The diameter of the inside of a tube, pipe or other object. Often abbreviated "ID".

How do you find the inside diameter?

Divide the circumference (perimeter) by pi. That gives you the diameter. Half of that is the radius. To get the area, square the radius, and then multiply by pi.

What is inner diameter of a coil?

Inner diameter is defined as the width of the inside of a coil spring's diameter measured from the center of the helix. Sometimes, inner diameter can be difficult to measure accurately. The formula to calculate inner diameter is I.D. = O.D. – 2d where 2d is two wire diameters.