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All (well written) patterns list the yarn requirements for a pattern. Maybe you have just shy of that number but really want to give it a go. If you aren’t sure if you have enough yarn, but you really want to try making something anyways…
Socks – Go for a toe up pattern. Stop when the yarn runs out. It’s better to have an inch less sock cuff that to run out of yarn just before you finish the toe.
Scarfs – Would you rather have a skinny 5 foot long scarf or a fat 3 foot long scarf? For the former, chain or cast on the whole length and work it lengthwise.
Sweaters – Top down sweaters mean you’ll definitely get through the yoke section. If you do the bottom first and run out of yarn, you can always leave it sleeveless. If you definitely want sleeves, do those before the lower body- that way you’ll have your sleeves and the sweater may just wind up a cropped bolero or waist length instead of hip length.
The Two at a Time Rule – Any time you are a) worried about having enough yarn and b) making two of the same thing (socks, mittens, sleeves), split all the yarn you have for that in half. Weigh your yarn, and start winding it into a ball. Stop to weigh the ball frequently until it weighs half the total amount. Snip the yarn, and you now have two roughly equal balls of yarn. Make the first sleeve/mitten/glove with one of the balls. If you run out, you know you definitely don’t have enough for the project. If you have a good bit left over from the first, then you should be good to go with the second.