Aran weight yarn is heavier than worsted without being too bulky. A lot of the time you can use an aran weight yarn to sub for a heavy worsted or bulky weight yarn. Aran weight yarn is a bulky yarn, typically in the range of 8-12 stitches per 4 inches on a 10-10.5mm needle. It is often used for knitting sweaters, blankets, and chunky throws. Check your gauge and get knittin!