Roll length from diameter
The annular-area approximation compares the area of the wound roll with the area of the core, then divides by effective material thickness.
Length = π × (OD² − core OD²) ÷ (4 × thickness)
Why real rolls differ
Nominal caliper is not always the same as effective wound thickness. Coatings, adhesives, compression, air gaps, winding tension, and material stretch can materially change the result. When possible, calibrate against a known roll.