How to Use the Radius Calculator

Pick which measurement is known — Diameter, Circumference, or Area — enter it, and the radius, plus the diameter, comes out below.

from diameter:       radius = diameter ÷ 2
from circumference:  radius = circumference ÷ (2π)
from area:           radius = √(area ÷ π)

Area is the one that surprises people, because it involves squaring the radius — working backward means a square root, not a simple division the way the other two do. Doubling a circle's area, for instance, only multiplies its radius by √2, not by 2.