Hi Daystar. You are right in thinking that a thicker yarn will have less yardage than a thinner yarn, even though they are of the same weight. One 50 gram (1.75 oz) ball of yarn might have 100 yards, while another might have 400 yards.
In the pattern instructions, does it give you a gauge? Like how many stitches to how many inches on what size needles? This will give you a clue about what weight (gauge, thickness) the yarn is. Even better, patterns usually tell you what yarn they used. Does yours not say that? Then you can find info on that yarn which will tell you how many grams or ounces it weighs and how many yards or meters are in it, and that way you can know the ounce to yard ratio.
hope this helps.