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Pack-Years Calculator

Measure lifetime cigarette exposure in pack-years from cigarettes per day and years smoked — the standard metric for lung-cancer screening.

Cigarettes per day
Years smoked
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How it works

Pack-years = (cigarettes per day ÷ 20) × years smoked · one pack = 20 cigarettes

A pack-year quantifies cumulative smoking exposure by treating one pack (20 cigarettes) a day for one year as a single unit, so that intensity and duration combine into one number. It is the metric clinicians use to gauge risk and eligibility for interventions — for example, US guidelines recommend annual low-dose CT lung-cancer screening for adults aged 50–80 with a 20-pack-year history who currently smoke or quit within 15 years. Two smokers can reach the same pack-year total by very different routes, and the figure captures dose rather than current habit.

Worked example

Someone who smoked 20 cigarettes (one pack) a day for 10 years has (20 ÷ 20) × 10 = 10 pack-years. A heavier smoker on 40 a day for 15 years reaches (40 ÷ 20) × 15 = 30 pack-years — past the 20 pack-year screening threshold.

Sources: Pack-year

Frequently asked questions

How many cigarettes are in a pack?

A standard pack contains 20 cigarettes, which is the denominator in the pack-year formula. If you count in a different pack size, convert to 20-cigarette packs first.

Why do pack-years matter?

They summarise total tobacco exposure, which correlates with risk of lung cancer, COPD and heart disease. Screening and some treatment guidelines are written in terms of a pack-year threshold, commonly 20 or 30.

How do I count varying amounts over the years?

Split your smoking history into periods of roughly constant intensity, compute pack-years for each, and add them. For instance 10 a day for 10 years plus 20 a day for 10 years is 5 + 10 = 15 pack-years.

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Educational estimate using standard published formulas — not medical advice, a diagnosis, or a substitute for clinical judgement. Dosing, diagnosis and treatment must be decided by a qualified clinician using validated tools and the full clinical picture.