From The Atlantic:
Having examined the finances of all 43 presidents (yes, 43; remember, Cleveland was president twice), we calculated the net worth figures for each in 2010 dollars. Because a number of presidents, particularly in the early 19th Century, made and lost huge fortunes in a matter of a few years, the number for each man is based on his net worth at its peak.
We have taken into account hard assets like land, estimated lifetime savings based on work history, inheritance, homes, and money paid for services, which include things as diverse as their salary as Collector of Customs at the Port of New York to membership on Fortune 500 boards. Royalties on books have also been taken into account, along with ownership of companies and yields from family estates.
The net worth of the presidents varies widely. George Washington was worth more than half a billion in today’s dollars. Several presidents went bankrupt.