This paper summarises the results of a more than 5-year practice-led study on the use of upcycling design and production methods in garment mass production. The efficiency of upcycling design approach is described by analysing the generation and potential use of various types of fabric leftovers from garment manufacturing. The results of this research show that depending on the size of the factory the fabric leftovers and textile waste generated in garment production ranges from 25–40% of the total fabric used. Experiments show that 50% of that material can be upcycled into new garments and for some types of leftover—mainly spreading loss and excess fabric—it can even be up to 80%. Implementing upcycling on the industrial level requires transparency to understand the waste created in garment production and create designs that suite the production system. It is important to consider that the upcycling design process differs from regular design—a garment is designed based on the parameters of the waste materials.