Pack year is a term used to describe the number of cigarettes a person has smoked over time. One pack year is defined as 20 manufactured cigarettes (one pack) smoked per day for one year.
I smoked 1 1/2 packs per day for 26 years, which equals 39 pack years:
1 pack per day x 26 years = 26 pack years
1/2 pack per day x 26 years = 13 pack years
26 pack years + 13 pack years = 39 pack years
See also: How Smoking Harms Us - Head to Toe