Last updated: 2026-06-21

Pack-Years

Pack-years are a standard way to summarise how much a person has smoked over their lifetime. One pack-year means smoking one pack — 20 cigarettes — a day for one year. The measure lets very different smoking histories be compared on a single scale, and it underpins eligibility for lung-cancer screening.

The formula

$\text{pack-years} = \dfrac{\text{cigarettes per day}}{20} \times \text{years smoked}$

Dividing the daily count by 20 converts it to packs per day; multiplying by the number of years gives the cumulative total. Because the two factors trade off, 20 cigarettes a day for 15 years and 40 a day for 7.5 years both come to 15 pack-years.

Why the number matters

Cumulative exposure tracks closely with the risk of lung cancer, chronic obstructive pulmonary disease, and cardiovascular disease. Screening guidelines use it directly: the US Preventive Services Task Force recommends annual low-dose CT screening for adults aged 50 to 80 with at least a 20 pack-year history who currently smoke or quit within the past 15 years.

Worked example

Someone who smoked 25 cigarettes a day for 24 years has:

At 30 pack-years this history is well above the 20 pack-year screening threshold.

Counting changing habits

Pack-years accumulate only while a person actively smokes, so a former smoker keeps the total built up to the point they quit. When the daily amount changed over the years, the most accurate approach is to work out pack-years for each phase and add them together. For other cardiovascular-risk measures see the Blood Pressure Classifier.