John D. Rockefeller Sr.
Seattle Times/JR Partners—Getty Images
By Jacob Davidson
July 29, 2015

Comparing the wealth of our contemporaries is hard enough. When Forbes ranks its annual 500 richest people, the publication not only has to sift through mountains of data, it also must decide what even qualifies as wealth. For example, should people who draw their money from the country they rule, such as Vladimir Putin, be included? (Forbes says no).

When comparing wealth across history, however, we run into even more challenges. How does one contrast riches in a pre-industrial age with the wealthy of today? It’s not just a matter of adjusting for inflation; coinage and currency is a relatively recent invention. Much of pre-modern wealth was held in stuff, and commodity prices have drastically fluctuated across history as the total amount of resource wealth has grown.

Another issue is that wealth is a relative measure that doesn’t equate very well to standard of living, at least across centuries. King Louis IV was one of the richest men of his day, but his money could not fix his rotting teeth. Meanwhile, in modern times, air travel and electric toothbrushes are available to hundreds of millions of people.

One way to account for the value of goods that did not exist in the past is to measure wealth through the amount of energy a person can command. This method resembles the labor theory of value, made famous by Marx and Adam Smith, which states that the worth of an item is the total amount of labor that goes into its creation.

If we expand the definition of labor to include non-manual power, like fuel, we can see how the average person became better off as technology improved.

“The big difference between the last couple of hundred years and all earlier times is the immense amount of energy that the industrial revolution unleashed,” says Ian Morris, a professor history at Stanford University. “Every time you flick on a light switch, it’s as if half a dozen slaves leap into action, all thanks to coal and oil powering our electricity plants.”

In his book, The Measure of Civilization, Morris calculates the average hunter‐gatherer captured between 5,000 and 10,000 kilocalories of energy per day from the environment. With the invention of agriculture, that number moved up around 10,000 kcal per day, and in the time of the Roman Empire (and once again in 12th‐century Song dynasty China) the average kcal per day jumped to 30,000.

By comparison, the average American burns through something like 230,000 kcal per day; nearly eight times as much as Romans and 23 times that of our ancient agricultural ancestors. “Basically, we’re all rich,” says Morris. “Which is why our lives are so different from those of prehistoric and ancient peoples.”

In that respect at least, the typical college student might be richer than the greatest Roman emperors, and the wealthiest individuals in history will nearly always be modern figures who can benefit from technological advances.

But when we speak of wealth, we generally don’t mean whether someone can buy a computer or a horse-drawn carriage. Most of the time, we’re using money as a substitute for economic power. And power is both relative and sensitive to time and place. The typical American may have access to more resources than a Roman emperor, but none have the same kind of economic influence that can reshape societies.

Luckily, there is a method of calculating wealth that can measure the influence of one’s money while accounting for changes in value over time. MeasuringWorth, a website run by a group of economics and history professors dedicated to comparing wealth throughout history, recommends measuring economic power by taking an individual’s wealth and comparing it to the total economic production in the economy—its GDP, in other words.

That’s the metric I’ve chosen to use when compiling this rich list, but it’s still not perfect. No scholarly list of wealth historical figures exists—the closest might be The Richest of the Rich, by Philip Beresford and William D. Rubinstein, which compiles the 250 wealthiest people in Britain since 1066—leaving us to peruse the history books for rich list “nominees.”

But once we picked an eligible group, we run into the same problem as Forbes: How do you separate the wealth of the monarch from the wealth of the state? In this list, with the generous help of several historians, I’ve done my best to select individuals with large private wealth, or monarchs in nations where wealth was especially concentrated amongst the ruling class.

Another issue: Wealth and GDP data get increasingly scarce the farther you go back in time. That’s a big problem because ancient leaders were much more proficient at extracting wealth from their society than our modern elites, making them far more powerful economically.

Branko Milanovic, a Graduate Center, CUNY economics researcher specializing in inequality, estimates that pre-modern societies dating from the Roman Empire to British India in 1947, were able to extract nearly 76% of the maximum amount of wealth that could be taken before their people would slip below subsistence levels. Modern nations, measured from 1998 through 2002, extracted a comparatively small 43% of that limit. For this reason, the most accurate list of the richest people to ever live (based on economic power) would likely include almost exclusively pre-modern rulers.

A final difficulty is measuring economic power between countries with different levels of economic output because one person might have a higher share of wealth in a less economically powerful nation. A truly correct international comparison would require knowing each person’s wealth as a percentage of global GDP, and old global economic data is even harder to come by.

But perhaps the governing rule of lists like these is to not let the perfect become the enemy of good, or at least the enemy of fun.

My ranking, based on hours of interviews with various economists and historians, does its best to order some of the wealthiest historical figures in order of their economic influence, and gives a rough estimate of their monetary wealth when possible. Most importantly, it tried to put their riches in context.

Read next: The 10 Richest People of All Time

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