How to use
- Measure the full roll outside diameter and the core outside diameter with the same unit. Use the core surface where material begins, not the spindle hole.
- Find the material caliper or thickness. Select µm, mm or mil separately so a thin film does not require a long decimal.
- Calculate, compare metres and feet, and copy the current estimate. Editing any input clears the old result.
- Treat the answer as a planning estimate and check a known roll or supplier tolerance before ordering or scheduling production.
Formula and interpretation
The material cross-section is the annular area: π × (outer diameter² − core diameter²) ÷ 4. Dividing that area by material thickness gives length in the same base unit.
Diameter is squared, so a small measuring error near the roll edge can noticeably change the answer. Use an outside diameter of the core, because the hollow bore is not part of the wound-material boundary.
Worked examples
Paper roll
A 300 mm roll on a 76 mm core with 100 µm paper gives about 661.49 m (2,170.26 ft).
Empty core boundary
When outer and core diameters are equal, the geometric material area and estimated length are zero.
Limits and practical checks
- Soft, compressible or stretched material can differ from the ideal geometry.
- Thickness variation, loose winding, trapped air and telescoping edges are outside the model.
- The tool estimates length only; it does not calculate weight, tension, usable labels or production waste.
Frequently asked questions
Which core diameter should I enter?
Enter the core outside diameter where the material starts, not the inner bore used by the shaft.
Why is thickness so influential?
The same annular area contains more length when each layer is thinner, so length is inversely proportional to thickness.
Can I use mixed units?
Yes. Diameter and thickness have separate unit selectors; results are shown in metres and feet.
Is this an exact inventory count?
No. Verify critical quantities by weight, unwind count or supplier data when winding density is uncertain.