How to determine where to start the gusset for a toe-up sock

Knitting a toe-up sock can be a super fun project, but it can be a bit of a challenge to get the fit just right. One of the most important steps in creating a comfortable sock is figuring out where to start the gusset. The gusset is the bit that goes between the foot and leg, and it's what gives the sock the flexibility to fit snug around the heel. 

My formula for determining where to start the gusset in a toe-up sock works for many of my sock patterns, including the Dublin Memories Socks, Barca Memories Socks, Rhubarb and Custard socks designs. It has a sole gusset and a heel flap.

My formula below assumes the total length of the foot and has three parts: 

• A: toe to start of the gusset 
• B: gusset increases 
• C: half of the heel flap

See the illustration below.

Note the formula above can likely work for any sock with a heel flap regardless of whether the gusset increases are done on the sides or the bottom of the feet. Let’s get started! 

1. Measure the foot's length: the Total

> As an example, the hypothetical wearer of Dublin Memories Socks has a foot length of 24.5 cm/9¾".

2. Make a gauge swatch and note both stitch and row gauge

> For illustration, we can use the gauge for Dublin Memories Socks which is 32 sts & 42 rows = 10 cm/4"

3. Calculate the number of rounds needed for gusset increases and convert to cm or inches using row gauge: The B. 

Usually, the number of increases needed for the gusset is around half the number of foot stitches. I like to move 4 increases to the heel shaping section, so I need to increase 4 sts less in the gusset, but this is optional.

Typically, the gusset increases are worked symmetrically on both sides (so 2 increases in one round) but every second round. That means the rounds needed to work the required increases are equal to the required increases. 

> In our example, the number of increases that are worked for size 2 (and 64 foot stitches) in Dublin Memories Socks is 28 increases over 28 rounds. Using the row gauge of 42 rounds, 28 rounds come out at about 6.5 cm / 2 3⁄4". 

4. Calculate the heel flap width in stitches, and convert half of that to cm or inches using stitch gauge: The C

Usually, the feel flap is around half of foot stitches. In my patterns, I like to make the heel a bit bigger so that the fit around the ankle is better and it fits people with higher arch.

> In our example, the number of sts used for the heel flap for size 2 in Dublin Memories Socks are 34. Half of that would be 17, and using the stitch gauge of 32 sts, it comes out at about 5.5 cm / 2".

5. Final step: The A = Total - (B + C)

Adding the length of the gusset increases and half of the heel flap gives us the remainder of the length from the start of the gusset. All we need to do is subtract from the total foot length and we have the length at which we need to start the gusset.

> In our example, A = 24.5 cm / 9 3⁄4" - (6.5 cm / 2 3⁄4 + 5.5 cm / 2 1/4") = 12.5 cm/5"

Et voila!

If you don't like calculating, I have done all the maths in all my patterns and provide the B + C measurement for you, so simply subtract that from your foot length and get your own customised gusset starting point.