There isn't a way to figure out how many skeins of worsted yarn would equal 18 oz. You would need to look at the yarn, and see how many ounces are in that particular skein and go from there.
The word "weight" in yarn refers to the size or thicknessof the strand of yarn, not an actual ounce, pound, gram or kilogram. Then the size or thickness would be measured by how many stitches are in 4 inches. The Yarn Council of America has a guideline for "weight" measurement for yarns.
The reason there are variables in the actual skein ounces, grams weight, is that different fibers of the same length would weigh differently--some are lighter or heavier than others.
The number of yardsin a skein of worsted yarn varies, but is roughly between 180 and 220 yards.
Using Zephyr 2/18 as an example, I would estimate 500 yards.
650 grams is 22.945 ounces, which is approximately 23 ounces of yarn. Assuming 50 yards per ounce (according to a chart at Annie's Attic), you'd have 1147, or to round it off about 1150, yards in 650 grams. To add to that, I have a skein of Red Heart Super Saver Worsted Weight yarn in hand. It says 7 oz. 198g, 364 yds, 333 m. Using that measurement, 650g would be about 1165 yards. That said, not all worsted weight yarn weighs the same, so 650 grams of one brand may not be equal to another brand. If you are purchasing yarn for a particular project, I suggest buying more than you think you need to be sure you have enough of the same dye lot for your project.
One ton of Jaggerspun Zephyr 2/18 = 10,040,000 yards (5,700 mi approx)
One ton of Jaggerspun Zephyr 2/18 = 10,040,000 yards (5,700 mi approx)
There are 18 of them!
18
18.
18
18 = 1,800%
18.
18/7
18.