   SEARCH HOME Math Central Quandaries & Queries  Hi Can you help with the equations to calculate the length of paper required to achieve a target outside diameter when wrapped around a core please? The inside diameter of the cardboard core is 76mm The thickness of the wall of the cardboard core is 5mm The thickness of the paper is 138microns The desired outside diameter of the finished roll is 320mm I hope you can help me with a solution, Thank you Hi Stephen,

The inside diameter of the cardboard core is 76 mm so the inside radius of the cardboard core is $\large \frac{76}{2} \normalsize = 38$ mm. If I understand what you mean by "the thickness of the wall of the cardboard core" then the outside radius of the cardboard core is 38 + 5 = 43 mm.

Now look at our response to a previous question from Tuomas. Using the notation in that response you have $r = 43 \mbox{ mm }, R = \large \frac{320}{2} \normalsize = 160 \mbox{ mm },$ and since there are 1000 microns in a millimeter, $t = 0.138$ mm. Now you can use the expression

$t \times L = \pi \; \left( R^2 - r^2\right)$

to find $L.$

Write back if you need more assistance.

Harley     Math Central is supported by the University of Regina and The Pacific Institute for the Mathematical Sciences.