Start with a swatch
Yarn use depends on your tension, your hook and the stitch pattern, so two people can need very different amounts for the same blanket. A small square of about 4 inches, made in the stitch you plan to use, tells the calculators how much yarn your fabric takes. Weighing it on a kitchen scale is the easiest way to measure the yarn.
Know your yarn weight
Most yarn labels show a weight number from 0 (lace) to 7 (jumbo) and a suggested hook size. The yarn weight chart lists the usual gauge and hook range for each weight, and the hook size chart converts between millimeter, US letter and old UK sizes.
Buy from one dye lot
Each batch of dyed yarn has a lot number on the label, and colors can vary slightly between lots. Buying everything you need at once, plus a little extra, avoids a visible color change partway through a project.