To do approximate calculations on planets, stars, and moons:
Diameter = 2 x the radius
Radius = half the diameter
Circumference = 3.14 x the diameter
If the diameter is 100,000 miles: the radius is 50,000 miles and the circumference is 314,000 miles.
If you only know the circumference, you divide it by 3.14 to get the diameter. So 314,000 miles divided by 3.14 is 100,000 miles.