An Easy Long-tail Cast-on Calculation

An Easy Long-tail Cast-on Calculation

You know who you are.  You need to cast on 300 stitches but either run out at stitch #299, or make it to 300 but have enough yardage left over to knit a small hat.  We've been using this method for decades and it's right every time.  That said, you need to be certain that your yarn is snug on the needle, but neither too tight or too loose.  It won't work as well if you pull too tight, or let your yarn get too loose and sloppy as you cast on.

Say you need to cast on 20 stitches on a US 6 needle.  The US 6 needle is also known as the size 4mm.  Thus, the needle circumference is 4mm * 3.14 (or Pi).  If you need 20 stitches of 4mm diameter stitches, then you’ll need a minimum of 20 x 1.26cm length.  Convert to inches (1” per 2.54cm) and you’ve got roughly 10” minimum for the tail to cast on.  Add a few more inches so that you’ll have yarn  to hold on to at the end; four to five inches should do it.

The metric to imperial conversion multiplied by Pi can be simplified to 2.54 * 3.14 = 7.98 or 8

Thus for an US6 or 4mm needle:

   20 stitches x 4mm/8  = 10” for the tail + 4” extra = 14” tail

   30 stitches x 4mm/8  = 15” for the tail + 4” extra = 19” tail


For a different size needle (US8, also known as the 5mm):

    20 stitches x 5mm/8  = 12.5” for the tail + 4” extra = 16.5” tail, round up to 17”

    30 stitches x 5mm/8  = 18.8” for the tail + 4” extra = 22.8” tail, round up to 23”


Try it yourself!

___ stitches   x   ___mm needle / 8 =   ___” minimum  + 4” =  _____”  total needed for tail


BTW, we don't take credit for coming up with this.  This formula has been floating around for decades.

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