Weigh it, measure it
Put the work in progress on a kitchen scale (needles or hook off), measure how far up the length you are, and count the unopened skeins plus any loose yarn in grams.
If the first part used 540 g for 24 of 60 inches, the whole blanket will use about 1,350 g. Skein compares that with what you have and tells you how many skeins to buy now, while the same dye lot is still on the shelf.
Questions
How much yarn do I need for a throw blanket?
A 50 × 60 in throw in worsted double crochet takes about 2,270 yards with a 10% margin: 7 skeins of 364 yd. Knit stockinette uses about 10% less, single crochet about 18% more.
How do I work out yarn from a swatch?
Make a swatch in your yarn and stitch, measure it and weigh it. Blanket area ÷ swatch area × swatch grams gives the grams for the blanket; the label turns grams into yards. A 6 × 6 in swatch of 18 g scales to about 3,630 yards for a 50 × 60 throw with 10% extra in 220 yd / 100 g yarn.
Does crochet use more yarn than knitting?
Usually, yes. Single crochet uses about 30% more yarn than stockinette for the same area, double crochet about 10% more. Granny squares are close to stockinette.
How many skeins for a striped blanket?
Split the yards by the number of colours and round each colour up to whole skeins. Three equal stripes of 2,270 yards in 364 yd skeins is 3 skeins each, 9 in all, where a plain blanket needs 7.
How do I know if I'll run out of yarn?
Weigh the blanket so far and measure how much of the length is done. Grams used ÷ fraction done is the whole blanket. Take away what you've used and what you have left, and the rest is what to buy.
Why buy all the yarn at once?
Dye lots vary. Skeins from a later batch can be a shade off and show as a stripe. Buy every skein of a colour together, from the same lot, and keep the receipt.