In more detail
pack-years = packs per day × years smoked, or cigarettes per day / 20 × years. Periods of different smoking levels are worked out separately and added together. The measure captures cumulative exposure, but it treats intensity and duration as interchangeable, and research suggests that how many years someone smoked matters more for lung-cancer risk than how much they smoked per day. Cigars, pipes and vaping do not convert cleanly into pack-years.
Why it matters
Lung-cancer screening guidelines use pack-years. For example, the US Preventive Services Task Force's 2021 recommendation covers people aged 50 to 80 with at least 20 pack-years who smoke now or quit within the past 15 years, offering a yearly low-dose CT scan; other countries and organisations use different thresholds. Pack-years also help doctors judge the risk of COPD and heart disease. Whether screening is appropriate is a decision to make with a doctor.
Example
15 cigarettes a day for 30 years: 15 / 20 × 30 = 22.5 pack-years
2 packs a day for 15 years: 2 × 15 = 30 pack-years
½ pack for 20 years, then 1 pack for 10: 0.5×20 + 1×10 = 20 pack-years