How Many Stitches to Cast on for a 20 Inch Hat?
For a hat sized to a 20-inch head circumference, worked in a stretchy 1x1 rib brim with about 1.5 inches of negative ease (an 18.5-inch cast-on circumference), the numbers work out like this at common gauges:
- Worsted weight, 4.5 stitches per inch, 1x1 rib: cast on 82 stitches (the raw gauge math gives 83, rounded down to the nearest even number for the rib pattern).
- Worsted weight, 4.5 stitches per inch, 2x2 rib: cast on 84 stitches (rounded up to the nearest multiple of 4).
- DK weight, 5.5 stitches per inch, 1x1 rib: cast on 102 stitches (this one lands exactly on an even number already, no rounding needed).
Why 1x1 rib rounds down here but 2x2 rib rounds up
Look closely at the two worsted-weight results above and notice they moved in opposite directions from the same starting point: 83 raw stitches became 82 for 1x1 rib but 84 for 2x2 rib. That's not an inconsistency — 82 and 84 are simply the two nearest valid stitch counts on each pattern's own number line (every 2nd number for 1x1 rib, every 4th for 2x2 rib), and 83 happens to sit exactly one stitch from both. Whichever of the two is numerically closer wins; when a pattern's steps are wider, as with 2x2 rib, the "closer" answer can land on the opposite side of the raw number than a narrower-stepped pattern would. See the methodology page for the general rounding rule this follows.
Why 1.5 inches specifically, and not 1 or 2
1.5 inches sits in the middle of the typical 1–2 inch negative-ease range for a ribbed brim — a reasonable default when you don't have a strong reason to lean toward either edge of that range. Push it toward 2 inches for a very stretchy, springy wool worked in a snug rib, where the fabric can comfortably pull in further without feeling tight; pull it back toward 1 inch for a less elastic fibre (cotton or acrylic) or a looser rib, where too much negative ease risks a brim that's genuinely uncomfortable rather than snug. See our hat project guide for the fuller explanation of how brim stitch pattern affects how much ease is appropriate.
Adjusting for your own gauge
These numbers assume the stated gauges exactly. If your own swatch measures a different stitches-per-inch than 4.5 (worsted) or 5.5 (DK), your correct cast-on will differ — sometimes by quite a few stitches, since gauge differences compound across a 20-inch circumference. Plug your own swatch's numbers into the Cast-On Calculator rather than treating any generic worked example, including this one, as a substitute for your own gauge.