Coil / Roll Stock Length
How much strip is left on the coil without unrolling it: the wound material is an annulus, so L = pi (OD^2 - ID^2) / (4 t) for coil outside diameter OD, core diameter ID, and thickness t. A 48 in coil on a 16 in core at 0.024 in (24 ga) holds 5,585 ft; halve the thickness and the same coil OD holds twice the length. Exact for a tight coil with no telescoping; the last wrap and core stub trim it slightly. A layout aid; weigh or measure-off before a critical cut.
Formula and source
L = pi (OD^2 - ID^2) / (4 t); L_ft = L / 12.
The coil / roll stock length annulus identity, first-principles; the coil OD, core ID, and thickness come from the material.
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