To estimate yarn for a rectangular blanket, divide the blanket’s area by your swatch’s area, then multiply by the weight of yarn in that swatch. Use measurements from the same yarn and stitch construction you plan to use for the blanket.
Take four measurements before you begin
Measure the swatch width and length, weigh the swatch without tools or labels, and choose the blanket’s body dimensions. Use one unit for all dimensions. If you intend to wash or block the finished piece, measure your sample in a comparable state. A tiny or unrepresentative sample can make the calculation misleading.
A complete example
A 12 × 12 cm swatch weighing 8 g has an area of 144 cm². A 90 × 120 cm blanket has an area of 10,800 cm²: 75 swatch areas. The base estimate is therefore 75 × 8 = 600 g.
Adding 10% gives 660 g. If the label says 100 g and 200 m per skein, that is an estimated 1,320 m. Round 6.6 skeins up to 7. Rounding up is separate from the allowance: seven skeins contain 700 g, not 660 g.
What this estimate leaves out
A separately worked border, fringe, joins and different stitch sections need their own consideration. The calculator does not infer those from the rectangle. Its 10% starting allowance is editable, not a universal recommendation. A swatch also cannot predict every change in your tension over a large project.
If your yarn supply is fixed, use the stash blanket planner to solve for length instead of required yarn. For measurement uncertainty, read why gauge changes an estimate.
Use the calculator ↗Examples illustrate the arithmetic; they are not physical project tests. Send a correction.