Size of circular knitting needle

How do you choose what length of circular needle to use for each project? Is there a formula to determine the size ( length of cable from tip of needle to tip of other needle ) of the circular needle you should use for different number of cast on stitches?

It depends what you’re knitting, and if you’re knitting in the round, or flat.

Are you following a pattern? Patterns will say what size circ to use.

If you’re knitting in the round, the circ needs to be smaller than the circumference of the item. Unless you’re knitting something really small, then you’d use 2 circs or a long one and the Magic Loop method. If you’re knitting flat, it needs to be long enough to hold the sts for the item without them being crowded. So for a blanket, you wouldn’t use a 16", that’s too short, but for ar 36" wide one, you can use a 24-32" one.