That depends on its color, opacity, condition, and many other variables.
You cannot find the diameter of something from its circumference unless it is a circle and, in that case, diameter = circumference/pi
It could mean a circle with a diameter of 12.5 mm
You need to qualify the 30 inches. Is it the diameter or circumference of a circle?
to cheak the diameter of something
my dick
There is no such unit of measurement as a diameter. A diameter is the distance across something - which can be as big or as small as desired.
the diameter times pi (3.14)
There is no such unit of measurement as a diameter, a diameter is the length across something. Perhaps you are thinking of a decimetre.
the diameter of a circle with a circumference of 19 (assuming the value of π is 3.14159) is approximately 6.046.
The diameter of something is the distance across it - perhaps you are thinking of the unit of length the decimetre, which is equal to 10 centimetres.
If you know the radius, double that to get diameter. If you have neither radius or diameter, then measure around the outside.
pi x diameter = circ., therefore circ. / pi = dia... It comes out to 7.something