You can estimate a blanket’s length from the yarn you already have by choosing a width first. The same amount of yarn can make a narrower, longer rectangle or a wider, shorter one; it does not determine one unique blanket size.
Set aside yarn before sizing the body
Weigh the yarn available for this project. Enter a separate reserve for finishing or another planned section. If you have 700 g and set aside 40 g, 660 g remains. With a 10% allowance above the base body estimate, the base body budget is 660 ÷ 1.10 = 600 g.
This division matters. Taking 10% off 660 g instead produces 594 g, which describes a different way of budgeting. Our planner explicitly uses the same allowance definition as the forward yarn calculator.
Turn that budget into an area
Suppose your 10 × 10 cm swatch weighs 5 g. Each gram represents 20 cm² of that fabric. A 600 g body budget gives 12,000 cm²: at 100 cm wide, approximately 120 cm long; at 80 cm wide, approximately 150 cm long. These are mathematical planning sizes, not tested pattern dimensions.
Check the pattern before you commit
The suggested dimensions may not match whole stitch or row repeats. Use the stitch multiple calculator to select a workable body width, then put that width back into the stash planner. Round the length down to a complete row repeat if your pattern requires it.
Do not combine unlike yarns into one measured density unless the swatch represents that combination. For stripes, a total weight can conceal a shortage of one color. Measure and budget each color separately. Check partial skeins before adding them to your supply.
Use the calculator ↗Examples illustrate the arithmetic; they are not physical project tests. Send a correction.