



 
Hi Ken, The formula you want is in Penny's response to an earlier question. Look at it to see if this what you want. If I am correct then using the notation in her response you have $R = 13.5$ inches, $r = 3$ inches and $t = 3.5$ inches. Thus if $T$ is the length of the material on the roll then \[L = \frac{\pi \; \left(R^2  r^2\right)}{t} \] Harley  


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