This pattern is meant to fit a dog that is 15 to 20 pounds. Although, you could make slight alterations to the size by using a different size hook!
The neck measures ~12.5 inches around, the chest measures ~21 inches around, and the length of the body is ~14 inches.
Medium Dog Sweater

Supplies: Worsted Weight Yarn in two colors and an H hook
Stitches Used: sc- single crochet, slst- slip stitch, ch- chain, hdc- half double crochet, BLO- work stitch in back loop only, sc2tog- single crochet two stitches together, trc- treble crochet, hdc2tog- hdc two stitches together.
Collar (working with color 1)
Row 1: ch 5, begin stitches in 2nd chain from hook, sc 4, ch 1, turn.
Row 2-Row 41: BLO sc 4, ch 1, turn.
Holding Row 41 and the foundation chain together to create ring of sweater collar, slst through both layers in each of the 5 stitches.
Main Sweater (change to color 2)
Turn collar so that the seam is inside, now begin working in the round.
Round 1: ch 1, sc 45 around edge of collar.
Round 2: hdc in each stitch around.
Round 3: *2 hdc in first stitch, 1 hdc in next stitch* repeat * to * two times, hdc 37, *2 hdc in first stitch, 1 hdc in next stitch* repeat * to * two times.
Round 4: *2 hdc in first stitch, 1 hdc in each of the next 2 stitches* repeat * to * three times, hdc 31, *2 hdc in first stitch, 1 hdc in next 2 stitches* repeat * to * three times.
Round 5: *2 hdc in first stitch, 1 hdc in each of the next 3 stitches* repeat * to * two times, hdc 38, *2 hdc in first stitch, 1 hdc in each of the next 3 stitches* repeat * to * two times, sc in last stitch of round, slst to first stitch of round. (49 stitches)
Round 6: ch 3, trc 2, hdc, slst in each of the next 10 stitches, hdc next 37, slst next 10, hdc, trc 2, slst to first trc of round. (63 stitches)
Round 7: ch 3, trc 3, ch 10, skip 10 stitches, hdc next 37 stitches, ch 10, skip 10 stitches, trc last 3, slst to first stitch of round.
Round 8: ch 2, hdc first 3 stitches of round, hdc in each chain from round 7, hdc 37, hdc in each chain, hdc last 3 stitches, slst to first hdc of round.
Round 9: ch 2, hdc2tog, hdc next 59 stitches, hdc2tog, slst for first stitch of round.
Round 10: ch 2, hdc2tog, hdc next 57 stitches, hdc2tog, slst for first stitch of round.
Round 11-Round 16: ch 2, hdc in each stitch around, slst to first hdc of round.
Round 14: slst first 9 stitches, hdc 41, leave remaining stitches of round unworked, ch 2, turn.
You will now work in rows.
Row 15-Row 17: hdc 41, ch 2, turn.
Row 18: hdc2tog, hdc 37 stitches, hdc2tog, ch 2, turn.
Row 19: hdc2tog, hdc next 35, hdc2tog, ch 2, turn.
Row 20: hdc2tog, hdc next 33, hdc2tog, ch 1, turn.
Row 21: sc2tog, hdc 23, sc2tog.
Change to color 1.
Row 22: ch 1, sc around entire bottom edge of work, slst to first color 1 sc.
Fasten off, weave in ends.

Need a smaller size? Check out the extra small version of this sweater!