The sum
You need three numbers: how much the knitting so far weighs, how far along it is, and how much yarn is left.
- Weigh the work on the needles, in grams, minus the needles (weigh them once and subtract).
- Estimate how much of the project is done — by rows, by inches, or by the pattern's sections. Be honest about what is left: a sweater's sleeves are about a third of the whole, a shawl's last ten rows are a quarter of it, a sock's foot is more than its leg.
- Yarn needed to finish = grams knit so far ÷ fraction done × fraction left. Compare with the grams left in the ball.
A hat that weighs 60 g with the crown decreases still to go (about 15% of a hat) needs 60 ÷ 0.85 × 0.15 ≈ 11 g more. If the ball has 30 g left, knit on. If it has 8 g, this is the moment to make the hat shorter, not three rows from the top.
The halfway trick for socks and pairs
For socks, mittens, sleeves — anything made twice — weigh the ball before you start and after the first one. If the first sock used less than half, the second is safe. If it used more, you know now, and the second sock gets a shorter leg while the two can still be made to match.
From grams to yards
Patterns count yards; scales count grams. Each yarn's yards per gram is on its page (yardage ÷ skein weight), and for a rough answer the weight class is enough:
| Weight | Yards per gram | Yards in 100 g |
|---|---|---|
| Lace | 7–15 | 700–1,500 |
| Fingering | 4–4.5 | 400–450 |
| Sport | 3–3.5 | 300–350 |
| DK | 2.5–2.9 | 250–290 |
| Worsted | 2–2.2 | 200–220 |
| Aran | 1.6–2 | 160–200 |
| Chunky | 1–1.4 | 100–140 |
| Bulky and super bulky | 0.5–1 | 50–100 |
So 100 g of worsted is about 210 yards, and 100 yards of worsted weighs about 48 g. Cotton and silk are denser than wool, and alpaca a little denser, so a 100 g cotton skein sits at the low end of its class.
Before you start: how much extra
The pattern's yardage is for its gauge and its size exactly. Buy the extra skein if any of these is true: your gauge is a little tighter than the pattern's; you plan to lengthen anything; you want a swatch you can keep; the yarn is hand-dyed and you will alternate skeins; or the yardage lands within 10% of a whole number of skeins. An unused skein comes back within 30 days; the sweater, blanket and hat and scarf guides have the totals, and the calculator on every yarn page rounds up for you.
From a swatch, before you knit
Weigh your washed gauge swatch and measure it; grams per square inch × the project's area is the yarn it will take, and the gauge swatch guide and the blanket guide walk through it. It is the most accurate estimate there is, because it uses your gauge, your stitch pattern and this yarn.
When the answer is no
Shorten what has not been knit yet: the body before the ribbing, the sleeve before the cuff, the shawl before the border. Switch to a contrasting color for the last section and call it a design — cuffs, a border, a toe. Or buy one more skein and alternate it in from the point it starts; a different dye lot blended over a few inches is invisible, and the dye lot guide says how. If the colorway is gone, write to us with the dye lot number; we can sometimes find one.