Yardage is the length of yarn a project needs, measured in yards or meters. It's the most reliable way to plan yarn amounts, since skein sizes vary by brand — a pattern calling for 1,000 yards needs 1,000 yards regardless of how it's packaged.
As a rough guide, a worsted-weight throw takes about 1,000–1,500 yards, a beanie 100–200, and a baby blanket 600–900. Always buy a little extra from the same dye lot so colors match if you run short.