take your gauge (it's usually x sts/4 in) and divide it to turn it into 1 inch. Going with the wristband from Monday, the gauge was 21sts/4in. 21 divided by 4 = 5.25 <--meaning, there's 5.25 sts per in.
Multiply this number, 5.25, by the circumference of your wrist. This will equal the amount of sts you need to cast on. If the number ends with a fraction, then round up to the nearest st. Using my wrist, it ended up being 31.5, so I rounded it up to 32.
|here's the formula|
|formula in action|
You could end it at that, and stitch away to your heart's desire, but if you want to center a design in the middle of the piece, then continue on to...
create a design using graph paper.
divide the number of sts in the design by half and divide the cast on number of sts (aka total sts in the piece) in half.
subtract these two numbers to find out how many sts are before and after the chart.
Now that we know there should be 8 sts before and after the chart, the pattern should be:
k8, chart, k8
p8, chart, p8
or change it up and do a seed stitch, rib...whatever you want.