I'm momemts away from finishing the socks for the Mr., and realizing that I will have some yarn leftover. This reminds me of the mystery of the stash - how much yarn is in each of those leftover balls?
The original ball band tells me that this particular yarn was 5oz, and 405yds. To know what's left, I can just weigh the remaining ball (on the little $10 kitchen scale) and use that weight to calculate the remaining yardage. 405yds in 5oz means there were 81yds per oz. I have a half an ounce left, so I have 40yds. If I stick this ball in a snack size baggie with a lable of yardage and the original ball band, I know all I need to know.
My kitchen scale allows me to switch back and forth from grams to ounces, which makes it all the more handy. It's one of the best knitting tools I've ever bought.
In cases where I have multiple colors of the same yarn type, (Cascade 220 for example) I can leave one ball band in a larger bag with all the remainder balls, and weigh out each color and list them on one piece of paper in the bag.
This is a fiddly process, but a really fast one. It saves me a ton of time when I'm sorting out yarns for projects. If a sweater calls for 1500yds of the main color, and one skein of contrast for a stripe on color and cuffs, it likely doesn't need hundreds of yards of contrast. Comparing the skein size in the pattern and my stash labels, I can tell if a leftover ball will do the job or if I need a new one. For colorwork projects and intarsia that only use a smidge of a color to fill in a flower or an eye or a leaf, again it's pretty easy to know if I have the yardage I need.
Give it a try. I hope it helps!