Use the label for this yarn
You need the full ball’s weight and length from its label or the manufacturer’s listing. Weigh the yarn you have left, then compare its weight with that reference. You can choose grams or ounces independently for the scale and label, and meters or yards for the label length. Unit changes convert entered values.
For the example shown, 45 g remaining from a label giving 200 m per 100 g is about 90 m, or 98.43 yd. The calculation keeps the full ratio and rounds only the displayed result. It also works for several partial balls of the same yarn weighed together.
Subtract what is not yarn
Weigh bare yarn if possible. If a cardboard core or wrapper is on the scale, enter its separately measured weight to subtract it. Use zero when it is already removed, or when you zeroed the scale with the empty container first. Do not guess a core’s weight from its size.
You can use a known length and weight of the same yarn as the reference instead of a full ball. A very light sample can be difficult for a kitchen scale to measure accurately. Use enough yarn for a stable reading, keep it dry and avoid stretching it while measuring.
An estimate, not a yarn-category conversion
This assumes the same length per unit of mass throughout the yarn. Label rounding, moisture, uneven thickness and scale precision affect the estimate. Two yarns described as DK or worsted can still have different lengths per gram; use this yarn’s own reference rather than substituting a weight-category average.
The result is an estimate of yarn length, not a prediction of how much a garment or blanket will use. Check your pattern’s length requirement and make a gauge swatch before planning a substitution. The tool does not add a safety margin to the estimated amount left.