Your stored food supplies have been exhausted. Some disaster has wreaked havoc with the commercial food supply. How much land would you need to grow ALL of your own food? For this particular article, I'm setting aside the question of raising your own chickens, fish, goats, cattle, pigs, etc. For the sake of simplicity, let's consider how much land it would take to grow a complete vegetarian diet, per person. Now I'm not a vegetarian, but I eat a fairly healthy diet, and if the necessity arises, I'd adapt.
Certain assumptions are necessary to this type of calculation. For example, we have to estimate the total calories per day per person, and the percent of calories attributed to protein, fat, and carbohydrates. [Technically, what we call 'calories' are kilocalories (kcal).] The USDA nutrition labels assume 1800 kcal per day for an adult woman and 2200 kcal per day for an adult man. So the average is that ubiquitous 2000 kcal diet figure found on so many product labels.