KnitCalcs
Hat Size Calculator — Cast On & Dimensions from Head Size
Calculate cast-on stitches, hat height, and crown decrease schedule from head circumference.
How We Calculate This
This calculator uses head circumference and gauge to determine the cast-on count and crown shaping for a basic hat.
The formula
Cast on = (Head circumference - Ease) x (Stitch gauge / 10)
Hat height = Head circumference x 0.4 (approximately)
Crown decrease rounds = (Cast on - 8) / Crown sections, decreasing 1 stitch per section every other round until about 8 stitches remain.
Frequently Asked Questions
Wrap a tape measure around the widest part of the head, just above the ears and across the forehead. This is the head circumference. Hats are typically knitted with 2-5cm of negative ease so they stay on snugly.
A fitted adult beanie from brim to crown is typically 18-24cm — this calculator defaults to head circumference x 0.4 (about 22cm on a 56cm head, 24cm on a 60cm head). For a slouchy hat, add 5-10cm. Children's hats are 14-18cm. Baby hats are 12-15cm. The height includes the brim/ribbing section.
A standard crown decreases over 4-6cm. Divide stitches into 6 or 8 equal sections and decrease 1 stitch at each section marker every other round. Continue until 6-12 stitches remain, then draw yarn through remaining stitches.
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Last updated: February 2026
All calculations are estimates based on standard yarn weights and gauge. Results may vary with individual tension, yarn brand, and needle size.