Depending on needle size used, the actual gauge and finished size of the hat may vary.

A gauge of 16 sts per 10 cm [4 in] on 5.5 mm [US9] needles will yield a hat circumference of:
(54–55 cm) – 56 cm – 57 cm – (58–59 cm)
(21.3 – 21.65 in) – 22 in – 22.5 in – (22.8–23.2 in)

A gauge of 18 sts per 10 cm [4 in] on 5.0 mm [US8] needles will yield a hat circumference of:
(48–49 cm)– 50 cm – 51 cm – (52–53 cm)
(18.9–19.3 in) – 19.7 in – 20 in – (20.5–20.9 in)