How much yarn do I need

Every yarn estimate you find online is an average of somebody else's knitting. Your tension, your needle and your stitch pattern each move the answer, and a cabled sweater can take half again as much yarn as a plain one at the same size. Weighing your own swatch skips all of that: it measures how much yarn one unit of your fabric actually costs, and the rest is proportion. You need a scale that reads to a gram — a kitchen scale is fine.

How it is worked out

yarn = (swatch weight ÷ swatch area) × project area

The swatch gives grams per unit of area for this yarn worked at this tension in this stitch pattern. Multiplying by the area of the project gives grams, and the ball band converts grams to metres. Because both areas are in the same units they cancel, so it does not matter whether you measured in centimetres or inches — only that you were consistent.

Source: weight per area, held constant across the same fabric

Questions people ask

A sweater is not a rectangle. What do I put in?

Add up the pieces as rectangles and be generous: for a drop-shoulder sweater, body width times length twice for front and back, plus each sleeve as a rectangle of its widest width. The overestimate from squaring off a curved armhole roughly pays for the yarn you lose to seams and to the tail you cannot knit with, so the two errors work against each other rather than adding up.

How much extra should I buy?

One ball more than this figure, and buy it in the same dye lot at the same time. Running out with two rows to go is the expensive mistake, because the shop will have moved on to another lot and the join will show. Yarn left over is not wasted — it is the swatch for the next thing.

Does the stitch pattern change the answer?

A great deal. Cables and ribbing pull the fabric in, so the same area of finished garment holds far more yarn than stocking stitch does; lace holds less. This is exactly why the swatch has to be worked in the pattern you will use. A stocking stitch swatch will underestimate a cabled sweater badly enough to leave you short.

Should I block the swatch before weighing it?

Block it, then measure it, then weigh it. Blocking changes the area — sometimes by ten per cent in lace and rarely by nothing at all — and the finished garment will be blocked too. Weighing before and measuring after would quietly mix two different fabrics into one number.

My yarn only gives yards, not metres.

Enter the yardage in the length field and read the answer as yards; the arithmetic is a proportion and carries whatever unit you put in. If you want metres from yards, one yard is exactly 0.9144 metres by the international agreement of 1959.

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