What is the height difference between sc, hdc and dc, and how do I swap them?

Measured in DK weight on a 3.5 mm hook, a short single crochet is 5.5-6.5 mm tall and about 4.5 mm wide. A half double is 8.5-9.5 mm tall, a double 11.5-13 mm. Row-for-row, ten rounds of sc is 6 cm; ten rounds of dc is 12 cm.

Width barely changes between the three: all sit around 4.5-5 mm because the base and the pull-through are the same. That is why swapping stitch height changes the shape proportionally — you get a taller, thinner-looking fabric from the same stitch count.

Say a pattern gives 20 rounds of sc for a 12 cm body. If you want the faster fabric of hdc, multiply the round count by 6/9, i.e. two thirds: 13 rounds of hdc. Keep the increase rounds at the same stitch counts, just compress the straight rounds between them.

The other direction needs care. Going dc to sc to make a firmer fabric means multiplying by 12/6 = 2, so a 12-round dc tube becomes 24 rounds of sc — feasible, but the base round now has to carry twice as many rounds of weight, so add one round of extra stitches at the widest point.

Amigurumi wants sc almost without exception: it is the only one of the three that holds stuffing in without gaps and keeps its shape after washing. Hdc is the compromise for bags and baskets where you want speed and a softer hand. Dc belongs in flat work — granny squares, lace, blankets — where its height creates drape rather than holes.

One exception worth knowing: for a coaster or trivet that must not curl, hdc is often better than sc, because the extra height gives the disc somewhere to go and it lies flat at a smaller stitch count.