Pack Year Calculator
The standard measure of cumulative smoking exposure.
Your history
Cumulative exposure
One number for dose and duration together
A pack-year is one pack — twenty cigarettes — a day for one year. The calculation is(cigarettes per day ÷ 20) × years smoked, so ten a day for twenty years is ten pack-years, and forty a day for ten years is twenty. It compresses intensity and duration into a single figure, which is what makes it usable in research and clinical guidelines.
Its main practical use is lung cancer screening eligibility. US guidance recommends annual low-dose CT screening for adults aged 50-80 with a 20 pack-year history who currently smoke or quit within the past 15 years. The threshold was lowered from 30 pack-years in 2021, which substantially widened eligibility, particularly for women and Black adults who tend to develop lung cancer at lower cumulative exposure.
Two limitations worth knowing. Pack-years treat dose and duration as interchangeable, and evidence suggests duration matters more than intensity for lung cancer risk — forty years of ten a day is riskier than ten years of forty a day, though both compute to twenty. And the measure says nothing about what was smoked or how deeply it was inhaled. Risk falls substantially after quitting but does not return to never-smoker levels.
Medical disclaimer. This calculator is for general information and education only. It is not medical advice, not a diagnosis, and not a substitute for professional care. Screening eligibility is set by clinical guidelines that consider more than pack-years alone; discuss your own risk and screening options with a doctor.Speak to a doctor or another qualified clinician about any health decision.Last reviewed .
Frequently asked questions
How do you calculate pack years?
Divide cigarettes smoked per day by 20 to get packs per day, then multiply by the number of years smoked. Ten cigarettes a day for twenty years is 0.5 × 20, which is ten pack-years.
What pack-year history qualifies for lung cancer screening?
US guidance recommends annual low-dose CT for adults aged 50-80 with at least 20 pack-years who currently smoke or quit within the last 15 years. The threshold dropped from 30 in 2021. Other countries set their own criteria.
Does it matter whether I smoked heavily for a short time or lightly for longer?
Yes, though pack-years cannot capture it. Evidence suggests duration of smoking matters more than daily intensity for lung cancer risk, so forty years of ten a day carries more risk than ten years of forty a day, despite both being twenty pack-years.
Do pack-years reset after quitting?
No. Pack-years are a cumulative lifetime measure and do not decrease. Risk does fall substantially with time since quitting, which is why screening guidance uses both pack-years and years since quitting.
What about cigars, pipes or vaping?
Pack-years are defined for cigarettes. Cigars and pipes differ in inhalation pattern and are not directly convertible. Vaping is not measured in pack-years at all, and its long-term risk profile is still being established.