The phrase “yarn weight” can mean two different things in everyday conversation. A weight category describes a thickness grouping. A number such as 100 g describes the mass of the ball. They answer different questions and should occupy different lines in your project notes.
Read three numbers separately
Imagine a label that says “medium”, “100 g” and “200 m”. Medium is a thickness category, 100 g is how much the ball weighs, and 200 m is its stated length. None of those numbers can replace either of the others.
A second ball might also weigh 100 g but contain 300 m. Buying the same number of balls would then give you a different total length. Conversely, equal lengths of two yarns may have different masses. This is why an instruction to “buy five balls” is useful only when you know the exact yarn and ball size.
Use the category as a starting point
The Craft Yarn Council uses categories from 0 through 7, ranging from lace through jumbo. These categories are helpful when narrowing alternatives, but they are broad groupings. Sharing a category does not guarantee that two yarns make identical fabric with the same hook.
Look at the intended fabric, the pattern’s gauge and the particular yarn’s label. A category can help you find candidates; a sample determines whether a candidate behaves usefully for your project. Do not convert a thickness category directly into an exact stitch count.
Calculate the label ratio
For the 100 g / 200 m example, the ratio is 2 m per gram. A 35 g remainder would contain an estimated 70 m if its measured mass is all yarn from that ball. Remove the label, container and any non-yarn objects before weighing. Label tolerances and scale precision mean this remains an estimate.
For the 100 g / 300 m example, the ratio is 3 m per gram. The same 35 g remainder would be about 105 m. This is a useful illustration of why there is no universal grams-to-metres conversion.
Convert length without changing the yarn
If a pattern lists yards and the label lists metres, convert one into the other before comparing. The length converter does that calculation. Keep enough precision during the calculation, then round up when deciding how many complete balls are required.
Do not round every ball’s converted length to a convenient number and then multiply a large quantity if accuracy matters. A small per-ball difference can accumulate. Use the label number in its original unit where possible.
Keep the useful label information
Along with mass and length, record fibre composition, care instructions, shade and dye-lot identification if provided. Keep the original label for the project. Those details help when you need to match a replacement, care for the finished piece or understand why a substitute behaves differently.
For a personal quantity estimate, make a sample and enter the label values into the yarn calculator. For a published pattern using another yarn, compare total lengths and gauge first.
References: Craft Yarn Council weight categories; Lion Brand: yardage per skein.