The formula
Estimated metres = remaining grams ÷ label grams × label metres. For 30g remaining from a 100g / 200m ball, the estimate is 30 ÷ 100 × 200 = 60m.
How precise is this?
This assumes the remaining yarn has the same length-to-weight ratio as the label. Scale resolution, moisture and differences within a ball can affect the estimate. It cannot identify an unlabelled yarn. Check that the stash matches the exact yarn and shade you intend to use.
Use your result
The planner subtracts your stash before rounding the remaining requirement up to whole balls or packs. The displayed estimate is rounded; the transferred value is rounded down to the nearest 0.001m to avoid increasing the estimate through display rounding.
How much yarn is left on a part-used ball?
Weigh what’s left, divide by the ball weight on the label and multiply by the label’s length. That gives the metres remaining. Take the band off first, and weigh only the yarn.
It works because a ball’s length is spread evenly through its weight. It won’t work for an unlabelled ball unless you know which yarn it is.
50g and 100g balls
A 50g ball of Paintbox Cotton DK has 125m. If 18g is left, that’s 18 ÷ 50 × 125 = 45m.
A 100g ball of Stylecraft Special DK holds 295m. If 42g is left, that’s 42 ÷ 100 × 295 = 124m, a little over two-fifths of a ball. Enter that in the planner as yarn you already own, and it comes off the pattern’s total before the new balls are rounded.
If the label gives yards
Use the yards in place of metres and choose yards in the converter. Stylecraft lists Special DK as 322 yards, so 42g left is 42 ÷ 100 × 322 = 135.2 yards. That’s the same length as above: 135.2 × 0.9144 = 124m.