Add or subtract products in inventory

Is there a special formula or steps to subtract products eg vinyl, material from inventory? I added the products but it does not become less, I only use pieces eg 20cm. So basically Can I input in cm2?

Inventory items are removed by selling them or writing them off. You sell by sales invoice or receipt. You write off with an inventory write-off. See the relevant Guides. For an overview of inventory management, read the three-part Guide beginning here: Manage inventory - Part 1 Introduction | Manager.