Lots of numbers get tossed around when talking about the food supply of ancient Rome.
A good case in point is Rosenstein’s impressive piece in JRS 2008. But I find it hard to think about what these numbers mean about assumptions about subsistence level and caloric/nutritional need. So I did a few calculations.
Assuming 200 kg of wheat per person per year means 1860 calories from that staple per day.
Assuming 266 kg of wheat per person per year means 2475 calories from that staple per day.
Add to that some olive oil.
Assuming 20 liters per person per year adds 440 calories per day.
Assuming 30 liters per person per year adds 660 calories per day.
This gives us a calorie range of about 2300 to 3100 with 15 to 25 percent of the calories coming from oil (I’m rounding all these numbers). High for a sedentary modern westerner. Crazy low for say a lumberjack who might need upwards 5000 calories per day.
What did the Romans think? Well, 5 modii of grain per month is what was put on sale at reduced prices or given away for free at some points in history. It is called in some rhetoric “prison rations” and is about what Cato prescribes for his more active slaves, the farm laborers. Notice that when Atticus wishes to be generous to the Athenians his donative is 7 modii per person, not 5.
Assuming 6.65 kg per modii, 5 modii provides about 3700 calories a day. We often read in modern scholarship that this is enough for two people. Two people that lay around all day. Or, two people that can supplement their diet with other food stuffs. However, neither of these two conditions really seems to me to well describe the urban poor of Rome. Being poor is hard work. Hell, even milling that grain into a more digestible flour that maximizes its nutritional potential takes some real labor.
Need some help imaging this amount of food? Each modii would result in about 13 loaves of bread, so we’re talking approximately 65 loaves a month.
Update 5/5/2014: A great footnote from Walthall’s 2011 ZPE article on Sicilian grain measures:
The modius is ~8.73 liters.