Quantities of food needed are based on: the number of people being served, the portion size you are serving, and the amount of shrinkage and waste involved in preparation.
To determine how much is needed:
(1) Determine portion size.
(2) Multiply the portion size by the number to be served. Convert to pounds. ⬅️ Edible portion required.
Determining how much raw product to buy:
Divide the edible portion by the percent yield (which is the amount provided by 1 pound of the item). Example: If the portion size is 6oz, and we need 30 servings, we will need 180oz. Converted to pounds, this equals 11.25lbs edible portion.
If 1lb (or 16oz) of the raw item produces 8oz cooked, the yield is 50%. Divide the edible portion by the percent yield. You will need to purchase 10lbs of raw product to end up with 5lbs edible. (If the question doesn't provide the yield, but you're given waste, shrinkage, or loss during cooking, subtract this amount from 100% to get the yield. E.g. if the shrinkage was 30%, the yield would be 70%)