Measure outside diameter (OD)
Measure the full wound roll across its widest outside points. If the roll is oval or uneven, take several measurements and consider using an average plus a tolerance range.
Use core outside diameter, not the hole
The roll-length geometry needs the outside diameter of the core because that is the radius at which material begins winding.
Thickness is the biggest sensitivity
Small thickness errors can produce large length errors. Nominal caliper can also differ from effective wound thickness because of compression, coatings, adhesives, and air gaps.
Calibrate when possible
If you have a roll with known length and measured diameters, use it to estimate an effective thickness for future rolls made from the same construction and winding process.
Use the calculator
Move from the explanation into a live calculation using the related PackFlow tool.