In this case, it’s angel hair, for a recipie I want to try, from a bag that’s larger than 12 oz. Is it best to buy an appropriate kitchen tool, or is there another way to do it that can skip that?
Like a scale? An invaluable tool in the kitchen, as I bake a lot. Is it a one pound bag? If so, fan out the contents on the countertop and separate into halves, then half one of the halves. There you go. Got any canned goods that weigh 12 ounces, make sure it doesn’t say 12 fluid ounces[that’s a volume, not a weight, measurement] Grab a handful of pasta in one hand, and the can in the other and guess-timate. It’s not too critical with pasta to be a bit off the mark. Hell, double the recipe and use the whole bag. Leftovers are great. Good luck.
Careful! Does the recipe call for 12 ounces of fresh pasta, or 12 ounces of dry pasta?
12 oz. of dry pasta is a lot more food than 12 oz. of fresh pasta is.
Can you estimate from the total weight of the bag? (If it’s 36 ounces, take a third?)
Wouldn’t a one pound bag of pasta weigh 16 oz? Then doubling the recipe and using the whole bag wouldn’t work, but your half then half again would give you 8 + 4 = 12.
How big is your bag. That will help us help you.
The recipie calls for dry pasta, and it’s a 48 oz bag. UNFORTUNATELY, I at some point took out an unknown amount to eat, so I’m afraid estimation based on total amount won’t do me any good. 
Any suggestions, or should I just go out and buy a pasta measurer?
Do you have a postage scale in the house? That’s what I use when I need to weigh something for cooking. I just get a paper plate, put it on the scale, readjust the balance to zero and place whatever I’m weighing on the plate. I have a small one, so it only measures up to 16oz, so if I need more, I just measure in batches.
Or you could use a bathroom scale and round off to the nearest pound.