do you mean you by a roll of 100 mt then sell rolls of 200 mt?

No, I mean that I can either buy 1 roll = 100m or 2 rolls = 200m, or 3 rolls = 300 m,... I admit that my wording was a bit confusing. I re-typed the question to make it clearer.

In this case i suppose you buy "units", the you can buy 1...2...3...N rolls. so the lenght (in this phase) is not that important because you're about to place 3 rolls in your warehouse.

Thank you for the quick answer. Unfortunately this approach does not work for me as I need the textile as raw material for production and keep track of how many meters are left. In this example I need 1.2 m of textile to manufacture one bag. So if I manufacture 10 bags I consume 12 m and have left 88 m on the roll.

sorry...i did hit the return button too early

I know what you are trying to do, and i have not found a good solution in openERP yet. You would have to define a unique identity for every roll, and track every roll individually. In openERP right now that is not possible without custom programming. Products get lumped together, unless you create a new product for every item of inventory.

what's wrong with tracking each roll individually? IMO, assigning each roll to a lot then specify that lot when using the textile can work fine

Your suggestion is fine if the parents and children are finite, but if you want to store more information than "child consumed X feet of parent" it gets complicated very quickly. Like if you are buying 30,40,50 mm wide rolls and producing 30,40,50 mm wide children, simple 6 products, but if you want to produce children and parents that vary 1 mm you have to create and store a product for every width variation - 40 products.