How to use the long-tail cast-on calculator
- Knit a swatch in the stitch you will start with, block it and count the stitches over 4 inches (or 10 cm).
- Type the width you want and that gauge. Switch to centimeters if you measure in metric.
- Pick a stitch pattern preset or type its multiple, for example 4 plus 2 for 2x2 rib worked flat.
- Choose how to round. Nearest gives the closest width; round up when the piece must not come out narrow.
- Add selvage stitches if your pattern asks for them.
- For a long-tail cast-on, do the wrap test and enter the length of 10 wraps. No yarn handy? The rough estimate uses your needle size instead.
How the stitch count works
Gauge tells you how many stitches fill one inch. Multiply that by the width and you have the exact count, which is rarely a whole number and rarely fits a stitch pattern. A pattern that repeats over 4 stitches with 2 extra only works at 6, 10, 14 and so on, so the calculator finds the nearest of those counts.
Rounding changes the width a little. The results show the width you will actually get so you can decide if the difference matters. On a scarf it rarely does. On a sweater body it adds up, because the front and back both change.
Formulas
- Stitches per inch = stitches per 4 inches ÷ 4
- Exact count = width × stitches per inch
- Repeats = (exact count − extra) ÷ multiple, rounded to nearest, down or up
- Cast on = repeats × multiple + extra + 2 × selvage stitches
- Resulting width = fitted stitches ÷ stitches per inch
- Tail (wrap test) = (length of 10 wraps ÷ 10) × cast-on stitches + 6 inches
- Tail (rough estimate) = π × needle diameter × 1.5 × cast-on stitches + 6 inches
Worked example: a ribbed scarf
Your swatch has 22 stitches over 4 inches, which is 5.5 per inch. You want an 18 inch wide piece, so the exact count is 18 × 5.5 = 99 stitches. For 2x2 rib, (99 − 2) ÷ 4 = 24.25 repeats. The nearest whole number is 24, so cast on 24 × 4 + 2 = 98 stitches. That is 98 ÷ 5.5 = 17.8 inches, a fraction under what you asked for. Rounding up to 25 repeats gives 102 stitches and 18.5 inches.
For the tail, 10 wraps around the needle measured 8 inches, so each stitch uses 0.8 inches. Then 0.8 × 98 = 78.4 inches, plus 6 for weaving in, comes to about 84 inches, a little over 2 yards. The rough estimate for a 4.5 mm needle gives 88 inches, close to the wrap test in this case, though it will drift for very thick or very thin yarn.
Stitches for a 20 inch width
Counts for common gauges, plain and fitted to 2x2 rib worked flat (multiple of 4 plus 2).
| Stitches per 4 in | Plain | 2x2 rib |
|---|---|---|
| 16 | 80 | 82 |
| 18 | 90 | 90 |
| 20 | 100 | 102 |
| 22 | 110 | 110 |
| 24 | 120 | 122 |
| 28 | 140 | 142 |
| 32 | 160 | 162 |
Long-tail rules of thumb compared
Tutorials give several quick rules for the tail. Here they are applied to the worked example above, a 98 stitch edge about 17.8 inches wide.
| Rule | From | Tail |
|---|---|---|
| 3.5 × edge width + 6 in | Sarah Maker | 68 in |
| 4 × edge width | Nimble Needles (short pieces) | 71 in |
| 3 × edge width + 10% | Nimble Needles (bigger pieces) | 59 in |
| Wrap test, 10 wraps = 8 in, + 6 in | This calculator | 84 in |
| Needle estimate, 4.5 mm, + 6 in | This calculator | 88 in |
The width rules come out 13 to 26 inches shorter than the wrap test here. Width rules leave out yarn thickness. The wrap test measures your own yarn on your own needle. Nimble Needles notes that the wrap test does not work on the cable of a circular needle, which is much thinner than the needle tip. Wrap the needle tip itself, or use Lion Brand's sample method: cast on 10 stitches, pull them off and measure the yarn they used. Type that length in the 10 wraps box.
Sources also disagree on the extra length for weaving in. Sarah Maker adds 6 inches, Nimble Needles 6 to 8, and Lion Brand 10 inches or more. The calculator adds 6, so add a few inches more if you like a longer end to sew with.
Tail per stitch by needle size
When you can't do the wrap test, the calculator estimates the yarn one stitch uses from the needle size. The table shows that estimate, and the tail for a 100 stitch cast-on including 6 inches to weave in.
| Needle | Per stitch | Tail for 100 sts |
|---|---|---|
| 2.5 mm | 0.46 in | 52 in |
| 3.5 mm | 0.65 in | 71 in |
| 4.5 mm | 0.83 in | 89 in |
| 5.5 mm | 1.02 in | 108 in |
| 6.5 mm | 1.21 in | 127 in |
| 8 mm | 1.48 in | 154 in |
| 10 mm | 1.86 in | 192 in |
One published figure to check it against: the Needle & Arts sock formula says 26 inches of sock yarn casts on 60 stitches plus a tail for darning in. On a 2.5 mm needle, the estimate here gives 34 inches for 60 stitches, about 8 inches more. That is a safe direction to be wrong in, but it is one more reason to prefer the wrap test.
If the tail runs out
Running short a few stitches from the end usually means starting over. Sarah Maker's advice is to pull out the stitches and cast on again with a longer tail. For a long edge, such as a blanket, Lion Brand suggests holding two strands, one from each of two balls or both ends of one center-pull ball. One strand acts as the tail, so it can't run out.
Tips for a better cast-on
- Cast on loosely or over a needle one or two sizes larger if your edge tends to be tight. A tight cast-on can make the bottom of a piece narrower than the rest.
- Place a stitch marker every 20 or 25 stitches as you cast on. Counting a long row goes faster and you catch mistakes sooner.
- Leave a little extra tail. Running out a few stitches from the end means starting over, and a spare 10 inches costs almost nothing.
- For a very long cast-on, you can skip the estimate and use the ends of two balls, one as the tail.
- Count again before you start the first row. Fixing a missing stitch is easiest now.
