What measurement is used to describe the inside diameter of ferrous pipe?

The measurement used to describe the inside diameter of ferrous pipe is the inner diameter. This dimension is crucial in plumbing and piping applications because it directly affects the flow capacity of the pipe. Understanding the inner diameter allows contractors to determine how much fluid or gas can pass through the pipe and ensures that the system maintains proper pressure and flow rates.

In plumbing, ferrous pipes, which are commonly made from steel or iron, have specific inner diameters that are standardized for different applications. Knowing this measurement helps in choosing the appropriate fittings, valves, and other components that will connect to the pipe without causing restrictions or leaks.

The other options do not appropriately describe the inside diameter. Wall thickness refers to the material's thickness that makes up the pipe itself, which contributes to the overall strength and durability but does not specify the internal dimension. The pipe schedule, which is a system of assigning numbers based on the wall thickness and pressure rating, does not define the actual inside diameter directly. Outer diameter is the measurement across the pipe from one outer edge to the other, which is not the same as the inner diameter. Thus, the inner diameter is the correct and relevant measurement for determining the capacity and compatibility of ferrous piping in installations.