Estimating amount of yarn needed

I want to knit a healing shawl like the one someone made for me. It is about 20 inches wide and 70 inches long and was knit of a lovely colored bulky acrylic yarn. It is just plain knitting with a fringe at each end.
If I use very large needles (about size 11 or 12) and a bulky yarn, how can I figure out how much yarn to buy?

You could weigh it and translate the ounces into the number of skeins of a similar yarn.