As of 2018, according to the US Treasury, what is the average life of a US $1 bill in months of circulation?

As of December 2018, the average life span of a US $1 bill in circulation is about 6.6 years or 78 months before it is replaced due to wear. The first $1 bill was issued in 1862 as a Legal Tender Note with a portrait of Salmon P. Chase, the Treasury Secretary under President Abraham Lincoln. The current design of the US $1 bill technically dates to 1963 when the bill became a Federal Reserve Note as opposed to a Silver Certificate. The most notable and recognizable element of the modern $1 bill is the portrait of the first president, George Washington, painted by Gilbert Stuart.

The $1 bill issued in 1929 (under Series of 1928) was a $1 Silver Certificate. The treasury seal and serial numbers were in dark blue. The reverse had a large ornate ONE superimposed by ONE DOLLAR. These $1 Silver Certificates were issued until 1934.

In 1933, $1 US Notes were issued to supplement the supply of $1 Silver Certificates. Its treasury seal and serial numbers were red. Only a small number of these $1 bills entered circulation.

Key design changes to the $1 bill were made in 1934 and 1935. The blue numeral 1 became gray, the treasury seal was made smaller and superimposed by WASHINGTON D.C., and a stylized ONE DOLLAR was added over the treasury seal. The reverse was also changed to its current design, except for the absence of IN GOD WE TRUST. In 1957 the $1 bill became the first U.S. currency to bear the motto IN GOD WE TRUST. In 1969, the $1 bill began using the new treasury seal.