Start with area
For scarves, wraps, and blankets, multiply finished width by finished length. A larger area needs more yarn, but stitch pattern and yarn weight change the final number.
Use a stitch factor
Plain stockinette, garter, and simple crochet stitches can start near 0.7 yards per square inch for a medium project estimate. Dense cables, bobbles, and textured crochet need a higher factor. Loose lace needs less.
Add a buffer
Add 10 to 15 percent for tails, gauge differences, mistakes, and matching. Add more if the yarn is discontinued, hand dyed, or sold in small skeins.
When accuracy matters
Make a swatch, measure its area, weigh it, then compare the weight used against the skein label. This is the most reliable method when the finished size or fit matters.