If you know the diameter, divide it by 2
If you know the circumference, divide it by 2π
If you know the area, divide it by π and take the square root
If you know the chord and sagitta, use the circle geometry formula
If you know the arc length and central angle, use radius = arc length ÷ angle in radians
If you know the coordinates of the center and a point on the circle, use the distance between them
If you know the equation of the circle, take the square root of the radius-squared term
If you know the sector area and central angle, use radius from the sector area formula