Spool math and demo

Good calculations, but perhaps a bit too complex.

Here is a simpler one, which provides almost exact results (based on the example at the bottom).

If the inner and outer diameters are D0 and D1, then calculate the cross-sectional area of the roll as follows (where r1=D1/2 and r0=D0/2):
Pi * (r1^2 - r0^2)

Now simply divide this by the thickness h.

Using the numerical example on this page,
approximate formula roll length: 235.4623694 m
exact formula roll length: 235.6194545 m
this formula: 235.619449 m (which matches the exact formula to 7 digits).

Cheers,

Pranab

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