How much yarn a blanket really needs
Turn width, length and gauge into a trustworthy yarn total, learn the yards-per-100 figure by weight, and avoid a mid-project shortage.
Area is the honest starting point
A blanket's yarn appetite tracks its surface area far more closely than its width or length alone. A 50 by 60 inch throw is 3000 square inches, and doubling either dimension roughly doubles the yarn. That is why this calculator converts your finished size into square inches first, then applies a per-area yarn rate. Thinking in area also makes it easy to compare projects: a wide, short lapghan and a narrow, long runner with the same square inches will use similar amounts of the same yarn.
The yards-per-100 figure does the heavy lifting
The single number that most affects your estimate is yards per 100 square inches. It bundles yarn weight, stitch pattern and gauge into one measured rate. A plain worsted garter fabric sits near the default 110, but bulky yarn worked loosely might be 70 or 80, while a dense textured stitch in the same yarn can climb past 130. The only reliable way to pin it down is to make a swatch, record the yarn it used, and scale that consumption up to 100 square inches before you enter it.
From yards to skeins, always rounding up
Once you have total yards, the skein count is that total divided by the yardage printed on your ball band, rounded up to the next whole skein. Rounding up matters because yarn is sold in whole units and dye lots vary. A 3300 yard blanket in 220 yard skeins needs 15 skeins exactly, but a 3350 yard version would need 16, since 15 skeins only supply 3300 yards. When the division lands close to a whole number, buy the extra skein anyway.
Why a spare skein is cheap insurance
Every estimate omits small realities: extra yarn pulled into selvedges, the tail you weave in, a border you decide to add, or a gauge that loosens as your hands relax. Any of these can push you a few yards past the calculator's figure. Buying one spare skein from the same dye lot at purchase time costs little and protects the whole project, because matching a discontinued color or a different lot later is often impossible.