New York City
New York city has a population of 19 million. The urban land area is 8,683 square kilometers. Taking only 50% of this land area as roof area viable for Urban Farming, this leaves 4241.5 square kilometers of roof area. Taking tomatoes as an example produce, a 1,000 square meter roof can produce 43 tons of tomatoes per year. Applying this yield ratio to the calculated roof area, 4241.5 sq km x 1 million sq m per sq km x 43 tons / 1000 sq m results in about 182 million tons of tomatoes. Dividing this yield among the 19 million inhabitants results in 9.6 tons per individual. Since this way too many tomatoes for a single person, the excess production capacity can be assigned to other types of produce.
Assuming a human eats 1 kilogram of food per meal, this is 3 kilograms of food in a day. For 365 days or 1 year, this is approximately 1,100 kilograms of food. Thus, to produce this amount of food for 19 million inhabitants is just 20.9 million tons. Using 43 tons per 1,000 sqr m area, the necessary roof area to produce this amount is 20.9 million tons / 43 tons * 1000 sqr m or just 486 sqr km. This is just 5.5% of the total urban area of New York City.