitta)) + (sagitta ÷ 2)