Make the sample representative
Yarn use changes with stitch structure, hook, tension, fiber, and the hands making it. A useful swatch repeats the final fabric closely enough that its weight per square inch can stand in for the whole project.
Work a generously sized swatch so edge stitches are a small share of the sample. After washing and drying, enter the full dimensions of the same complete swatch you place on the scale; mixing a center-only area with the weight of the whole swatch overstates yarn use.
- Use the final yarn and hook.
- Work the actual stitch pattern, including color changes if practical.
- Wash and dry it as the finished item will be treated.
- Measure the complete dry swatch without stretching, then weigh that same piece on a gram scale.
Scale area, then show the buffer
If a 4 × 4 inch swatch weighs 18 grams, it uses 1.125 grams per square inch. A 40 × 50 inch blanket has 2,000 square inches, so the unbuffered estimate is 2,250 grams. A visible ten-percent buffer brings the purchase target to 2,475 grams.
Keeping the buffer separate matters. It lets you change the risk allowance without pretending the underlying estimate is more precise than it is.
Convert grams to skeins last
Divide the buffered grams by the net weight on one skein, then round up to a whole skein. When substituting yarn, compare length as well as weight: fibers with different densities can put very different yardage into the same 100 grams.
Buy the needed quantity from one dye lot when consistency matters. The spare skein is not waste if it protects a multi-week project from an impossible color match.
Sources & further reading