A yardage estimate starts from a simple idea: every square inch of finished fabric takes a roughly consistent length of yarn to fill, once you know the yarn's weight and the stitch pattern's density. That consistent length is the coverage factor, expressed as square inches of fabric per yard of yarn.
Coverage factor by yarn weight
Thicker yarn covers more area per yard because each stitch pulls through more fiber, so a bulky-weight coverage factor is a much bigger number than a lace-weight one. These factors come from averaging typical gauge across many patterns at each weight category, which is why they're a solid planning estimate rather than a promise, the same way the yardage-per-skein reference gives ranges instead of one fixed number.
Why stitch pattern matters as much as weight
Stockinette is the baseline most coverage factors assume. Stitch patterns that add texture, like cables, ribbing, or garter, pull in extra yarn for the same visible area, because the fabric compresses front-to-back or the yarn takes a longer path through each stitch. A cabled panel can use 20-40% more yarn than stockinette at the identical gauge and area, and colorwork with stranded floats on the back adds even more, since two strands are moving across every row instead of one. That's why a stitch multiplier sits alongside the weight-based coverage factor rather than replacing it: the two adjustments are independent and stack together.
Worked example
A 40in by 50in blanket panel in worsted weight, plain stockinette: area is 2,000 square inches. At a typical worsted coverage factor, that panel lands in the range of 800-950 yards before any buffer. Switch the same panel to a cabled stitch pattern and the yardage climbs toward 1,000-1,150 yards for the identical finished size, purely from the stitch multiplier.
Why estimates include a buffer
A 10% buffer on top of the base calculation covers the normal sources of variance: swatch tension that doesn't perfectly match your finished piece, edge stitches and seams, and the reality that most knitters and crocheters tension slightly differently under a full project than on a small swatch. Running short by a few yards on a large project is far more disruptive than buying one extra skein up front, so the buffer defaults toward caution rather than a razor-thin estimate.
None of this replaces a swatch. Coverage factors are averages across many projects and yarns, and a specific fiber, especially anything with unusual loft or drape, can sit outside the typical range. Use the estimate to shop with confidence, then treat your actual swatch gauge as the final check before you commit to a skein count.