Sock Size Calculator
Enter a foot measurement and your gauge, and get a cast-on count and foot length sized to that actual foot — not a generic small/medium/large that may or may not fit.
Why socks are knit smaller than the foot they fit
Sock yarn's stretch is what makes negative ease work: knit a sock's circumference a bit smaller than the actual foot circumference, and the fabric stretches to fit snugly rather than sitting loose and sliding around inside a shoe. 10% negative ease is the standard starting point for a typical ribbed or stockinette sock in wool-blend sock yarn; firmer or less elastic fibers (cotton blends, some plant fibers) hold up better with slightly less negative ease, since they have less stretch to give.
Rounding the cast-on to a usable number
The raw math rarely lands on a clean number, so this rounds to the nearest multiple of 4 — a standard convenience for socks worked on double-pointed needles or two circulars, since most heel-flap and gusset construction methods divide the stitches evenly across instep and sole. If your stitch pattern has its own repeat (like a 6-stitch cable panel), you may need to nudge the count further to a multiple that fits your pattern repeat cleanly, even if it strays slightly from the exact gauge math.
What this doesn't cover
This gives you the leg/foot cast-on and the length to knit straight before shaping — for the heel itself, use the Heel Turn Calculator, and for the toe shaping, the Toe Decrease Calculator. Running all three in sequence turns "what size do I cast on for this foot" into a complete, sized-to-the-person sock plan.