How roll length is estimated
The calculation compares the cross-sectional area of the finished roll with the core, then divides by the material thickness. All inputs are converted to millimetres internally before the formula is applied.
Web length = π × (OD² − Core OD²) ÷ (4 × material thickness)
Why thickness matters so much
The formula assumes each wound layer has a consistent effective thickness. A stock that is 10% thicker produces a meaningfully shorter roll at the same OD, so use measured or manufacturer-specified total caliper when possible.