Roll length calculator: diameter to meters (estimated)
Estimates the wound length of a fabric roll from its outer diameter, inner core diameter and fabric thickness.
Estimated — indicative value.
Frequently asked questions
How do you find roll length from diameter?
Without unwinding, measure the outer diameter, the inner core (tube) diameter and the single-layer fabric thickness in mm; plug them into L = π × (D_outer² − D_inner²) ÷ (4 × thickness) and divide by 1000 to convert to meters. The result is an estimate based on a uniform-spiral assumption.
Why is this result an estimate?
The formula assumes a perfect spiral wind and constant thickness. In practice winding tension, loft, moisture and fabric compression shift the figure, so expect about ±5-10% and unwind to measure when an exact length is required.
How do I measure thickness accurately?
Measure the thickness of a single fabric layer with a thickness gauge (micrometer), under a standard load if possible. Thickness is inversely proportional in the formula, so halving it doubles the estimated length — small errors here matter.
What if I ignore the inner core diameter?
The inner core (cardboard tube) holds no fabric; treating its diameter as zero overstates the length. In the example, ignoring the core gives ≈141.4 m versus the true estimate of 125.7 m — roughly 12% high.