Weight does NOT necessarily equal yardage (although it would be helpful if it did!). Different fibers are heavier than others (for example, cotton is heavier than wool). Also, some yarns are spun tighter than others. One may be a bulky weight with lots of air in it, while another with the same weight may be spun tightly and knit up finer.
As for a website or book to tell you exactly how to measure it...other people would know better than I would. But it wouldn't hurt to buy an extra ball of yarn, would it?