www.fiber2yarn.com: Metric Conversion possibly every useful info all in one place. wpi, us-uk terms conversion, yardage estimation for various projects.

tells me for a 60″ square afghan crocheted in dk i need 3600 yards or approx 3lbs of yarn.

update: the site has been reorganised. the main yardage estimation table/calculator is here.
information pages index is here.