Designing a knitted hat: size guidelines
| Size | Beanie Circumference | To beginning of crown |
| preemie | 11-12 inches | 4 inches |
| infant | 13-15 inches | 4.5 inches |
| baby/toddler | 14-16 inches | 4.5 inches |
| child | 15-17 inches | 5 inches |
| adult small or kid large | 17-19 inches | 5 inches |
| adult medium | 18-20 inches | 5 inches |
| adult large | 21-23 inches | 5.5 inches |
Update January 31, 2012: If you want to go beyond the basics, I just found a great, very detailed, head sizing chart at Tottoppers. You can also buy Kate Oates's Math for Hats pdf booklet.
Simplest Knitted Beanie Recipe
Multiply your gauge (stitches per 1 inch) by the desired circumference of the beanie. Round off the number of stitches to a multiple of 4. Cast on that number of stitches. Use a set of double pointed needles, 2 circular needles, or 1 short circular needle, and join in the round. Work about 1 1/2 inches of K2, P2 rib, then switch to stocking stitch. On the first row after the ribbing, increase or decrease a few stitches so that your total number of stitches is a multiple of 5. Knit without shaping to beginning of crown (see above measurement list for height of the hat to crown).
Decreasing for the crown:
Row 1: *Knit ((total number of sts/5) - 2), k2tog. Repeat from * to end of row.
for example, if you have 80 sts, you would [K14, k2tog] five times, and then have 75 sts.
Row 2: Knit
Row 3: *Knit ((new total number of sts/5) - 2), k2tog. Repeat from * to end of row.
Row 4: Knit
Continue decreasing like this (switching to dpn's if you started with a circular needle) until you have about 1/3 of your original stitch count. Then decrease every row, instead of every other row, until you have 10 stitches left. Then break your yarn, and thread the tail through the remaining stitches.