How to use your result
A swatch gives a personal material-use estimate. This calculator scales the weight of a rectangular sample to a larger rectangle, then adds your chosen allowance and rounds up to whole balls. The method assumes similar stitch density, yarn and finishing treatment throughout.
Total grams = base grams × (1 + allowance ÷ 100)
Estimated metres are total grams multiplied by the label’s metres per gram. The metre estimate inherits both swatch error and label tolerances. Whole balls are rounded up only after the allowance is added. The purchase length tells you the total label length in those whole balls; it will usually exceed the estimated amount used.
Follow the example
A complete 10 × 10 cm sample weighs 6 g. A 100 × 140 cm rectangle has 140 times its area, giving a base estimate of 840 g. A chosen 15% allowance increases that to 966 g. With 100 g balls containing 200 m, the estimate is 1,932 m used and a purchase quantity of 10 balls, containing 2,000 m in total.
Where estimates go wrong
Do not weigh an entire 15 × 15 cm swatch and enter only a 10 × 10 cm central measuring window. That pairs the weight of a larger piece with the area of a smaller one and overestimates yarn use. Central-window measurements are useful for gauge; area-based yarn estimates need matched weight and dimensions.
A scale with whole-gram steps can be imprecise for a very light sample. A 1 g difference on a 5 g sample is 20%, before any other uncertainty. Make a larger sample, or weigh several identical samples together and use their combined area. Remove the scale container weight first.
What this tool does not estimate
Shaped garments, three-dimensional objects, highly irregular lace, changing stitch patterns and separate colour allocations need a different method. A border or joining seam with different density should be estimated separately. Your allowance should cover your actual uncertainties; increasing it cannot make an unsuitable sample representative.
Work through the yarn-estimation guide and check what changes when substituting yarn.