If you remember the yarn name, you can look it up and see how much yardage was in them originally, then figure out how many yds per oz and go from there. Not all fibers weigh the same - 2oz of cotton may only be 90 yds while 2 oz of wool or acrylic may be 120 yds or more. So you can't compare weights unless the fibers are the same.
Another way to figure how much you have is to measure out 20 yds or so and see what that weighs. It's usually around 5g, but again, will vary according to fiber.
sue- knitting heretic