This video explains how you can purchase items in a different unit of measure.
Each item (product and material) in Katana has a Unit of Measure (UoM) that you can define on the item card. This is the unit of measure that you use for tracking the stock for this item (i.e. stock unit of measure).
If you wish to purchase items in a different unit of measure, you can define a purchase unit of measure for the item, and enter the conversion rate from the purchase unit to the stock unit.