Method / assumptions / examples
How this calculation works
The result is deterministic: the same measurements always return the same estimate. Here is the relationship and where real-world results can differ.
Formula
OD = √(core² + (4 × length × caliper) ÷ π)
The formula compares the cross-sectional area of the wound material with the annular area between the core and finished roll. Length and caliper must be expressed in the same unit before calculating.
Worked example
Examples
On a 3 in core, the estimated finished outside diameter is about 14.2 in.
Increasing effective caliper by only 0.5 mil adds roughly 1.2 in to the diameter on the same 5,000 ft roll.
Common mistakes
What to check before using the result
- Use the material’s effective caliper under normal winding tension, not only the nominal spec-sheet value.
- Adhesive, liner, coatings, air entrapment, and compression can all change the real finished diameter.
- Treat the result as an estimating value and validate critical rewind jobs against a known roll.
FAQ
Frequently asked questions
Does winding tension affect roll diameter?
Yes. Tension changes how tightly layers pack and can compress some materials. Use an effective caliper measured from a comparable finished roll when fit is critical.
Can I use microns and metres?
The underlying formula works with any consistent units. This version accepts shop-floor units and also reports the result in millimetres.
Does this include the core?
Yes. Enter the core’s outside diameter. The result is the complete finished outside diameter, including the core.