A yarn-weight label tells you the yarn category, but it cannot tell you how much your particular stitch pattern and tension will consume. For a blanket, make a representative swatch, measure the yarn actually used, and scale that measurement to the planned blanket area. This is a planning estimate: borders, changes of stitch, and different color sections need separate allowances.
Make a useful swatch
Use the same yarn, hook and stitch you intend to use for the blanket. Work enough rows that the sample includes the normal stitch pattern, then measure the usable width and height without stretching it. If you plan to block the finished blanket, measure the swatch after applying the same treatment. A tiny, unusually tight first row is a weak basis for a large purchase.
Scale consumption by area
Suppose a 5 × 5 inch swatch uses 20 yards. Its area is 25 square inches and its consumption rate is 20 ÷ 25 = 0.8 yards per square inch. A 50 × 60 inch blanket covers 3,000 square inches. At the same stitch density, the base estimate is 3,000 × 0.8 = 2,400 yards.
Add a 10% planning buffer: 2,400 × 1.10 = 2,640 yards. If a skein contains 300 yards, 2,640 ÷ 300 = 8.8, so you need nine whole skeins before subtracting identical full skeins you already own. The Crochet Blanket Yarn Calculator runs this method and also accepts a known finished blanket as a reference.
Check the assumptions before shopping
A border does not necessarily scale in the same way as the center: its yarn use depends on perimeter, width and stitch. Multi-color designs can run short in one shade even if total yarn is sufficient. Swapping yarn or hook after measuring the swatch also changes the consumption rate. If you have a partially used skein, weigh or measure its usable yarn instead of treating it as a full new skein.
Keep the base estimate, buffer and rounding separate so a later change in blanket size is easy to recalculate. If this project is cross stitch rather than crochet, use the Cross Stitch Fabric Calculator; fabric count and stitch dimensions answer a different question.
Two scenarios worth comparing
If you change the target blanket to 40 × 50 inches without changing the swatch, its area becomes 2,000 square inches. The same 0.8 yards-per-square-inch rate gives 1,600 base yards; with 10% buffer, 1,760 yards or six 300-yard skeins. If instead you keep the 50 × 60-inch target but use a stitch that consumes more yarn per square inch, the old 2,640-yard plan is no longer reliable. Swatch the new stitch rather than adjusting only the blanket dimensions.
For a blanket made in separate panels, measure seams and joins independently when they consume a meaningful amount. Record the exact yarn line and dye lot before buying, because a mathematically sufficient yardage total does not solve an unavailable matching-color problem.