About this tool
The Roll Length Calculator is a free, in-browser tool that estimates how much material is wound on a roll — tape, film, paper, foil, fabric, or wire — from three easy measurements: the outer diameter of the full roll, the inner (core) diameter, and the thickness of the material. It also works in reverse: enter a target length and it solves for the outer diameter you would build up to. All math runs locally in your browser, so nothing you enter is uploaded.
The method uses the annulus-area formula. The cross-section of the wound material is a ring of area π/4 × (OD² − ID²); dividing that area by the material thickness t gives the length: L = π × (OD² − ID²) / (4 × t), with OD, ID, and t all in the same units. The inverse solves the same relation for diameter: OD = √(ID² + 4 × L × t / π). The tool keeps units consistent (millimeters or inches) and reports the length in meters and feet, along with the number of wraps, (OD − ID) / (2t). It assumes tight, even winding with no air gaps, so very loose or foam-cored rolls read slightly long.
As a worked example, a roll with a 100 mm outer diameter, a 50 mm core, and 0.1 mm material thickness holds L = π × (100² − 50²) / (4 × 0.1) ≈ 58,905 mm — about 58.9 meters, or roughly 193 feet, built up over about 250 wraps. Because thin materials produce very long lengths, small errors in the measured thickness have a large effect; measure t as accurately as you can, ideally by averaging several layers.