- Circumference around the ball of the foot
- Circumference at the widest part of the foot: around the heel and instep (see picture at right for illustration)
- Length of foot (Best way: put a ruler on the floor, extending out from a wall; stand on the ruler, back of the heel pressed against the wall)
- Length from the tip of the longest toe to the center of the ankle (see this blog post for advice).
We've already calculated how the sock will fit around the ball of the foot: gauge x circumference x 88% for negative ease. Now calculate how many stitches the sock will need to be at its widest. My foot is 9 inches at the ball, 13 at the heel/instep, or 68 stitches at the ball and 98 at the widest point. I will need to increase 30 stitches over the gusset section.
Gusset sections begin about halfway along the foot. My foot is 10 inches long, so I will start my gussets when the sock is about 5 inches. I can safely continue knitting for a few inches, give myself some more fabric so I can accurately check row gauge. My sock is 13 rows (or rounds) per inch, so I'll want to start my gussets near round 65.
The increases need to be completed before the center of the ankle. For me, that's at 7.5 inches, or round 98. Isn't that handy? I'll need to make 30 gusset increases, and I've got just over 30 rows to do it.
I can knit plain up to row 68, and ponder what I want to do with the gusset section.