Pack-Years Calculator (Smoking)
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/8/2026
Pack-years are calculated with (cigarettes per day ÷ 20) × number of years smoking, the reference pack containing 20 cigarettes. Smoking 20 cigarettes a day for 10 years represents 10 pack-years.
Explanation
The pack-year is the reference unit used in medicine to quantify a person's cumulative lifetime tobacco exposure, combining both the intensity (cigarettes per day) and the duration (number of years) of smoking. The calculation starts from the classic 20-cigarette pack: smoking a full pack a day for one year equals 1 pack-year; smoking two packs a day for one year equals 2 pack-years; and smoking half a pack a day for two years also equals 1 pack-year — this is what allows smoking histories that differ greatly in intensity and duration to be compared on the same scale. This indicator is notably used as a criterion in low-dose CT lung cancer screening recommendations: some international guidelines, such as those from the US Preventive Services Task Force (USPSTF), use a cumulative pack-year threshold (combined with age and, depending on the version, time since quitting) to identify people for whom this screening is recommended. This calculator only gives the pack-year figure, a result to discuss with a healthcare professional who can place it within the screening or follow-up criteria that apply, which evolve with current guidelines and generally aren't based on this figure alone.
Example: 20 cigarettes a day for 10 years
Inputs
Cigarettes per day: 20. Years smoking: 10.
Calculation
Pack-years = (20 ÷ 20) × 10 = 1 × 10 = 10.
Result
This exposure represents 10 pack-years.
Frequently asked questions
Why do 10 cigarettes/day for 20 years give the same result as 20 cigarettes/day for 10 years?
Because the calculation multiplies intensity by duration: both profiles represent the same cumulative total number of cigarettes smoked over the period (10 × 365 × 20 ≈ 20 × 365 × 10). The pack-year is precisely designed to make these two histories comparable on a common scale, even though they correspond to very different day-to-day habits.
How do you calculate pack-years if smoking intensity varied over the years?
This calculator assumes constant intensity over the whole period entered. For a history that varied (for example, 10 cigarettes/day for 5 years, then 20/day for 8 years), calculate the pack-years for each period separately with this calculator, then add up the results for each period.
Does this figure alone determine whether lung cancer screening is recommended?
No, pack-years are just one of several criteria used in screening recommendations (age, time since quitting if applicable, presence of other risk factors). These specific criteria evolve with current medical guidelines: this result should be brought to a healthcare professional, who can interpret it within the guidelines applicable to your situation.