16 hdc = 4", using Size H crochet hook, and worsted weight yarn. This is what I used, but size can vary depending on your adjustments.
Note: Ch 2 at beginning of hdc rows counts as a stitch.
Row 1: Sc in second ch from hook and in each ch across (25 sc)
Row 2: Ch 2, turn, sc in back loop only (BLO) of first 5 sc, hdc in BLO of next 20 sc (25 sts)
Row 3: Ch 2, turn, hdc in BLO of first 20 hdc, sc in BLO of next 5 sc (25 sts)
Row 4: Ch 2, turn, sc in BLO of first 5 sc, hdc in BLO of next 20 hdc (25 sts)
Repeat Rows 3 and 4 until piece measures *half way around teapot; fasten off; this is Side 1 of the cozy. Make Side 2, following same instructions as Side 1. Place Side 1 and Side 2 together, wrong sides facing, and join sides together at top and bottom of each side, leaving openings for the teapot spout and handle. Make a crochet chain or knit an I-cord, to create a drawstring for the top of the cozy. Weave finished drawstring through row, at 5th sc from edge. Gather top of cozy and tie into a bow.
*Measure piece against your teapot for a good custom fit.