I tend to have big salad eaters, so with tubs that size, I'd probably have to use 4. Wish I could tell you weight, but I usually eyeball heads of lettuce. A medium to large head should serve at least 4 - 6, depending on what else you're adding, as boulangere says, or how many other dishes you're serving. Also, is it dinner or lunch? Kids included in that number or no? etc. Also depends on type of lettuce - a large head of romaine would probably serve 6 or more, whereas a "large" head of butter lettuce only four or so.

super helpful y'all. many thanks, c and w!! :)

I count about 100 grams for a side dish salad of mixed greens. That would be 3 - 4 ounces or about 8 to 9 pounds for 40 people.

Revised answer. Just washed and weighed the mesclum purchased at my GreenMarket. No waste. 100% edible. 50 grams or scant 2 ounces is plenty for this pristine mix.