I think this was brought up in another place on HN recently, and somebody argued it's probably a couple orders of magnitude off in the conversion to "today's dollars" apart from the accuracy of the figure in sesterces.
My thought is $15 billion in today's dollars is an absurd amount for "a years supply of grain for Rome", since Rome was supposedly around a million people. $15,000 per person for food alone is more than the global per capita GDP in 2018!
>$15,000 per person for food alone is more than the global per capita GDP in 2018!
So in the United States, one acre of wheat yields an average 37.1 bushels of wheat. That's about 210,000 kcals of flour per acre using modern machinery.
Grain was harvest by hand until the first century when the vallus was invented so I'm going to just estimate a difficulty increase of 4 people being able to harvest an acre in a day without a vallus or a cradle.
Beloch's 1886 estimate for the population of the empire during the reign of Augustus suggests there were 23 million European Romans, if we assume 'Rome' means the city I haven't a clue what the population would have been, if we assume the whole empire then Beloch's estimate was 54 million.
Let's just assume 1 million people with 20% of their caloric intake being grain and assume an even 2000 kcals per person.
So we need a bit over 1900 acres assuming no spoilage per million people. Wheat has a fairly narrow harvest window as you need to harvest it once it reaches a certain moisture level and if you leave it too dry too long it can easily be damaged/detached from wind and rain.
Let's assume a team of 4 people can realistically harvest, and prepare for further drying, 5 acres in the window.
That means we need 3800 people per million mouths fed a diet that gets 20% of it's kcals from wheat using ancient harvesting methods. Obviously this is an estimate but I tried to be generous in favor of the harvesting and my kcal number is likely low. Yields would also be markedly reduced back then without chemical fertilizer/herbicides/pesticides.
So 3800 people per million mouths earning 800-1000 sesterces a year ( https://www.slideshare.net/javierredondas/jobs-salariesrome ) let's assume another 10% in cost for milling (probably low) so we'll assume 3,762,000 sesterces per 1 million people for 20% of their kcal needs.
So anywhere from 3-12 million dollars per million people. I'm guessing this would have been more like 15-45 million dollars per million people so even with that figure we're looking at maybe 2 billion dollars for all of the Roman empire.
So yeah, I like the (IIRC) 1.5 billion dollar figure mentioned in the book I referenced elsewhere which is why I left it at 1 billion and then some in my comment.
My thought is $15 billion in today's dollars is an absurd amount for "a years supply of grain for Rome", since Rome was supposedly around a million people. $15,000 per person for food alone is more than the global per capita GDP in 2018!