Swatch yarn calculator

How to use this calculator

1. Measure and weigh the same dry swatch area; avoid mixing whole-swatch weight with centre-only measurements.
2. Enter project dimensions and the label length and weight for the swatched yarn.
3. Choose reserve and uncertainty separately. Use the upper estimate for shopping if that matches your risk preference.

How much yarn will my rectangle use?

Scale the weight of the swatch by the ratio of project area to swatch area, then use the same yarn’s label length per weight. Use the whole measured and weighed area consistently. This is an estimate for that fabric, not a universal yarn-weight chart.

Turn a selected yarn-length estimate into whole skeins after subtracting usable stash by colour. Per-colour skein shopping →

The calculation

Base grams = project area ÷ swatch area × swatch grams. Range = base × (1 + reserve/100) × (1 ± uncertainty/100). Yarn length = grams × label length per gram.

A worked example

A 10 × 10 cm swatch weighing 5 g scales to 175 g for 50 × 70 cm. With 10% reserve and ±10% uncertainty: 173.25–211.75 g. At 200 m / 100 g: 346.5–423.5 m.

Before you cast on

The range is a user-chosen sensitivity scenario, not a statistical confidence interval or guarantee. Use the same yarn, stitch pattern and finishing treatment. A scale’s resolution, edges and tails matter; reserve is extra yarn beyond the project and does not automatically account for a swatch you keep.

Measure a representative, washed and dried swatch without stretching it. See method sources and measurement guidance.

Display values round to 2 decimals; calculations keep full precision. Counts are whole numbers. No calculator can promise fabric behaviour.