This KS2 maths article explains how to measure the radius, circumference and diameter of a circle.

How to Solve for Radius in Excel. The radius of a circle or a sphere is the distance between the center and the edge. The radius can be used to calculate a circle's diameter, area and circumference.

The results confirmed that -- more than any other screening technique -- neck circumference was the most reliable alternative to BMI, when adjusted to a child’s age and gender.