Use the pattern’s stated yarn quantity when your yarn and gauge match. When adapting a simple project, a weighed swatch can provide a provisional estimate.
Yarn length per gram = labelled ball length รท labelled ball weight. Multiply by the estimated grams required for the project. For a simple rectangle, scale swatch weight by the ratio of project area to swatch area.
Example: a 4-by-4-inch swatch weighing 5 g represents 16 square inches. A 20-by-20-inch panel has 25 times that area, giving approximately 125 g in the same stitch and gauge. A yarn labelled 200 m per 100 g gives about 250 m for that panel. Add separately for borders, seams, swatch yarn that cannot be reused and changes in texture.
This method is less reliable for shaped garments or mixed stitch patterns. Use a representative swatch for each texture and check the pattern designer’s estimate. Round purchases up to whole balls and consider dye-lot continuity. See how to choose knitting yarn .