Hi! Thanks for the reply!!

Could you please explain if the fractional amount formula - in particular - 300*pi is my tube diameter or just "300".. I suspect its obviously the tube but I'd like to confirm.

Also, which of the wraps formula is for each type of extension / compression? are the 1 / 2 / 3 from left to right?

I've got this all down on a spreadsheet and its thrown one of my previous workings into doubt now (assuming this set of formula are correct for calculating number of wraps). I use this to work out my final roll diameter -

SQRT(((materiallengthLM*(4000*materialthickness))/PI)+tubediameter˛)

Using your formulas I am taking the number of wraps, doubling them and multiplying by the material thickness and adding the tube diameter to coarsely work out the finished roll diameter when wound up and for a 300mm tub with 0.5mm material thickness and 100LM roll length its coming out 17mm smaller with my diameter formula.

Can I assume 100% that your formula is correct and I might not be working out final roll diameter correctly? Obviously when I started calculating roll diameters I wasn't thinking about compression or extension of material.

I'm questioning both because I'm getting differing answers now to roll diameter and also because I had expected there to be a larger difference in the lengths on the insides and outsides than the formula is giving me.

But then obviously, like I originally said.. Its been a LONG time since I was being taught math!

Thanks again for your help with this!