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.  YOLO and all that, but there is a better way. 

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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