Start with the exact yarn
Retain the yarn name, product line, color, dye lot when supplied, fiber content, care instructions, and label weight and length. A photograph of the original label is more useful than a guessed category-to-yardage conversion.
Weigh the remaining yarn
Use a scale with suitable resolution and weigh the yarn alone. Exclude cones, labels, needles, buttons, and packaging. Measure in a comparable dry condition and note that scale and label tolerances affect the result.
Calculate a proportional estimate
Remaining length = remaining weight / full label weight × full label length. Use the same mass unit for both weights. The method assumes approximately consistent length per gram within this yarn; uneven construction or a mix of yarns can break that assumption.
Worked example
A label states 220 yards per 100 grams. With 42 grams remaining, the estimate is 42 / 100 × 220 = 92.4 yards, or about 84.5 meters. This is an arithmetic example, not a claim that 92.4 yards is enough for a hat, cowl, or socks.
When the label is missing
Look up the exact product or establish a ratio by measuring a known length and weight. Yarn weight categories and WPI do not establish universal yards per gram. Do not turn a thickness estimate into a precise remaining-yardage claim.
Plan the next project
Compare the estimate with a representative swatch or the actual pattern requirement. Account separately for joins, tails, borders, finishing, and waste. Record which yarn and measurements produced the estimate so it can be checked later.