How this calculator works
NCI defines a pack-year using 20 cigarettes daily for a year. Divide daily cigarettes by 20, then multiply by years. For changing habits, calculate each interval and add the pack-years. This page also estimates total cigarettes using 365.25 days per year and converts those to retail packs of your chosen size. Those auxiliary totals are approximate for calendar periods. Pack-years describe exposure; they do not establish screening eligibility or an individual probability of disease. The result is an estimate for information only and is not medical advice.
Formula
Pack-years = (cigarettes per day / 20) x years. Cigarettes per day = 20 x pack-years / years. Years = 20 x pack-years / cigarettes per day.
Worked example
Ten cigarettes daily for 20 years gives 10 pack-years. Forty cigarettes daily for 5 years also gives 10 pack-years, even if the retail packs contain 25 cigarettes.
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Frequently asked questions
Does a pack of 25 change the clinical denominator?
No. A clinical pack-year always uses 20 cigarettes per day. The retail pack input changes only the pack-count outputs.
Can I include time after quitting?
No. Include only smoking intervals. Years since quitting are a separate part of a clinical history.
Can I convert cigars or vaping into pack-years?
There is no equivalent conversion in the NCI cigarette definition. This calculator does not invent one.