Pack-Years: How to Calculate Your Smoking History and When It Qualifies You for Lung Cancer Screening

"I've smoked for 30 years" means something very different at five cigarettes a day than at two packs. To compare very different smoking histories, medicine uses a unit that combines both how much and how long: the pack-year. Spanish-speaking clinicians often call it the smoking index (índice tabáquico) and Portuguese speakers the smoking load (carga tabágica). It is the same calculation under different names.

The pack-year formula

The US National Cancer Institute (NCI) defines it as the number of packs smoked per day multiplied by the number of years a person has smoked. One pack-year equals smoking one pack a day for one year, or two packs a day for half a year. The reference pack holds 20 cigarettes, so in practice: pack-years = cigarettes per day ÷ 20 × years smoked.

If your smoking changed over the years

Hardly anyone smokes the same amount all their life. The right way to do it is to split your history into periods of steady smoking, work out the pack-years for each one and add them up. Someone who smoked 10 cigarettes a day for 10 years (5 pack-years) and then 30 a day for another 10 (15 pack-years) has 20 pack-years, even if their rough average looks lower. That is why our calculator accepts up to three periods.

20 pack-years and lung cancer screening

The reason this number has become so important is screening with low-dose computed tomography (CT). In 2021 the USPSTF widened its criteria: it recommends a low-dose CT every year for adults aged 50 to 80 with a history of 20 pack-years or more who currently smoke or quit within the past 15 years. It used to require 30 pack-years and start at 55.

The American Cancer Society (ACS) went a step further in 2023: it keeps ages 50 to 80 and 20 pack-years, but no longer looks at how long ago someone quit. For the ACS, a former smoker with 20 pack-years is still a candidate even if they quit two decades ago. Both stress that the decision to start should be made in a conversation with a clinician, because screening also has downsides, such as false positives.

What about vaping, cigars or pipes?

Pack-years are designed for cigarettes. There is no validated way to convert e-cigarette, cigar or pipe use into pack-years, which is why our calculator does not make one up. If you use other forms of tobacco or nicotine as well as cigarettes, tell your doctor exactly that: it is useful information even if it does not fit the formula.