What is an Inside Diameter?

The inside diameter is the measurement across the inner space of a pipe, tube, or coil, from one internal wall to the opposite. It’s commonly specified in inches either as fractions or decimals and is also referred to as inner diameter. This measurement is essential in steel applications where flow capacity, fit, and wall thickness are critical.

How to Measure Inside Diameter

The standard formula to calculate inside diameter is:

Inside Diameter = Outside Diameter – (2 × Wall Thickness)

This method allows manufacturers and engineers to reverse-engineer internal space from outer measurements, ensuring parts are sized and fitted correctly.

Inside Diameter vs Outside Diameter (OD)

While the inside diameter measures the usable internal space, the outside diameter (OD) includes the full span from one outer wall to the other. Both values are important in steel product specifications especially when determining wall thickness, structural fit, or how multiple components interact.

ID OD