13 billion $100 bills are currently in circulation.
12.2 billion $1 bills are in circulation.
In 1998, there were 7.5 billion $1 bills in circulation, 4.2 billion $20 bills and 3.2 billion $100 bills.
In a period of just under 25 years, $100 bills went from being under half of the amount of $1 bills available to now surpassing $1 bills as the number one form of US currency for the first time ever.
Which there are different factors to this, with one being international use, where the Fed estimates 80% of $100 bills and 60% of all US currency is held internationally.
That said, the number one reason for the change in the supply of $100 bills as the number one form of currency is inflation.
$1 in 1998 would be $1.82 today.
This change in the value of money has impacted prices for the last century across the board.
An example of this being Hershey Bars.
Hershey Bars in 1908 had a price of $0.02 per bar, which was later raised to $0.05, which held at that price for almost half a century, until the price/size increase in 1969 to $0.10 a bar.
Today, $1.59 is the average Hershey Bar price nationally, showing a near 1600% price increase in 53 years, compared to the 50+ years prior only having a 400% price increase.
This is an example of how much currency has changed, where coins were once the primary way to purchase things such as groceries and now simply act as change for purchases with bills.
Looking at this, it brings up the question of how long do bills such as the $1 or the $20 have until becoming obsolete to inflation.
Today, 1.2 billion $2 bills are inflation, which is about only 10% of $1 bills.
The $50 also only have 1.7 billion in circulation, which is under half the $20.
This rising inflation makes a case that within the next 10 years, both the $1 and $20 bills will lose relevance and the $2 and $50 will replace.
There’s also a very real chance we see the introduction to things such as $200 billion or a $40 bill to counter this.
The obvious case against this is digital/card payments, where only 19% of payments are done with cash currently, which is down from 30% in 2017.
That said, there will always be some demand for cash and it’s very likely a new line of bills are introduced, while almost 100% certain the $1 and $20 get surpassed by the $50, $5 and $2 in the next 10-20 years.